A. M. KUZMIN

MICROCLINE

(Experiments in the systematics of alkali feldspars)

In the present work the author has subjected our knowledge of the alkali feldspars in general, and of microcline in particular, to a comprehensive study. Microcline is here regarded as a substance occurring in a metamic state, and as a mineral formation into which monoclinic and triclinic alkali feldspars pass on their path of polymorphic transformations during the transition from one set of physicochemical conditions to another. Therefore the author does not consider microcline an independent mineral, and regards it as a parallel and lattice-parallel intergrowth of a large number of lamellae of orthoclase, anorthoclase, and agiroclase. A comprehensive study of the alkali feldspars gives the author grounds to propose a new systematics for them.

Introduction

Microcline, as is known, belongs to the most widely distributed minerals of the Earth’s crust. It occurs in magmatic and metamorphic rocks, in whose composition, alongside other elements, potassium and sodium play an essential role. It also occurs in pegmatitic and feldspathic vein formations.

In my studies of the magmatic rocks of Transbaikalia, the Kuznetsk Alatau, and Altai, I, like many other investigators, encountered microcline—and it more than once attracted my attention from both the morphological and the genetic and paragenetic points of view. The question of the relation of microcline to orthoclase and anorthoclase has interested especially many scholars. The study of cases of the joint occurrence in rocks of microcline and other alkali feldspars, and the assimilation of the enormous literature on them, prompted me as well to express certain judgments, from new points of view, about microcline and its position in the series of potassium-sodium feldspars. The abundant literature concerning it merely indicates that the solution of the question of the relation of microcline to orthoclase, adularia, and anorthoclase has not only theoretical but also practical significance—for example, for understanding the conditions of formation of the corresponding rocks, for determining stratigraphic position, and for identifying magmatic rocks [20], etc.

It seems to me that the judgments set forth below solve, considerably more simply and naturally, the problem of what microcline is and what its relation is to the other potassium-sodium feldspars.

Taking this opportunity, I express my deep gratitude to Prof. Yu. A. Kuznetsov, who read this work and made a number of comments that I took into account in its final preparation.

Statement of the Problem

Answers to the questions posed above appear to be a rather difficult matter. These difficulties are determined in part by the fact that, with contemporary ideas about microcline and its position in the series of alkali feldspars,

feldspars have come together very closely. But in the published literature, from time to time, authors return directly or indirectly to the questions: what is microcline? What is orthoclase? What are their interrelations? In the literature, likewise, from time to time there appear the most varied judgments about their interrelations. This testifies to the fact that the question of microcline has not been resolved to this day, and that uncertainty as to the correctness of existing judgments concerning the relation of microcline to orthoclase and to other alkali feldspars prompts investigators to take up this question and to express their opinions, if they possess new material which seems to them either to confirm established judgments or to contradict existing views. In any case, the relations between microcline and orthoclase at the present time, as A. N. Zavaritskii says, cannot be considered established [37].

In order to answer the question of what microcline is and what its position is among the other alkali feldspars, I found it necessary to consider the history of the question of how our ideas about microcline took shape, and to give a comparative characterization of microcline and other alkali feldspars with respect to their morphological, physical, and chemical properties, and thereby to show what place microcline occupies among them and whether it is an independent mineral.

In resolving the question of microcline as a possible polymorphic modification, it was necessary to touch seriously and more broadly, insofar as my knowledge allowed, on the question of polymorphism and of transitional states of matter that may occur in transformations of one modification into another.

Basically, in considering the question of microcline, for the sake of objectivity I made extensive use of the literature data on alkali feldspars which, it seemed to me, deserved confidence and which are widely used in asserting the independent status of microcline.

History of Microcline

Microcline as an independent mineral was separated from the orthoclases by A. Breithaupt in 1830 and was named, more than 100 years ago, by the name under which it is still known to us [103]. The basis for its separation was the ascertainment of the fact that the cleavage planes along \(\{001\}\) and \(\{010\}\) are inclined to one another at an oblique angle, very close to a right angle. This feature was first established for green feldspars from Greenland. In addition, the suggestion was made that microcline is widely distributed among orthoclases.

Later A. Des Cloizeaux, in 1866 [41] and in 1876, confirmed that some potassium feldspars have a triclinic character, and retained for them the name microcline [107, 108]. For similar alkali feldspars he noted a characteristic and peculiar microscopic twin structure, which was partly expressed in simple fine-twin, it should be understood, parallel fibrous formations; or the fine-twin structure had an intersecting lattice character, especially characteristic of amazonites from various deposits.

Thus, the works of A. Breithaupt and A. Des Cloizeaux determined the independent significance of microcline as a triclinic mineral among the alkali feldspars known up to that time, although somewhat earlier A. Des Cloizeaux had regarded microcline as monoclinic [108].

At the same time, the question of the relations between orthoclase and microcline came to the fore. E. Mallard, it seems, was the first to raise this question [122]. The same investigator, while studying anomalies among minerals—especially minerals that readily form polysynthetic twins, mimetic twins that assume forms of higher

symmetry, came to the conclusion that among crystalline media there may be cases in which mimetic crystals will be composed of individuals so small, optically indistinguishable in size, that such crystals will appear physically homogeneous. From this point of view E. Mallard regarded orthoclase as possibly a mimetically complex twin of triclinic crystals; the twinned structure was, in this case, imagined as submicroscopic and in a number of cases so fine that it becomes even optically indistinguishable [122].

The considerations just set forth concerning the character of orthoclase are supported by A. Michel-Lévy, who likewise regards orthoclase as a submicroscopically polysynthetic twin composed of very thin lamellae of triclinic feldspar, which intergrow according to the albite and pericline twinning laws and thus form, in some cases, microcline, and in others—orthoclase [123]. In the former, the microcline lattice and fibrousness are to one degree or another visible under the microscope; in orthoclase the microcline structure becomes so fine that it is not visible even with more perfect methods of optical investigation. However, A. Michel-Lévy and A. Lacroix, despite the fact that orthoclase is regarded by them as a submicroscopically twinned formation, nevertheless consider it necessary in their description of rock-forming minerals to distinguish microcline and orthoclase from one another as two independent minerals [124].

What has just been said about orthoclase as a derivative of microcline is warmly supported by P. Groth and, thanks to his authority, becomes firmly and for a long time established in mineralogy and petrography [113; 114; 115].

K. G. Tschermak regarded the substance \((\mathrm{K},\mathrm{Na})_{2}\mathrm{Al}_{2}\mathrm{Si}_{6}\mathrm{O}_{16}\) as dimorphous, and orthoclase and microcline, in form and in optical properties, as heteromorphous [130]. Thus, in orthoclase and microcline he saw two polymorphous modifications.

R. Brauns, in contrast to the existing views concerning microcline, expressed a different point of view. In studying the optical anomaly of many minerals he came to the conclusion that the monoclinic forms of alkali feldspar—orthoclase—should be considered the normally equilibrium state of the substance; for example, adularia occur in this state in their freshest varieties. In opposition to this, he considered the microcline structure, and therefore microcline as well, to be secondary, arising in various ways [102]. Other scholars at that time also adhered to this point of view on microclines. Thus, I. Lehmann and F. Rinne explained the appearance of the microcline lattice in orthoclase as the result of pressure, while F. Klockmann and A. Beutell attributed the transformation of orthoclase into microcline to the influence of chemical agents appearing later.

Thus, A. Breithaupt, A. Des Cloizeaux, E. Mallard, A. Michel-Lévy, K. G. Tschermak, R. Brauns, and P. Groth are the founders of those views to which many scholars adhered and still adhere when considering the relations between orthoclase and microcline. An especially significant role in the formation of these views on potash-soda feldspars belongs to E. Mallard, for the considerations he expressed—that orthoclase is nothing other than submicroscopically twinned microcline—have persisted stubbornly up to the present time. Even now no one has ventured to reconsider this question radically as a whole. At least E. Bayer, in his works on the twinned structure of feldspars, on the basis of extensive material studied with the aid of the Fedorov universal-stage method, comes to the conclusion that under ordinary conditions the alkali feldspars are finely polysynthetically twinned formations, the character of whose manifestation depends on the degree

of the dispersion of a given twinned aggregate composed of a triclinic alkali feldspar [98].

Thus, after roughly 50 years, E. Bayer wholly repeats the established assertions, adding nothing essentially new to contemporary ideas about orthoclase and microcline. To this I would add that the inclination to see in orthoclase a submicroscopic structure is so persistent that it leads directly to unexpected conclusions. Thus, T. Barth, having found a rare case of monoclinic albite, without seriously reflecting on the matter, seeks to regard its monoclinic character as the result of submicroscopic twinning of albite lamellae [100].

From what has been said it is clear that certain ideas, which lack serious foundations, are sometimes capable of blinding a researcher and holding him captive for a long time, thereby giving no occasion for their reconsideration. Such an idea was precisely the thought of regarding orthoclase as submicroscopic microcline, and none of the scholars subjected this notion to serious criticism up to the present time. M. A. Usov was one of the few who, contrary to the established opinion, could not agree to consider microcline identical with orthoclase and orthoclase as submicroscopically twinned microcline. Against such an assertion, in his opinion, speaks the circumstance that “both in individual blocks of the microcline lattice and in the apparently homogeneous mass of the crystal of the latter, the extinction angle and the angle between the optical axes remain identical” and distinct from those of orthoclase [83]. This sound judgment, however, remained isolated and was supported by no one. Moreover, M. A. Usov himself did not take a firm position on this question.

In contrast to the foregoing, A. E. Fersman considers the relations between orthoclases and microclines to be complex, and the transitions of one into the other to occur “rather easily: an increase in temperature gives transitions of microcline into orthoclase, but for the reverse process the participation of extraneous, often purely mechanical, energy is necessary” [86].

In the history of microcline, A. K. Boldyrev was the first to reveal yet another new detail, to which before him due attention had not been paid. Microclines with the structure characteristic of them “are fresh and always fresher than other feldspars,” and they “in the overwhelming majority of cases are completely transparent” [17]. The same characteristic feature of microclines is emphasized by M. A. Usov [83] and V. N. Lodochnikov [54]. This freshness and transparency, as properties opposed to the decomposed orthoclases, A. K. Boldyrev tends to explain by the phenomenon of recrystallization of alkali feldspars, proceeding under the conditions of a hydrochemical process, of increased temperature and pressure, caused by dynamometamorphism [17].

Moreover, the solution of the question of orthoclase and microcline in its full scope becomes still more complicated as soon as one proceeds to establish the interrelations among sanidine, orthoclase, microcline, and anorthoclase. Thus, L. A. Vardanyants, on the basis of a study of alkali feldspars in the intrusive rocks of the Caucasus, comes to the conclusion that anorthoclases with microclines form a single isomorphous series, and therefore he proposes to regard anorthoclases only as microcline. He even recommends, with respect to the Caucasian neo-intrusions, abandoning the term anorthoclase in favor of microcline [21]. In opposition to him, D. S. Belyankin, with regard to the alkali feldspars of the same Caucasian rocks, speaks of their anorthoclasization, and on this basis regards microcline as lattice anorthoclase [15].

The two contradictory judgments of two major specialists just cited, concerning one and the same subject, testify only to the fact that we all understand microcline and its place among the other alkali—

feldspars. Hence, naturally, the need had arisen to take a critical approach to all the material on microcline and, at the same time, to approach the solution of the question: what is microcline? What is its place among the alkali feldspars known to us?

On the basis of what has been said, I believe that the question of microcline was raised by me in timely fashion, and that its solution from the standpoint of systematics is necessary and correct, if only because the term microcline has lately become increasingly indefinite, since essentially all alkali feldspars are being accepted by most investigators as microclines, and tendencies are appearing to regard them as anorthoclases; i.e., the concept “microcline” is being reduced to a group concept, and microcline in this group is the principal individual, whereas the other alkali feldspars, with the exception of albite, are varieties, different forms of its manifestation. One can hardly agree with such conclusions. In any case, one cannot “remain in the position of the old terminology” of the alkali feldspars, as D. S. Belyankin writes—or, rather, in the old concepts about them—for “this leads to many misunderstandings” [15].

Chemical composition of potassium–sodium feldspars

According to modern views, the potassium–sodium feldspars include sanidine, orthoclase, microcline, anorthoclase, and albite. Among the orthoclases, water-clear varieties such as adularia are distinguished. In addition, natronorthoclase is distinguished as a monoclinic variety of anorthoclase, and barbierite as monoclinic albite.

In chemical composition, the alkali feldspars are isomorphous compounds $KAlSi_3O_8$, $NaAlSi_3O_8$, $CaAl_2Si_2O_8$, $BaAl_2Si_2O_8$ (Fig. 1), with complete and incomplete miscibility depending on physicochemical conditions.

Fig. 1

In composition, under natural conditions, the alkali feldspars prove to be predominantly potassium feldspars, and they are much more widespread than alkali feldspars in which the sodium component predominates over the potassium component.

In addition to the foregoing, numerous special analyses of orthoclases, adularias, microclines, and sanidines have shown that in their composition,

apparently, as a constant isomorphous impurity, in addition to the corresponding aluminosilicates of Na, Ca, Ba, and Sr, the molecules RbAlSi₃O₈ and CsAlSi₃O₈ [26; 27; 28; 79; 80; 77; 59, etc.] take part; their amount for the rubidium constituent component ranges from 0.01 to 0.6%, and in individual cases reaches a relatively high percentage (up to 3%), which gave V. I. Vernadsky grounds for distinguishing rubidium microclines of composition (K,Rb)AlSi₃O₈. The amount of cesium in alkali feldspars ranges from traces to 0.24% Cs₂O.

It is further assumed that, under high-temperature conditions of mineral formation, the substances KAlSi₃O₈ and NaAlSi₃O₈ may, in contrast to the plagioclases, give several isomorphous series [18; 38; 83]. With decreasing temperature, the feldspars formed—initially homogeneous—decompose in a number of cases with the formation of perthites and antiperthites [72]. However, perthitization and antiperthitization of feldspars may with equal success also be a secondary phenomenon, connected with the introduction of albite or oligoclase material, and these secondary formations of perthites cannot always be readily distinguished from primary exsolution perthites.

Among general views on orthoclase and microcline as physicochemical media, one may cite the views of I. Vogt. He proceeded from the fact that KAlSi₃O₈ and NaAlSi₃O₈ at high temperatures form a continuous isomorphous series, which at lower temperatures becomes unstable and decomposes into the potassic-sodic and calcic-sodic parts of the feldspars, of which the latter forms perthitic intergrowths [131] in orthoclase and microcline.

The same view was held by P. N. Chirvinsky [92] and V. I. Luchitsky [56].

K. G. Uorren, in order to explain the wide distribution of microcline as compared with orthoclase, found it necessary to introduce into the equilibrium diagram a point of “inversion,” which would allow for a corresponding change in the crystal structure of these two representatives. The assumption of a possible “inversion” was explained by the lower solubility of the albite component in orthoclase material, which also caused the appearance of an intermediate formation in the form of microcline.

E. Spencer, likewise considering changes in potassic-sodic feldspars in connection with changes in temperature conditions, explains their diversity by the redistribution of potassium and sodium atoms and as a function of the sizes of their ions [78]. On this basis he distinguishes an orthoclase–microperthite series of feldspars, in which there are definite relationships between chemical composition and physical properties. This series of alkali feldspars belongs to the exsolution type, the microperthitic part of which, upon more or less prolonged heating to 750–800°, can again enter into solid solution, with a decrease in specific gravity and refractive index. From the orthoclase–microperthites, adularia and microcline E. Spencer would, “strictly speaking,” separate out, on the grounds that their 2V and Ng—Np are too large, while, on the other hand, sanidines and orthoclases, in contrast to orthoclase–perthites of the same composition, have a small 2V and lower refractive indices [78].

F. Machatschki conceived the chemical structure of alkali feldspars as spatially consisting of tetrahedra [SiO₄] and [AlO₄]. The character of the lattice itself was determined by the sizes of the corresponding cations. Thus, in his opinion, potassium and barium possess relatively large ionic radii, and, when placed in the lattice, they provide monoclinic symmetry. Conversely, the ionic radii of sodium and calcium are relatively small, and their placement in the spatial lattice will cause an additional deformation of the structure and lead to the necessity of creating triclinic symmetry [119].

H. Sh. Wang adheres entirely to this same position in resolving the question of the relations between orthoclase and microcline, and points out, moreover, that, owing to the change in the sizes of the potassium and sodium cations at high temperatures, they fully satisfy the requirements of monoclinic symmetry. With decreasing temperature, the different sizes of the potassium and sodium cations are in contradiction with their monoclinic distribution and therefore make necessary the existence, at lowered temperatures, of “agirinous” microcline. In accordance with this, it is assumed that microclines contain relatively more sodium than orthoclases [132]. Incidentally, the author gives no chemical data whatever in support of the idea just stated. He also asserts, without proof, that sanidines, in comparison with orthoclases, are somewhat richer in sodium [132].

Anorthite and celsian molecules are usually admixed with K-Na feldspars in comparatively small amounts. In individual cases the substances $\mathrm{KAlSi_3O_8}$ and $\mathrm{BaAl_2Si_2O_8}$ are capable of forming an independent isomorphous series of hyalophanes¹. Besides the barium molecule, in alkali feldspars the strontium component $(\mathrm{SrAl_2Si_2O_8})$ is apparently also present, in small quantities, rather often. The discrepancies in silica, when recalculating analytical data into the elementary components of the feldspars, long ago attracted the attention of the Russian scientists D. S. Belyankin and B. M. Kupletskii, who suppose that in the composition of alkali feldspars the kaliophilite and carnegieite molecules play some role [8; 49].

Until recently there existed the opinion that anorthoclases, in chemical composition, are calcium-sodium feldspars in which sodium considerably predominates over potassium [72; 127; 131], and that anorthoclase stands closer to albite than to orthoclase [136].

M. A. Usov was inclined to regard anorthoclases as albite-oligoclase in solid solution with potassium feldspar [84] and imagined them as metastable formations which, on decomposing, form plagioclase with antiperthitic intergrowths of potassium feldspar or form “chessboard albite” with remnants of orthoclase in it [84].

However, from the practice of studying anorthoclases of Caucasian rocks and anorthoclases from rocks of the Kola Peninsula and other places, no such conclusion follows, and it is supported by none of the investigators. As for chessboard albite, its appearance, as I have more than once had occasion to observe in studying the intrusive rocks of the Tashtagol iron deposit, is closely connected with albitization of the rock, to which both alkali feldspars and plagioclases are subjected to the same degree, with the formation of chessboard albite in their place. Foerstner’s anorthoclase we could regard as a particular case of two intersecting series in the region of albite-oligoclase: series of potassium-sodium feldspars with an isomorphous admixture of calcium, on the one hand, and of lime-sodium feldspars with an isomorphous admixture of potassium, on the other². The practice of studying Caucasian magmatic rocks led to the necessity of distinguishing, among alkali feldspars by optical properties, varieties close to Pantelleria anorthoclases. Later, as will be shown below, in chemical composition the Caucasian anorthoclases prove to be predominantly potassic. Therefore, at the present time, substantial changes are being introduced into ideas about anorthoclases, sharply changing the position of anorthoclase in the feldspar series. Thus, D. S. Belyankin, relying on extensive practical experience in

for the study of Caucasian anorthoclases, was the first to draw attention to the fact that among anorthoclases there are both predominantly potassic and sodic varieties [3; 10], among which the potassic anorthoclases predominate over the sodic ones.

S. P. Solov′ev, with respect to the alkali feldspars from the intrusions of Kabardino-Balkaria, indicates that the potassium molecule of the feldspars in anorthoclases, with a \(K_2O\) content from 10.47 to 12.91%, decidedly predominates over the sodium molecule, which ranges from 2.55 to 3.36% \(Na_2O\). An analogous observation is also true for anorthoclases [73], which are common components of Caucasian intrusive and effusive rocks. A considerable predominance in Caucasian anorthoclases of \(K_2O\) over \(Na_2O\) within the limits indicated by S. P. Solov′ev also follows from earlier works by other investigators, in particular from many works by D. S. Belyankin and V. N. Lodochnikov [55].

O. A. Vorob′eva, for microcline and anorthoclase from the rocks of the Khibiny and Lovozero tundras [30, table 9], gives a series of chemical analyses from which it follows that the contents of sodium oxide and potassium oxide in them vary within the following limits:

microcline: \(Na_2O\) from 0.45 to 6.75%, \(K_2O\) from 9.48 to 15.62%
anorthoclase: \(Na_2O\) from 0.13 to 5.64%, \(K_2O\) from 8.74 to 15.68%

According to V. G. Makhlaev, the alkali feldspars from the alkaline rocks of the Khibiny, represented by microcline and anorthoclase, are characterized by a predominance of the potassium constituent over the sodium constituent, the ratio \(K_2O : Na_2O\) in them ranging from 1.40 to 23.33 [58].

As follows from the data just cited, in chemical composition anorthoclases differ in no way from microcline, and in them the potassium content sharply predominates over sodium. Foreign scholars, too, with some delay, are inclining toward the opinion that anorthoclases are more often potassic than sodic, as V. N. Lodochnikov writes [55].

In addition to what has been said, we shall refer to L. A. Vardanyants, who, on the basis of a study of alkali feldspars in intrusive rocks of the Caucasus [21], comes to the conclusion that microcline and anorthoclase form a single isomorphous series, and hence he proposes that anorthoclases be regarded as nothing other than microclines.¹

However, before turning to a consideration of the chemical peculiarities of other alkali feldspars, we must pause somewhat over the views of N. G. Winchell and A. N. Winchell [136], who regard anorthoclase as a calcium-sodium mineral, and over the views of M. A. Usov, who conceived anorthoclases as albite-oligoclase occurring in solid solution with potassium feldspar [84], and see how far these views correspond to reality. To solve this problem we shall turn to K. Hintze, who in his work [116] gave 36 chemical analyses for anorthoclases. From their study it follows that in 27 cases \(CaO\) is more often present from traces to 1.40%. The \(K_2O\) content ranges from 1.3 to 5.7%. Such feldspars are indeed anorthoclases. The remaining anorthoclases of K. Hintze, with a calcium oxide content, as a rule, of more than 2% and with a relatively small amount of potassium oxide, must be oligoclases. In the anorthoclases studied by M. Fuchs, the amount of \(K_2O\) ranges from 1.2 to 5.7%, and calcium oxide from traces to 1.3%, and in one case, when the \(CaO\) content in the anorthoclase corresponded to 1.8%, \(K_2O\) was 3.6%, while \(2V\) and the extinction were anorthoclase-like [112]. Oligoclases similar to anorthoclases in extinction angles contain calcium oxide in the range from 2 to 6% and differ from anorthoclases also in physical properties. On this basis feldspars with a content

¹ In this interpretation it is unclear what L. A. Vardanyants understands by anorthoclase.

calcium oxide below 1.5%, with a relatively high content of potassium oxide and with very small extinction angles in sections parallel to (001), which have been assigned to oligoclases, most probably must be anorthoclases. For example, feldspars from the Helsingfors region, identified as oligoclases and containing calcium oxide in the range from 0.39 to 1.25% and potassium from 2.40 to 6.99% [116], correspond in character to anorthoclases. Feldspars from the Lake Laach region with 0.30% CaO and 3.40% K₂O, identified as oligoclase [116], by their properties should likewise be regarded as anorthoclases.

Familiarity with anorthoclases studied by M. Fookes, G. Förstner, D. S. Belyankin, V. N. Lodochnikov, L. A. Vardanyants, S. P. Solov’ev, S. A. Vorob’eva, and others makes it possible to conclude that, in chemical composition, anorthoclases do indeed constitute a continuous isomorphous series \((\mathrm{Na},\mathrm{K})\mathrm{AlSi}_3\mathrm{O}_8\); moreover, within some regions, potassium anorthoclases play the predominant role in the composition of igneous rocks, while in others—on the contrary—rocks with sodium anorthoclase are widespread.

Thus, on the basis of what has just been said alone, we may say that microclines and anorthoclases are potassium-sodium feldspars. Judging by their properties—for example, optical properties, characteristic for each of them—it may also be assumed that they are independent minerals with definitely expressed properties and, chemically, constitute two parallel series of potassium-sodium feldspars.

With regard to the chemical composition of sanidines, we must note that, in accordance with the summarized data of E. Mackinen and I. Fogg, it follows that the feldspar under consideration is not only predominantly potassic but, in a number of cases, is substantially sodic [120; 131], i.e., sanidine, in its chemical and physical characteristics, apparently constitutes a third series of alkali feldspars, in which the potassium and sodium contents likewise vary within wide limits in different occurrences. However, the majority of known examples show that in sanidines potassium predominates over sodium.

And, finally, in the chemical composition of adularias, studied from ore and Alpine veins, as follows from I. Fogg’s data, in terms of content the orthoclase molecule, amounting to 82.5 to 98%, quantitatively predominates over the albite molecule (from 2 to 17.3%) and the anorthite molecule (from 0.8 to 7.5%), taken together [131]. From what has been said it may be concluded that, in chemical composition, adularias will be closer to those alkali feldspars in which potassium decidedly predominates over sodium. But there are also grounds to suppose that adularia and cleavelandite constitute the same kind of continuous independent series, perhaps even two series of minerals of monoclinic and triclinic syngony, such as sanidine, microcline-orthoclase, and anorthoclase each constitute separately. Barth’s natronorthoclase¹ is only an intermediate member of the adularia-cleavelandite series.

The chemical composition of orthoclases is analogous to the composition of microclines, sanidines, and adularias. The potassium oxide content in orthoclases varies over wide limits. P. N. Chirvinsky, proceeding from 82 analyses [92], gives the average chemical composition of orthoclase and microcline from granitic rocks, in which K₂O = 11.44% and Na₂O = 2.79% [92]. Incidentally, P. N. Chirvinsky regards microcline as secondary, formed at the expense of orthoclase, which passes into more stable forms under the influence of various factors

¹ This term was first proposed by M. A. Usov [84] to designate hypothetical barberiite, the existence of which, as follows from chemical analyses of orthoclases, was considered probable, but to which at that time no corresponding name had been assigned.

According to I. F. Foth, microclines and orthoclases from granites and pegmatites, on the basis of 106 analyses, consist on average of 71.45% of the orthoclase molecule, 25.57% of the albite molecule, and 2.98% of the anorthite molecule [131], i.e., the majority of the potassium-sodium feldspars of the indicated rocks are predominantly potassium minerals.

The chemical composition of feldspars undergoes changes after their formation. F. Klockmann and A. Beutell explained the secondary nature of microclines chiefly by the subsequent alteration of orthoclases under the influence of chemical processes. This point of view was later supported by a number of scholars, who connect the appearance of a twinned structure in microcline with frequent cases of albitization of orthoclases, and see in this the reason why, around albitic perthitic intergrowths, a finely twinned microcline lattice readily develops, which fades out with distance from the veinlets of perthitic albite. B. Popov connected the microcline structure with the perthitization of originally homogeneous orthoclase; moreover, he explained the very process of formation of perthites by the circulation of solutions in protoclastically crushed feldspars. He also pointed out that microcline lamellae according to the pericline law of twinning develop near perthitic intergrowths; with increasing distance from them, the microcline lamellae are predominantly oriented according to the albite law [66; 67].

V. I. Luchitsky, considering the state of the question of the formation of microcline [56], likewise comes to the necessity of connecting its occurrence with processes of perthitization of orthoclases. As proof he cites the example of the Ništad rapakivi in Finland studied by him, in which the orthoclases lack perthites and even traces of cryptoperthites. If, in individual cases, albitic intergrowths do appear in the orthoclase, then microclinization appears simultaneously with them [56].

A. K. Boldyrev, as noted above, drew attention to the remarkable fact that microclines, in comparison with other feldspars, always look fresh, and he explained their formation, presumably, by recrystallization and the creation of fresh microclines in place of destroyed feldspars, with an obligatory hydrochemical process involving the introduction of potassium or even sodium, of which there must be “much” in the lattice alkali feldspar [17].

P. Niggli distinguishes among the alkali feldspars a monoclinic series, to which he assigns \([\mathrm{SiO}_4 \cdot \mathrm{SiO}_2 \cdot \mathrm{SiO}_2]^{\mathrm{Al}}_{\mathrm{K}}\)—orthoclase, adularia, and sanidine. To the substance of only the just-mentioned minerals there is isomorphously admixed the substance \([\mathrm{SiO}_4 \cdot \mathrm{SiO}_2 \cdot \mathrm{SiO}_2]^{\mathrm{Al}}_{\mathrm{Na}}\). He points out, among other things, that there are monoclinic homogeneous potassium-sodium feldspars in which the content of \([\mathrm{SiO}_4 \cdot \mathrm{SiO}_2 \cdot \mathrm{SiO}_2]^{\mathrm{Al}}_{\mathrm{Na}}\) reaches 70% by weight and which are known as sodium orthoclases or sodium sanidines [126].

To the triclinic series of feldspars, according to P. Niggli, belongs microcline of composition \([\mathrm{SiO}_4 \cdot \mathrm{SiO}_2 \cdot \mathrm{SiO}_2]^{\mathrm{Al}}_{\mathrm{K}}\); between microclines and albite, as he writes, there are mixed crystals of composition \([\mathrm{SiO}_4 \cdot \mathrm{SiO}_2 \cdot \mathrm{SiO}_2]^{\mathrm{Al}}_{(\mathrm{K},\mathrm{Na})}\); among them one may distinguish sodium microclines, anorthoclases, and potassium albite [126].

The chemical composition of alkali feldspars, as isomorphous mixtures of substances corresponding to the compositions \(\mathrm{KAlSi}_3\mathrm{O}_8\), \(\mathrm{NaAlSi}_3\mathrm{O}_8\), and \(\mathrm{BaAl}_2\mathrm{Si}_2\mathrm{O}_8\), on the basis of available graphical triangular diagrams [97; 126; 131], can also be represented graphically in the form of a tetrahedron (Fig. 1), at whose vertices the just-mentioned components will be placed, while the faces of this tetrahedron will give an idea of possible isomorphous mixtures among each three components. The interior space of the tetrahedron, in turn, will give an idea of feldspars more complex in chemical composition, as isomorphous compounds of four principal components.

figures. The unhatched areas define the possible field of greatest distribution in the plane of the face of the tetrahedron of feldspar points, the position of which is established on the basis of chemical analyses.

It should be noted, however, that, although we have sufficient data for constructing such a triangle for the triad \(KAlSi_3O_8—NaAlSi_3O_8—CaAl_2Si_2O_8\), considerably fewer such data are available for the other triad \(KAlSi_3O_8—NaAlSi_3O_8—BaAl_2Si_2O_8\), and only very rare data are available for the two other triads. The latter is probably to be explained by the fact that, in analyzing feldspars, the necessity of testing them for the presence of such components as barium, strontium, rubidium, and cesium is often overlooked.

Before concluding this chapter, two further questions should be raised.

First. If anorthoclases form a continuous isomorphous series of composition \((K,Na)AlSi_3O_8\), also containing, as an isomorphous admixture, \(CaAl_2Si_2O_8\), then, on cooling, according to the theories developed to explain the formation of orthoclase-perthites and microcline-perthites, they should form exsolution perthites. Such a picture, as a rule, is not observed by investigators of anorthoclases. On the contrary, one of the characteristic features of anorthoclases is their optical homogeneity. Perthitization, uneven extinction, and lattice structure in anorthoclases are rare phenomena. This property of homogeneity is also a characteristic feature of sanidines and of adularias. In that case it would be proper to regard all perthites of alkali feldspars as later formations, arising as a result of the penetration of albite-oligoclase solutions into cracks of fractured grains of orthoclase and microcline. The perthitization of microclines probably occurs simultaneously with microclinization.

The second question concerns the topochemical relations between the overall chemistry of the rock and the chemistry of the alkali feldspars. These relations are usually considered direct. For example, M. A. Usov explains the increased content of sodium oxide over potassium oxide in a number of rocks of Dzhungaria by the fact that microcline-anorthoclase or natronorthoclase¹ “contains many albite molecules” [82; 83]. Therefore, the conclusions on the question touched upon that V. G. Makhlaev makes are extremely interesting and timely. He studied the ratios of potassium and sodium in rocks and in the alkali feldspars characterizing them in porphyritic nepheline syenite, foyite, khibinite, vein nepheline syenite, feldspar ijolite, aegirine–hornblende syenite, and micaceous nepheline syenite, in which \(K_2O : Na_2O\), in the order in which they are listed, varies from 0.61 to 3.84, while in the rock-forming feldspars it varies from 1.40 to 23.33 [58]. The figures cited clearly show that, despite the predominance of sodium oxide in the chemistry of the rocks (this especially concerns the first three of V. G. Makhlaev’s above-listed rocks), in microclines and anorthoclases the potassium part, as a rule, quantitatively predominates over the sodium part.

Similar regular relations between potassium and sodium in intrusive rocks and in the alkali feldspars contained in them could also be established for other points. However limited our information on this question may be, we are entitled to conclude that, in the case of a predominance of \(Na_2O\) over \(K_2O\) in the bulk composition, there is still no basis for assuming that, in the alkali feldspars as well, the albite part will predominate over the orthoclase part. Most likely, it will be correct to suppose that, in a considerably greater—

¹ M. A. Usov does not give chemical analyses of alkali feldspars from the rocks of Dzhungaria. Their assignment to anorthoclases and natronorthoclases is made on the basis of optical data available from Rosenbusch and Förstner.

in most cases inverse relations will obtain. Hence, naturally, one may suppose, for example, that the microcline-anorthoclases and natronorthoclases of Dzhungaria, established only on the basis of optical data [82; 83], will in reality be anorthoclases and orthoclases, predominantly potassium rather than sodium minerals.

Thus, chemically, the potassium-sodium feldspars constitute an isomorphous series from representatives of the composition $\mathrm{KAlSi_3O_8}$ to representatives of the composition $\mathrm{NaAlSi_3O_8}$. This series, however, as is assumed, is fully manifested under certain physicochemical conditions. Among this series, the most widespread are feldspars in which the potassium constituent always predominates over the sodium constituent.

Crystallographic relations between orthoclase and microcline

The crystallographic relations between orthoclase and microcline, as noted above, were first studied by A. Breithaupt, who, measuring the angles between the planes $P(001):M(010)$, found that this angle in the varieties which he called microcline is not equal to a right angle and deviates from it by $0^\circ22'$ [103]. A. Des Cloizeaux, as a result of a careful study of orthoclases from many localities, found that this angle ranges from $89^\circ20'$ to $89^\circ50'$ and thus confirmed A. Breithaupt’s data [107].

Later results of the study of the relationships $(001):(010)$ for microcline are available, for example, from A. Michel-Lévy, who regarded the microcline lattice as a consequence of twinning of thin plates—lamellae of feldspar according to the albite and pericline laws [124].

The refinement of the morphological properties of microcline was carried out by O. B. Böggild, and the determination of the geometrical constants for orthoclase in its adularia [41] and sanidine varieties [43] was made by N. Koksharov; from these it follows:

Table 1. Geometrical constants of alkali feldspars

Name	$a:b:c$	$\angle \alpha$	$\angle \beta$	$\angle \gamma$	Note
Sanidine	$0.62277 : 1 : 0.55191$	$90^\circ$	$116^\circ01'47''$	$90^\circ$	[43]
Orthoclase	$0.65851 : 1 : 0.55538$	$90^\circ$	$116^\circ3'14''$	$90^\circ$	[41]
Microcline	$0.6637 : 1$	$89^\circ18.5'$	$115^\circ50'$	$92^\circ9.5'$	[105]

O. B. Böggild determined the angle between $(001):(010)$ for microcline to be $89^\circ48'—89^\circ43'$, while the angle between the faces $(100)$ and $(010)$ proved to be $88^\circ6.5'—87^\circ56'$. According to K. Hintze, the magnitude of the angle between the third and second pinacoids in microclines varies on average within the limits from $89^\circ18'$ to $89^\circ46'$ [116].

However, for typical microclines from various localities other values are given for the angle between $(001):(010)$. Thus, according to F. Klockmann, this angle is $89^\circ53'$ for microcline from Hirschberg; N. V. Ussing gives, for microcline lacking a visible lattice, the angle $(001):(010)$ as $89^\circ30'$ [129]. The same angle for amazonite from Pike’s Peak is $89^\circ44'$. Other constants are also given for Hirschberg microcline: $a:b:c = 0.6495 : 1 : 0.5546$; $\angle\alpha = 90^\circ07'$, $\angle\beta = 115^\circ50'$, $\angle\gamma = 89^\circ55'$.

The geometrical constants for orthoclase given by N. Koksharov still retain their value, and if deviations from

his data, as, for example, for natro-sanidine from San-Pietro: \(a:b:c = 0.6429:1:0.5524\), \(\angle \beta = 63^\circ 37.5'\), close to the data of F. Klockmann, then these deviations most likely depend on the chemical composition and structural features of the minerals rather than on inaccuracies in measurements of their crystals, as K. Hintze thought. In any case, the available data from measurements of crystals of alkali feldspars make it possible to divide the latter into two subgroups. One of them includes crystals in which the angle between the faces \((010):(001)\) is \(90^\circ\). This will include the monoclinic crystals of the K–Na feldspars of the sanidine, orthoclase, and adularia series.

The other subgroup of alkali feldspars consists of the anorthoclase, microcline (agiroclase), and adularia series of feldspars, for which the angle between the faces \((001):(010)\) is not equal to \(90^\circ\).

Here one should also note cases of possible parallel intergrowth of orthoclase and microcline. Information on this is available from K. Hintze [116] and A. Des Cloizeaux [107]. D. Belyankin cites a case of twin intergrowth of anorthoclase (with the properties of microcline, A. K.) with orthoclase [10]. M. Reinhard and P. Bechlin, considering the microcline lattice in microcline, came to the conclusion that the parallel twin lamellae of microcline are nothing other than a parallel intergrowth of microcline lamellae with orthoclase lamellae [128].

Such parallel intergrowth of microcline and orthoclase lamellae is rather often observed in microclines. I myself have had occasion to establish analogous cases in microclines of many igneous and metamorphic rocks of the Shilka-Nerchinsk district of Eastern Transbaikalia. This feature of microclines was expressed in the fact that, when microcline lamellae were set parallel to the crosshairs of the ocular, some of them showed straight extinction, while others located next to them showed oblique extinction. Such relationships of lamellae in the microcline lattice, confirmed on the Fedorov stage, I, independently of M. Reinhard and P. Bechlin, had to regard as a parallel “intergrowth” of orthoclase with microcline, and to consider that this phenomenon is widespread in microclines.

Thus, on the basis of the study of morphological properties, the alkali feldspars are divided into monoclinic and triclinic series, and the possibility emerges of regarding the microcline lattice as a parallel “intergrowth” of monoclinic alkali feldspars with triclinic ones.

Typomorphism of Alkali Feldspars

The habit of crystals of alkali feldspars is one of the most important indicators of the physicochemical conditions of their formation, and especially of temperature.

Indications of the typomorphism of feldspars are found in A. Lacroix, who, on the basis of the character of the forms, distinguished seven principal types [118].

To the first type he assigned crystals of prismatic habit, elongated along the third (\(c\)) crystallographic axis. The faces present are, accordingly, developed uniformly and are represented by the forms \(M\{010\}\), \(T\{110\}\), \(P\{001\}\), and \(y\{\bar{2}01\}\). Crystals of this kind, according to the ratio of the forms \(\{010\}\) and \(\{110\}\) ^1), acquire a hexagonal habit (Fig. 2A). Crystals of similar habit are predominantly encountered in the form of Carlsbad twins (Fig. 2AA).

The second type of crystals, elongated along the first axis \(a\), is determined by the predominant and more or less equal development of the forms \(P\{001\}\)

^1) No fundamental distinction is made between \(T(110)\) and \(l(\bar{1}10)\), \(n(021)\) and \(e(021)\).

Fig. 2

and M{010}, which gives the crystals a square appearance. On such crystals, moreover, the forms T{110} and y{201} are common, of which in some cases the first predominates, in others the second (Fig. 2 B).

The third type of crystals of alkali feldspars has a tabular appearance and is determined by the predominant development of the form M{010}. In this type of crystal one should distinguish, in essence, two subtypes. In one of these two subtypes the character of the crystals is determined by the more or less equal development of the forms P{001} and y{$\overline{2}$01}, which gives the tabular (Fig. 2 V) crystals an overall square appearance. If the crystals are elongated along the axis a, they approach type II (Fig. 2 B). In the second subtype, unlike the first, the forms P{001} and x{$\overline{1}$01}, with their more or less equal development, impart a sharply expressed rhombic character to the platy crystals (Fig. 2 G).

The three types of crystals just listed, according to A. Lacroix, are characteristic of all feldspars; as for the remaining types of crystals, they are characteristic only of particular varieties of potassium-sodium feldspars [118]. Thus, the fourth type of crystals is characteristic of adularia and is determined by the predominant development of the prismatic form T{110}, which gives crystals of this kind, in the zone [001], a rhombic character (Fig. 2D).

The fifth type of crystals is determined by the development of the faces P(001) and M(010). The crystals are elongated along the second crystallographic axis b. In their appearance such crystals resemble pericline crystals of albite. This pericline type of crystal among alkali feldspars is encountered rarely [118].

The sixth type of crystals is tabular, more or less uniformly elongated in the plane (001) (Fig. 2 E).

And, finally, the seventh type of crystals is determined by the development of the faces M(010) and x($\overline{1}$01), elongated along the axis of the zone [101]; in their character the crystals of this type also acquire a square appearance¹ and at first glance resemble the crystals of Fig. 2 B. A similar type of crystal among feldspars is most characteristic of albites [118].

In the crystal habit of potassium-sodium feldspars, G. Kalb distinguishes only four main types [117], which in character are typical of definite genetic conditions of formation (Fig. 3). Thus, as the first type of crystals (a) he singles out crystals typical of high-temperature conditions of mineral formation, to which he assigned porphyritic segregations in the trachytes of Drachenfels. Characteristic of this type is the presence of the faces M{010} and P{001}, which are developed more or less equally; the face y{201} present, as compared with T{110}, has predominant development (Fig. 3a). The crystals are elongated along the first axis and have a square appearance.

To the next type (b) were assigned crystals whose habit is essentially determined by the development of the faces M(010), P(001), T(110) and x($\overline{1}$01), y($\overline{2}$01). The crystals are isometric in the plane b and c and somewhat compressed along the axis of the zone [100] (Fig. 3 b). Such crystals are typical of conditions in which pneumatolytic processes play an essential role, leading to the formation of feldspar druses in granites and pegmatites [117].

Of hydrothermal origin, the potassium feldspars—adularias, occurring in Alpine mineral and ore veins, are known in two types. In one of them—the Zillertal type—the faces T(110), x($\overline{1}$01), P(001), and M(010) acquire predominant importance (Fig. 3 v); in the other type—the Felso-

¹ A. Lacroix does not illustrate this type of crystal with a corresponding figure.

… preferential development is acquired by the faces T(110) and x(\(\overline{1}01\)), which give the crystals the appearance of trigonal rhombohedra, somewhat flattened along the crystallographic axis \(a\) (Fig. 3г).

G. Kalb illustrates the general qualitative relationships between the types of crystals in quantitative expression, depending on physicochemical conditions, by the formal scheme of change in the habit of feldspars presented in Fig. 4 [86].

Fig. 3. Change in the habit of potassium feldspars.
а—Drachenfels type—(001), (010), (\(\overline{2}01\)), (110); б—Striegau type—(010), (001), (110), (\(\overline{1}01\)), (\(\overline{2}01\)); в—Zillertal type—(110), (\(\overline{1}01\)), (001), (010); г—Felsöbánya type—(110), (101). (After Kalb)

Fig. 4. Scheme of the development of forms on crystals of alkali feldspars as a function of physicochemical conditions (A. E. Fersman, Pegmatites, I)

Diagram labels:

• Type
• A. Drachenfels
• Б. Striegau
• В. Zillertal
• Г. Felsöbánya
• Phase
• A—orthoclase
• E—orthoclase-perthite
• H—adularia
• I—orthoclase (ore veins)
• Predominance of faces
• \(\rho\)
• M
• y
• T
• x
• Probable temperature
• 800°
• 500
• 350
• 250

Finally, on the basis of habit P. Niggli groups the crystals of alkali feldspars into four principal types, which in Fig. 5 are illustrated fairly well by the corresponding drawings [126]. Thus, he distinguishes first of all the adularia type of crystals, for which the typical forms are T{110} and x{\(\overline{1}01\)}. In the simplest crystals with the faces T{110} and x{\(\overline{1}01\)}, their form will resemble a rhombohedron, somewhat compressed along the \(a\) axis, and in character will correspond to crystals of the Felsöbánya type [117]. With the appearance of the form P{001} and its growth, the faces of other simple forms develop at the same time, as is illustrated by a series of examples (Fig. 5 A–Д). However, the simplest case is represented by a crystal elongated along the \(c\) axis (type IV of A. Lacroix), on which chiefly the faces of the form T{110} are developed, together with more or less equally developed forms P{001} and x{\(\overline{1}01\)} (Fig. 5, В–Д). This type of crystal proves to be close to the Zillertal type [117].

The subsequent forms given by P. Niggli are characteristic of various varieties of orthoclase. According to the habit of the crystals, we may distinguish the following three types.

The orthoclase hexagonaloid¹ type of crystals (Fig. 5, Е–Л) is characterized by a prismatic habit; the crystals are elongated along the third crystallographic axis \((c)\). In their simple forms in the zone \([001]\), with equal dimensions of the faces \(T\{110\}\) and \(M\{010\}\), crystals of alkali feldspars have a hexagonaloid character.

Fig. 5. Habit of crystals of alkali feldspars (after P. Niggli)

Feldspars show a hexagonaloid character. Deviation from the simplest combinations and from more or less isometric crystals, for which the predominant development of the forms \(T\{110\}\) is thus characteristic,

Table 2. Typomorphism of alkali feldspars

Type No.	Crystal types	Type-determining forms — principal forms	Type-determining forms — accompanying forms	Additional forms	Note
I	Vertical-prismatic hexagonaloid1 . . .	$(010), (110)$	$(001), (\bar{2}01)$	$(100), (130), (021), (\bar{1}01), (\bar{1}11)$	Type I of A. Lacroix
II	Square-prismatic positive . . . . .	$(001), (010)$	$(110), (\bar{2}01)$	$(130), (\bar{1}01), (100)$	Type II of A. Lacroix Type of Drachenfels
III-A	Sanidine platy-square . . . . . . .	$(010)$	$(001), (\bar{2}01)$	$(110), (130), (\bar{1}11)$	Type III of A. Lacroix
III-B	Sanidine platy-rhombic . . . . . . .	$(010)$	$(001), (\bar{1}11)$ or $(001), (\bar{1}01)$	$(110), (111), (\bar{2}01), (13\bar{1})$	” ” ”
IV	Prismatic hexagonaloid pegmatitic . . .	$(110), (010)$	$(001), (\bar{1}01)$	$(\bar{2}01), (021), (\bar{1}11)$	Striegau type
V	Square-prismatic negative . . . . .	$(010), (\bar{1}01)$	$(110), (001)$	$(\bar{1}11)$	Type VII of A. Lacroix
VI	Basal-tabular . . . . . . . . . . .	$(001)$	$(010), (110)$	$(\bar{1}11), (\bar{2}11), (\bar{1}01)$	Type VI of A. Lacroix
VII-A	Adularia-rhombohedral . . . . . . . .	$(110), (\bar{1}01)$		$(001)$	Felsöbánya type
VII-B	Adularia prismatic vertical (rhombo-prismatic) . . . . . . . . . . . .	$(110)$	$(001), (\bar{1}01)$	$(021), (\bar{2}11), (010)$	Type IV of A. Lacroix Zillertal type

¹ In Carlsbad twins, alkali feldspars often assume a tabular and platy character along $(010)$ or tend to become hexagonaloid.

M {010} and P {001}, will be determined by the relatively strong development of the faces of one of the forms of the vertical prism {110} or {010}, or of the faces of one of the forms {101} or {201} with respect to {001} (Fig. 5, E–K).

The sanidine type of crystals is characterized by a tabular habit. Crystals with predominant development of the faces M{010}, P{001} and y {201} (Fig. 5, L–P) correspond to type II of feldspar crystals of square-platy habit (Fig. 2, B) according to A. Lacroix, the most characteristic for sanidine. Crystals with faces M{010}, P{001} and o{111} in sections parallel to (010) have a platy-rhombic habit and correspond to type III of A. Lacroix’s crystals, also characteristic of sanidine (Fig. 5, O).

The orthoclase square or pseudohexadric type. The crystals belonging here, shown in Fig. 5, P–U, in their habit correspond to type II of A. Lacroix or to the type of Drachenfels.

The basal-tabular type of crystals, occurring among potassium-sodium feldspars, consists of tabular forms whose character is determined by the corresponding relatively strong development of the basal pinacoid (Fig. 5, F, X).

A. Lacroix, for sanidine from the Puy-de-Dôme region, describes crystals on which the faces P{001}, M{010} and y {201}, and only these, were developed. The face of the basopinacoid, in its development, predominates over the other two [118].

The data given on the morphology of crystals of the alkali feldspars, in accordance with the conditions of their formation, give a clear idea of their typomorphism. However, in describing feldspars occurring both as porphyritic and porphyry phenocrysts in igneous rocks, and in druses of granites and pegmatites and in the pegmatites themselves, it is usually forgotten to note their crystal habit. Study of this feature of feldspars should help decipher many features of rocks with respect to particular moments of their formation.

Therefore, for the sake of uniformity, we must recommend adhering to the typomorphic classification of the forms of alkali feldspars that we have worked out, specified in Table 2 and requiring no further explanation¹).

Optical properties of sanidine, orthoclase and microcline and of other potassium-sodium feldspars²)

The optical properties of alkali feldspars were first studied by A. Des Cloizeaux, who showed that microcline, in contrast to orthoclase, is asymmetric [107]. He also first noted that some amazonites have a thin-lamellar structure characteristic of microcline, clearly distinguishable in optical study of this mineral [108]. For microcline—amazonite from Murzinka, the same investigator established the position of the plane of the optical axes, which makes an angle NgNp:P (001),

¹) By the “forms determining the habit” of crystals we mean forms with the most strongly developed faces, of which some are “leading,” determining the type of habit, while others are “accompanying forms,” developing as necessary for the completion of the crystal; their subordinate significance relative to the leading forms is evident. “Additional forms,” as a rule, are weakly developed or absent; their presence is only a detail that in no way affects the habit of the crystals. The topomineralogical significance of additional forms is as yet unknown, but undoubtedly depends on the character of the energetics of the medium in which they were formed.

²) In the chapter under consideration, data concerning the magnitude of refractive indices are omitted, for the reason that this part of the properties of feldspars requires a very large and painstaking effort in systematizing all the material, assessing its reliability, and correlating it with the chemical composition of the minerals. D. S. Belyankin’s conclusions on the refractive indices of a series of alkali feldspars, essentially anorthoclase feldspars, are very valuable, but they are only partial and not generalized.

equal to 10°26′, and \(NpNm : M(010)\) forms an angle of 17°48′ [107]. H. V. Ussing gives approximately the same constants for “simple”—untwinned microcline from Haserna near Meissen, in which the angle \(NgNp : (001)\) is 12°8′, and \(NpNm : (010)\) is 17°48′ [129]. The works of H. V. Ussing at the same time establish that the lattice structure in microclines is not always manifested and that, in essence, it is not obligatory for microcline as such. In this latter case, in order not to take non-latticed microcline for orthoclase, it is necessary, as V. N. Lodochnikov recommends [55], to carry out an optical analysis that would make it possible to establish the true position of the optical indicatrix with respect to \(P(001)\) and \(M(010)\). Without this it is easy to fall into error: to take triclinic alkali feldspars for orthoclase. Such mistakes in petrography are very often made by foreigners [132].

For microclines the extinction angle \(Np : (010)\) on the plane \(P(001)\), according to many data, ranges from 14 to 20°. In latticed microcline, according to V. N. Lodochnikov, the extinction angle \(Np : (010)\) ranges from 0 to 19° [55]. M. Reinhard and R. Bechlin, in latticed microclines, along with bands with oblique extinction, found bands with an extinction angle for \(Np : (010)\) equal to zero, i.e., such bands in their optical properties correspond to orthoclase. In general, for the majority of microclines, especially non-latticed ones, this angle ranges within 15—17° [128].

My investigations of microclines from the porphyritic granites of Eastern Transbaikalia, using the Fedorov method, showed that, indeed, in sections parallel to (001), the value of the angle \(Np : (010)\) within one and the same grain at its different points at first glance seems to fluctuate, in general, over wide limits from 0 to 20°. However, upon more detailed and careful investigation it proves that, by extinction, a number of bands can be distinguished for which the angle \(Np : (010)\) has strictly definite values. Thus, among them one can distinguish bands—spindles with straight extinction, which correspond to orthoclase. Then one can distinguish bands that extinguish within the range from 0 to 3—4°, more rarely up to 6°, and correspond, apparently, to anorthoclase. Finally, the third group of twin bands has extinctions of more than 10° and, mainly, fluctuates within the limits 14—17°, i.e., in the character of the position of the indicatrix such lamellae correspond to agiroclase. Thus, our investigations fully confirm what has been stated above about microcline and at the same time show that microcline represents regularly intergrown plates of physically different feldspars, or that the complexity of the structure of microcline may be regarded as the breakdown of an originally homogeneous \(\gamma\)-orthoclase in the process of its polymorphic transformations into \(\delta\)-orthoclase, anorthoclase, and agiroclase.

The extinction angle in non-latticed microcline on the plane (010) \(Nm : \perp (001)\) is an angle of 5—7° [12; 15], as in orthoclases, and with incomplete optical analysis microclines devoid of fibrous structure may mistakenly be taken for orthoclase. The angle of the optical axes for microclines, on the basis of data from the above-mentioned investigators, as a rule, for its different individuals ranges from \((-)\) 70° to \((-)\) 88°. According to E. Bäyer, for microclines the value \(2V\) may be less than 60°.

V. N. Lodochnikov, moreover, emphasizes facts known to all, that potassic feldspars may be patchy, that the lattice within the limits of one and the same grain is expressed unequally and unevenly, up to its complete absence. A. K. Boldyrev among microclines of pegmatites distinguishes monocrystalline microclines (our agiroclases), indistinctly striped or indistinctly latticed, plagioclase-like, i.e., distinctly polysynthetic, usually with wedging-out bands, transitional between striped and latticed, and distinctly latticed [18]. Thus, a regular-aggregate structure is evident for microclines.

The magnitude of the extinction angle within one and the same microcline grain varies over wide limits, from 0 to 20°; likewise within one and the same grain the magnitude \(2V\) also proves to be variable [55]. In confirmation of the data just cited, one may point, for example, to microcline from the Anabar granites, which is represented both by non-grating microclines and microcline-perthites and by microclines with a grating and patchy structure, expressed to varying degrees. The extinction angle within a grain has different values in its different parts [71]. In an analogous way, as my studies have shown, microclines in the porphyritic granites of the Shilka-Nerchinsk district of E. Transbaikalia behave. The optical properties within one and the same microcline grain from pegmatites of the Ilmen Mountains change [77].

Finally, with respect to microcline we shall cite one more authoritative observation: A. N. Zavaritskii, in assigning potassium feldspars to orthoclase or microcline, carries out a careful optical analysis of them. This latter gives him grounds to come to the conclusion that microclines, for example, in the rocks of the Berdyaush pluton occur together with orthoclases, whose microclinization he considers obvious [38]. With respect to the microclines of the Berdyaush rapakivi, A. N. Zavaritskii notes that they are heterogeneous, patchy- or wavy-extinguishing. The heterogeneity within ovoids often increases toward the periphery of the grain, in which \(2V\) varies from \((-)\ 59\) to \((-)\ 90^\circ\). Along with this, in the rocks of the Berdyaush pluton there occur rather often microclines devoid of a microcline grating and of fibrousness [69].

From the numerous optical data for microcline cited in the literature it follows that microcline, in the form in which we usually understand it, is not a strictly definite mineral. Among minerals referred to microclines, we must distinguish, as independent minerals, triclinic feldspars with an angle \(Np:(010)\) varying within the limits 14–20°, which appear to us optically homogeneous. These latter should be called agiroclases. To microclines proper we must assign those varieties of feldspars for which a grating, fibrous, patchy, and other kinds of manifestation of physical heterogeneity of their structure are characteristic.

For sanidines there are optically two types, or two modifications. In one of them the plane of the optical axes \(NgNp\) is situated, as in orthoclases, perpendicular to (010); in another modification of sanidine the plane of the optical axes is parallel to (010) [108; 126]. \(Np:a\) on the plane (010) forms an angle of about 3–6°. The angle of the optical axes in size is usually small and varies within the limits from \((-)\ 20\) to \((-)\ 35^\circ\). V. N. Lodochnikov notes in this connection that in measurements sanidines sometimes behave as optically negative uniaxial minerals, and the magnitude of the angle of the optical axes in such a case drops to zero [55].

The optical properties of adularia and orthoclase are determined by the following data. Transparent varieties of orthoclases are usually assigned to adularia. Orthoclases themselves are for the most part nontransparent, cloudy, white, reddish, gray, yellowish, or greenish in color. The plane of the optical axes in both minerals is situated perpendicular to (010), \(Ng\) coincides with the axis \(b\), while \(Np:a\) usually forms an angle of 4–5°, \(Nm:c\) is equal to 19–23°, or \(Nm:\perp(001)\) is equal to 5–7° and no more [55]. However, A. N. Zavaritskii considered it necessary to assign to orthoclases alkaline feldspars with an extinction angle \(Nm:\perp(001)=9–11–12^\circ\), which at all inclinations on the Fedorov stage retained straight extinction and in their properties did not correspond to microcline [38]. The magnitude of the angles of the optical axes for them varies within the limits from \((-)\ 60\) to \((-)\ 80^\circ\) and in individual cases may reach \((-)\ 80–90^\circ\) [55; 38].

E. Spencer, and after him H. S. Wang, assert that, with one and the same composition of orthoclases, sanidines, adularias, and microclines, orthoclases and sanidines have higher refractive indices than microclines. The refractive indices of adularias, in magnitude, are closer to those of microclines than to those of orthoclases. On this basis he regards adularias as submicroscopically twinned formations reproducing the properties of microcline [132].

Finally, the optical properties of anorthoclases [3; 5; 21; 30; 55; 58; 112; 118; 124; 127; 136, etc.] are determined by the following data: the plane \(NpNm\) is close to \((010)\). The extinction on the plane \((001)\) is determined by the relation \(Np : (010)\), which forms a small angle¹, varying within the limits from 0 to \(2^\circ\) [56], or usually from 1 to \(3^\circ\), decreasing in one direction to \(0^\circ\) and increasing in the other to \(6^\circ\). Extinction on \((010)\) with respect to \(Np : (001)\) forms an angle from 4 to \(10^\circ\); \(Nm : \perp (001) = 5\text{—}6^\circ\). The angle of the optical axes in anorthoclases varies from \((−)\,32^\circ\) [29] to \((−)\,54^\circ\), and even to \((−)62^\circ\); and O. A. Vorob’eva gives data of B. M. Kupletskii according to which anorthoclase from pegmatites of nepheline syenites has \(2V = (−)77^\circ\) [30]. The principal optical properties indicated here for anorthoclases are equally valid both for anorthoclases in which potassium predominates over sodium and for anorthoclases in which there is more sodium than potassium [116, 112].

V. N. Lodochnikov, concerning anorthoclases, remarks that “such microclines, which have an angle of the optical axes from \((−)\,30\) to \((−)\,55^\circ\), should be called anorthoclases” [55], although sometimes in thin sections they behave like microcline; from the latter, anorthoclases differ only “by extinction angles that are generally somewhat smaller, bringing the mineral closer to the monoclinic minerals” [55]. Thus, the principal point for assigning triclinic potassium-sodium feldspars to anorthoclases is the optical orientation of the ellipsoid with respect to cleavage along \((001)\) and \((010)\), different both from orthoclases and from microclines [55].

Incidentally, concerning \(2V\) as a criterion for distinguishing alkali feldspars from one another. V. N. Lodochnikov and D. S. Belyankin studied potassium-sodium feldspars and came to the conclusion that in a number of cases, at one and the same value of \(\angle Nm : \perp (001)\), the angle \(2V\) has different magnitudes even within one and the same grain [53]. The angles of the optical axes, for example, in alkali feldspars from the Chu River, vary from \((−)\,44\) to \((−)\,76^\circ\) in different parts of one and the same crystal and from \((−)\,44\) to \((−)\,85^\circ\) in different crystals [5]. At the same time, according to D. S. Belyankin, it is noteworthy that such considerable fluctuations in the magnitude of the optical axes in different crystals are not accompanied by a noticeable change in the chemical composition² of the potassium-sodium feldspars [5]. On this basis D. S. Belyankin arrives at the following conclusions:

1. The value \(2V\) cannot be used for judging the chemical character of potassium-sodium feldspars [5].

2. Therefore the table of M. A. Usov, in which the alkali content is linked with the change in the angle of the optic axes in alkali feldspars [83, Table V], is in this part devoid of any significance [5].

In addition to D. S. Belyankin’s conclusions, let us cite V. N. Lodochnikov’s indication that \(2V\) cannot be used to distinguish alkali feldspars from one another. Therefore the data of E. Bayer given below with respect to \(2V\), in light of the conclusions just presented, have only conditional significance.

Thus, from consideration of the optical properties of potassium-sodium feldspars there emerges the need to distinguish two series of sanidines in accordance with the position of the optical indicatrix. Orthoclases of adularia character and adularias proper are apparently optically close to one another, but differ in morphological and genetic respects. Orthoclases in crystals are usually opaque. This opacity may be caused not only by ordinary chemical alterations in the form of pelitization, sericitization, pegmatitization, and adsorption, but opaque orthoclases may become so as a result of a rearrangement of their space lattice in connection with changes in physicochemical conditions. Microclinization of orthoclases is most likely the result of such a rearrangement. We seem to have no special investigations of orthoclases from this point of view.

On the basis of all that has been said about anorthoclases, non-latticed microclines—agiroclases—and microclines proper with heterogeneous and latticed extinction, it follows that there are no data to speak of the identity of microcline and anorthoclase, as L. A. Vardanyants thinks, nor of the anorthoclasization of microclines, a process that has not been shown at all clearly either by D. S. Belyankin himself or by his followers. The microclinization of anorthoclases, noted by L. A. Vardanyants in Caucasian rocks, is a natural process. Likewise there are no grounds to regard the anorthoclases of the Caucasites, which have a moderate value of the angle of the optic axes and show agyrity, as sanidines [7]. Such an anorthoclase [54] indicates only its genetic relationship with sanidine and is its polymorphic variety.

Adularias are precisely those minerals of the potassium-sodium feldspar subgroup whose optical and crystallographic properties have been well studied and which, as is customary, are then attributed to orthoclases, since adularias are regarded as a variety of orthoclase. Moreover, they were most likely studied in its most transparent varieties. However, as early as E. S. Fedorov pointed out that among the adularias from St. Gotthard there are optically anomalous varieties of adularia, the properties of which indicate the presence among them of representatives of the triclinic system [87].

K. I. Viik, among the adularias from St. Gotthard, found a variety that contained \(\mathrm{Na_2O}\)—5.47% and \(\mathrm{K_2O}\)—11.90% and corresponded in composition to \(\mathrm{Or_3Ab_2}\). The angle between (001) and (010) proved to be \(89^\circ 36'\), the magnitude of extinction on the plane (001) ranged from 0 to \(3^\circ\), and on the plane (010)—from 5 to \(8^\circ\) relative to the edge (001):(010) [134]. The data cited allowed the author just mentioned to speak of the triclinic character of the adularia he studied, which he called “kalinatronmicroclase.” The latter, in its optical properties, as it seems to us, stands closer to anorthoclases and is in essence an anorthoclase adularia.

E. Bayer, for the adularias he investigated from various localities, notes that in a number of cases they have an evident twin structure and often prove to be optically heterogeneous, a circumstance that is taken as an indication of the triclinic nature of these feldspar varieties.

Moreover, the constant \(2V\) of adularias in their different individuals is often sharply different in magnitude [98]. Similar indications of the adularescence of adularias are also found in T. Barth [100] and other investigators who have studied adularias.

Thus, among adularias two principal varieties are also outlined: monogyral adularias—monadularias—and adularias with a tendency toward aegirine—agiro-adularias. In turn, among the aegirine adularias—agiroadularias—one may distinguish anorthoclase adularias—anadularias—and agiroadularias analogous to non-lattice microclines, agiroclase adularias.

To conclude the consideration of the optical properties of the alkali feldspars, let us cite one more opinion of E. Baier. Like many others, he is inclined to regard the magnitude of the angle of the optic axes (\(2V\)) as a characteristic feature for potash-soda feldspars; but he attaches special significance to this magnitude, since it varies strongly depending on the composition of the feldspars, their age, and especially on their microstructure [98]. This last feature affects the magnitude of \(2V\) to a greater degree than does the sodium content. E. Baier sees confirmation of his ideas above all in the degree of dispersity of the micropoikilitic structure of the alkali feldspars. Thus,

\[ \begin{aligned} &\left. \begin{array}{l} \text{sodium-free sanidines, uniformly extinguishing} \ldots \\ \end{array} \right\} && 2V = 0^\circ,\\[4pt] &\left. \begin{array}{l} \text{Na-bearing sanidines, quite uniformly extinguishing} \ldots \\ \text{sodium-poor orthoclases of deep-seated rocks, quite} \\ \quad \text{uniformly extinguishing} \ldots \end{array} \right\} && 2V = 0{-}40^\circ,\\[4pt] &\left. \begin{array}{l} \text{sanidines with a slight lattice} \ldots \\ \text{orthoclase with a high sodium content, extinguishing} \\ \quad \text{undulose and patchily} \ldots \end{array} \right\} && 2V = 40{-}56^\circ,\\[4pt] &\left. \begin{array}{l} \text{sanidine with a distinct lattice} \ldots \\ \text{orthoclases slightly patchy} \ldots \end{array} \right\} && 2V = 56{-}68^\circ,\\[4pt] &\left. \begin{array}{l} \text{sodium-rich anorthoclases} \ldots \\ \text{clearly expressed microcline} \ldots \end{array} \right\} && 2V > 68^\circ . \end{aligned} \]

From the data presented, therefore, it follows that the magnitude of \(2V\) in the series of alkali feldspars is chiefly a measure for determining dispersity, their structure, a measure of their internal rearrangement. Dispersity, expressed at the first stage in the form of uneven extinction of crystals of alkali feldspars, then in the form of undulose and patchy extinction, and finally in the form of distinct microcline fibrousness or lattice structure, is at the same time also a measure of the cloudiness and opacity of the potash-soda feldspars themselves.

Thus, from all that has been said above it follows that the microcline lattice, patchiness, and uneven extinction are characteristic not only of microcline, but also of anorthoclases, adularias, sanidines, and orthoclases. Microclines, it must be considered, optically do not constitute a modification of potassium feldspars, and therefore microclines should not be singled out as an independent mineral. As was shown above, microclines with a lattice structure represent a twin intergrowth of lamellae of different alkali feldspars, in particular lamellae of clearly monoclinic and triclinic potash-soda feldspars. Microclinization, one must suppose, affects all kinds of alkali feldspars in their historical development when physico-chemical conditions change, during the transition of one modification into another.

Be that as it may, optical investigations of the alkali feldspars make it possible to distinguish two modifications of sanidine: \(\alpha\)-sanidine and \(\beta\)-sanidine. Orthoclases represent an independent modification, which

could be designated as $\gamma$- and $\hat{c}$-sanidines. Anorthoclases, in their optical properties, may be regarded as a triclinic modification into which sanidines and orthoclases are transformed. Non-lattice microcline must be considered an independent mineral and given the name agiroclase, which probably represents a polymorphous modification of the orthoclases. Microcline, as has already been said, does not have the status of an independent mineral. Finally, among the adularias we must distinguish two polymorphous varieties: monadularias and agiroadularias.

X-ray studies of feldspars

X-ray studies of microcline and orthoclase have so far not yet given an indisputable answer to the question of whether in orthoclase and microcline one should see two independent mineral varieties, monogyral and agiric, or whether only one of them exists as independent, while the other is merely its special manifestation.

The first X-ray studies showed that there is no substantial difference in structure between orthoclase and microcline. However, this judgment met with corresponding objections from I. Vogt [131]. Others also doubt the value of the reports just cited, and G. A. Ohlming, on the same matter, says that the identity of structure revealed by means of X-rays does not prove the identity of microcline and orthoclase [63].

V. G. Taylor, on the basis of his X-ray investigations of sanidine, gives a space structure corresponding to a monogyral-planaxial type of symmetry, and notes that orthoclase crystals, in their structure, stand close to triclinic ones similar to microcline.

E. Schiebold, in 1927, assigned such alkali feldspars as adularia and sanidine, on the basis of X-ray study, to monoclinic feldspars, since the parameters¹ $a_0$, $b_0$, $c_0$ of the elementary parallelepiped of repetition and the angles $\alpha$, $\beta$, $\gamma$ between them had the following character [93; 94].

Table 3. Geometric constants of potassium-sodium feldspars

	Parameters	Parameters	Parameters	Angles	Angles	Angles	Localities
	$a_0$	$b_0$	$c_0$	$\alpha$	$\beta$	$\gamma$	Localities
Adularia	8.61	13.07	7.26	90°	116°03′	90	St. Gotthard
Sanidine	8.42	12.92	7.14	90°	115°35′	90	Lake Laach
Microcline	8.44	13.00	7.21	90°07′	115°50′	89°55′	Hilsberg

Microcline, on the same basis, as is evident from the data given in the table, was assigned by him to the triclinic feldspars. The space lattice of the monoclinic feldspars had a monoclinic-prismatic type of symmetry $C^3_{2h}$, whereas the space group of the triclinic feldspars was only $C^1_i$. This investigator points out, among other things, that from X-ray photographs it was in no case possible to draw a conclusion about an extremely fine twinned submicroscopic structure of a monoclinic feldspar, since all the X-ray photographs obtained in the symmetrical position showed complete identity, whereas the Laue diagrams of microcline leave no doubt as to its triclinic character.

Later E. Schiebold, in his article “The Structure of Silicates” [94], regards the potassium feldspars—orthoclases and adularias—as pseudomonoclinic. The parameters of the unit cell and the angle \(\beta\) for sanidine remain the same as those indicated earlier, i.e., sanidine is still considered by him to belong to monoclinic syngony [94].

A. E. Fersman, in connection with the X-ray metric characterization of the alkali feldspars, points out that the crystallochemical differences between orthoclase and microcline, established by E. Schiebold, come down to the position occupied by aluminum atoms in the space lattice: a symmetrical arrangement of the aluminum atom causes the appearance of monoclinic symmetry [86]. This latter, in A. E. Fersman’s opinion, is possible precisely at high temperatures near the melting points of K–Na feldspars, or under conditions of quiet crystallization at low temperatures [86], properties which he considers characteristic precisely of orthoclase.

We shall further note that E. Schiebold, in opposition to W. G. Taylor, believes that the latest X-ray metric studies have confirmed the acute-angle nature of orthoclase and have provided material for explaining the higher degree of its symmetry without resorting to the aid of the hypothesis of E. Mallard and Michel-Lévy concerning submicroscopic twinning, which found no confirmation in the investigations he carried out. According to E. Schiebold, X-ray analysis gives no basis for identifying orthoclase and microcline, and therefore he is inclined to regard orthoclase, including sanidines, as belonging to pseudomonoclinic structural type I, and microclines to triclinic type II, on the grounds that strictly monoclinic feldspars are in general impossible within the limits of the structures interpreted. From X-ray metric investigations he draws another conclusion, namely: since the structures of both varieties of alkali feldspars are close to one another and, according to the views of E. S. Fedorov [88], have a pseudocubic or pseudotetragonal nature, sharp differences should therefore hardly be expected in specific gravity, position and character of cleavage, and other properties [94]. From this a corresponding conclusion is also drawn with respect to the polymorphism of the potassium–sodium feldspars, and it is said that the question of the dimorphism of the alkali feldspars, long advanced to explain the morphology and optical properties of orthoclase and microcline, in the light of microstructural structures assumes an entirely new character [94].

Finally, S. P. Solov’ev, for microcline, sanidine, and anorthoclase, presents data from X-ray investigations in the form of Debyegrams for purposes of comparing the alkali feldspars of ancient and young igneous rocks [73], and from the cited X-ray characteristics the author concludes that “in general their crystal structures are quite similar” and that “each of the X-ray patterns of the feldspars studied possesses certain individual features, although the differences between them are insignificant” [73]. Apart from the general conclusions just cited, and judgments which say only that in sanidine on the Debyegrams some lines are observed that are absent in microcline, while the latter has a number of lines absent in sanidine, and that anorthoclase, in its Debyegrams, has distinctive features in comparison with microcline and sanidine, we find in S. P. Solov’ev no other more definite judgments about the character of the structure of the alkali feldspars studied.

The data presented on X-ray metric studies of the alkali feldspars allow one to note that the method of X-ray analysis, with respect to chemically complex compounds such as aluminosilicates are in most cases, and to media that crystallize in crystals of monoaxial and biaxial syngony, is not yet sufficiently perfected, while the interpretation of X-ray patterns and of the fine structure of complex compounds requires additional hypotheses and often has a subjective character.

E. Shibold, relying on E. S. Fedorov’s ideas concerning the pseudocubic or pseudotetragonal symmetry of potassium-sodium feldspars, considers it “convenient” to regard monoclinic feldspars as having a quadrupled primitive unit cell whose edges are parallel to the crystallographic axes \(a, b, c\) [94]. The molecule of the cell is considered as consisting of four tetrahedra arranged with respect to one another in an augite-centered manner. This (not especially convincing) point provides grounds for speaking of the impossibility of the existence of strictly monoclinic alkali feldspars [94]. The spatial combination of these augite-like molecules, by their nature, with respect to one another in the cell, and of the latter in the space group, makes it possible simply to explain the high degree of symmetry of orthoclase in comparison with microcline.

It seems to me that E. Shibold, in his conclusions concerning the impossibility for alkali feldspars of forming crystals of the monoclinic system, makes an error that consists in his substituting the concept of the structure of the molecule of the cell for the concept of a crystal as a geometrically homogeneous medium, whose symmetry depends both on the character of the unit cell itself and on how this cell is placed in the space of the crystal relative to its neighbors. Therefore, if the displacement of molecules with symmetry \(C_1^1\) in the cell takes place with respect to their neighbors as a reflection, then in such a case we shall have a crystalline cell whose degree of symmetry will be \(C_{2h}^{3}\) in reality, and not merely apparently, not pseudomonoclinic.

Further, unfortunately, in works on crystal goniometry it is not always possible to establish to which varieties of alkali feldspars the goniometric data belong, which must have a substantial effect on the conclusions that might be drawn.

Be that as it may, the following basic propositions follow from the goniometric investigations.

First, orthoclase, in its structural features, is in no case identical with microcline; i.e., the question of the identity of these two mineral species is thereby removed—a view that for about 70 years has been maintained, and is still maintained by many, especially foreign, scholars, down to the present time.

Second, taking into account the remark made above, we obtain the right to regard it as proven that among the alkali feldspars there are monoclinic and triclinic feldspars.

Third, on this basis the other question—of the submicroscopic and twin structure of orthoclase—is also removed, as not confirmed radiographically.

And finally, fourth, the question of the dimorphism of the feldspars has received new and firm confirmation.

Experimental investigations of the relations between microcline and orthoclase

For the purpose of resolving the question of the relations between orthoclase and microcline, a number of experimental investigations were carried out, the main aim of which was to prove the reversibility of one mineral into the other under appropriate temperature conditions. G. Förstner, studying sodium-bearing orthoclase from the liparites of the island of Pantelleria, found that when such feldspars are heated within the range from 100 to 500°, their triclinic varieties become monoclinic; the magnitude of the angle of the optical axes and the magnitude of the extinction angles change appreciably [111]. E. Makinen is distrustful of G. Förstner’s experiments.

Experimental investigations of the same character were later carried out, for example, by Rose and O. Mügge with microperthitic orthoclase, which

after prolonged heating somewhat below its melting temperature proved to be homogeneous. At the same time, S. Kōzu and I. Endo showed that the structure of adularia, after 78 minutes of heating at a temperature of 1060° and subsequent cooling, showed no changes. In contrast to adularia, moonstone from Ceylon, representing a very fine perthite (cryptoperthite) of composition 74.4% orthoclase, 23.1% albite, and 2.5% anorthite, after 98 hours of heating at 1180° and rapid cooling showed that its orthoclase and albite crystal lattices within the grain had everywhere become identical, and the mineral physically homogeneous.

E. Dittler and A. Köhler showed that microperthite from Arendal, rather rich in sodium (42% orthoclase, 56.2% albite, and 1.8% anorthite), after heating for 500 hours and more at \(1000^\circ \pm 25^\circ\), became almost homogeneous, which was expressed in identical extinction on (010) and \(Np : a = 6{-}8^\circ\). Microcline-perthites from Perth with 7% albite, antiperthites with 7.6% orthoclase and 2.9% anorthite, and cryptoperthites containing 42.9% orthoclase, 58.5% albite, and 0.6% anorthite from Frederiksvaern, after prolonged heating, showed a clear tendency to become homogeneous [109]. According to the same experimental studies, microcline from Perth with a fine-latticed structure, after heating for 500 hours at 1000°, became homogeneous, and its extinction on (001) became straight, parallel to (010). In microcline-perthites from Pikes Peak, when they were heated for 700 hours at 1000°, the microcline lattice disappeared completely, and the mineral itself acquired the character of orthoclase¹, in which the perthitic intergrowths formerly present, disappearing, seemed to dissolve in the orthoclase substance [109].

Thus, the experimental data on heating microclines and perthites of alkali feldspars, although few in number, clearly show that, first, \(K \cdot Na\)-feldspars crystallizing at high temperatures precipitate in the form of physically homogeneous crystals and, second, at high temperatures microclines and perthites are transformed into crystals devoid of the microcline lattice and acquire optical properties corresponding to monoclinic substances. Such transformations of microcline may indicate that, in its formation, microcline necessarily passes through an orthoclase stage.

Another type of experimental work that was of great importance for judging the nature of microcline and its relation to orthoclase consisted of studies whose aim was to produce the appearance of a microcline lattice in orthoclase. Thus, as early as 1885, I. Lehmann, by means of artificially reproducible pressure, caused the appearance of a microcline lattice in orthoclase, and G. Förstner in the same way caused twinning to appear in sodium-bearing orthoclase, which on the mineral looked like plagioclase striation [111].

The results of these experimental works on the artificial reproduction of the microcline structure in orthoclase were readily accepted by many investigators, and the view that the microcline lattice appears in the process of pressure during tectogenesis became widespread. Thus, we see, for example, that F. Rinne at the end of the last century also regarded the microcline lattice as a secondary formation, owing its origin to pressure; he saw confirmation of his arguments in the preferential manifestation of the microcline structure in areas subjected to crushing. He adhered to the same point of view in his later works as well, and continued to believe that orthoclase acquires a microcline character as a result of tectogenesis.

R. Brown considers the primary form for K–Na feldspars to be their monoclinic form, occurring in nature in an undisturbed state in the form of adularia, which, as a result of prolonged pressure, is transformed into the microcline form, while its microcline lattice is secondary [102].

This point of view on the secondary nature of microcline was later supported by A. E. Barlow. He considers it impossible to ignore the role of the pressure to which the rock is subjected: the duration and magnitude of this pressure must correspondingly affect the character of the manifestation of the microcline lattice in orthoclase. At least in orthoclases that have undergone pressure and crushing, cases may be observed in which, on individual fragments, a typical microcline lattice appears along their periphery, while in the central part of the same fragments it is absent; in its properties this part of the alkali feldspars corresponds to orthoclase. The appearance of a microcline lattice near perthitic intergrowths, according to B. Popov, may also be closely connected with the development of pressure that caused crushing, preceding albitization and the crystallization of albite material in these cracks [66]. Somewhat earlier, B. A. Popov regarded the phenomenon of microclinization as recrystallization of potash feldspar associated with its perthitization and albitization [67].

B. A. Popov places the origin of the microcline lattice in close dependence on the formation in potash feldspar of microperthitic lamellae of albite regularly intergrown with microcline. He regards such an appearance of microcline structure as a secondary phenomenon, developing as a result of slow recrystallization of potash feldspar caused both by the course of formation of microcline lamellae of albite and by the direct orienting action of already formed albite lamellae [67].

P. N. Chirvinsky, on the basis of a large number of observations by Russian and foreign investigators, explains the phenomena of microclinization of orthoclases by the transition of the latter into more stable forms under the influence of various factors, chiefly pressure, and then cooling. On this basis he considers the formation of microcline in rocks subjected to strong dynamometamorphism to be evident [92]. N. K. Lize believes that orthoclase, under the action of continuously varying pressure, acquires a microcline lattice [52].

Finally, G. O. Olling considers that the microclinization of orthoclase depends on pressure, and expresses the opinion that the microcline lattice appears even at the moment of breaking off the plate for a thin section and preparing the thin section itself, and that its occurrence can be avoided if all precautionary measures are taken [63; 97].

One should also dwell on another experimental study, belonging to A. Des Cloizeaux, who studied the effect of heating on the optical properties of sanidine, which was checked by the change in the position of the plane of the optic axes and the magnitude of the apparent angle \((2E)\) of the optic axes themselves. For carrying out such a study, sanidines from various deposits were taken. The experiments showed that in sanidines in which the plane of the optic axes before heating was situated perpendicular to \(M(010)\), after heating in a gas flame at a temperature of about \(600^\circ\), or in a gas blowpipe lamp (about \(900^\circ\)), and rapid cooling (as in quenching), the position of the planes \(NgNp\) changed and became parallel to the plane \(M(010)\). Sanidines in which the plane of the optic axes was parallel to \((010)\), after heating to the same temperature and under the same conditions, did not change the position of the optical indicatrix. As for the apparent angle of the optic axes, its magnitude upon heating, as a rule, increases. However, in individual cases, when the position of the plane of the optic axes changes from perpendicular to \((010)\) to parallel to it, the angle \(2E\) at first somewhat—

decreased somewhat, and then increased again in its dimensions. This evidently indicates that one modification of sanidine passes into another with increasing temperature.

Similar experiments were evidently carried out more than once by various investigators. Some succeeded, while others did not obtain positive data, depending on a number of methodological shortcomings of the experiment itself, which was still far from developed. But, be that as it may, we have no grounds for distrusting the experiments of A. Des Cloizeaux. Even for him such experiments were not always successful. Nevertheless, the available experimental data make it possible to say that in sanidines formed in magmatic orthoclase rocks at temperatures above 600–800°, the plane of the optic axes is arranged parallel to M (010), a position which may be preserved in sanidines by quenching. With relatively slow cooling, the plane Ng Np rotates into a position perpendicular to (010).

In intrusive rocks, when they remain for a long time in a hot state, the angle of the optic axes increases in the crystallized sanidine, and on cooling there is a rotation of the plane of the optic axes from a position parallel to (010) into one perpendicular to it. Can sanidines be found in intrusive rocks? On the basis of what has been said above, this is unlikely. However, such cases of the occurrence of sanidine in Caucasian rocks were at one time reported by D. S. Belyankin, but most probably he took for sanidine anorthoclase, which actually represents one of the polymorphic products of alteration of sanidine feldspar. There is also one further reference known to me in the literature to the occurrence of sanidine in intrusive rocks of China [133].

A number of investigators have shown that in adularia, at high heating temperatures (900–1100°), the plane of the optic axes becomes parallel to (010), as in sanidines. According to G. Mervin, adularia at a temperature of about 900° passes into sanidine.

E. Spencer, concerning the behavior of microcline on heating, says that it is transformed into sanidine [78].

Thus, experimental data have confirmed that one of the initial products in the transformations of feldspars from one modification into another is sanidine, while the other, as A. E. Fersman believes, will be orthoclase.

Polymorphism of alkali feldspars

Questions concerning the polymorphism of orthoclase and microcline are resolved differently by investigators, depending on how they conceive the character of orthoclase and the phenomenon of polymorphism. Investigators who regard orthoclase as cryptocrystalline or submicroscopic microcline cannot speak of their dimorphism.

K. Hintze considers the dimorphism of potassic-sodic feldspars hardly probable, on the grounds that both minerals—orthoclase and microcline—have similar crystallographic parameters of the principal forms, identical density, and very close refractive indices and other similar physical properties.

Another group of investigators, headed by G. Tschermak, regard orthoclase and microcline, as indicated above, as polymorphic modifications [130]. A. Des Cloizeaux also tended to regard microcline and orthoclase as dimorphic [108].

A third group of investigators was inclined to regard microcline as a secondary formation arising at the expense of orthoclase as a result of pressure or as a result of its chemical changes and recrystallization.

The statements just cited concerning the relations of microcline and ortho-

... between orthoclase are firmly maintained to the present time. K. Hintze said in this connection that the solution of this question is subjective in character, since for an objective answer to this question the corresponding data are as yet lacking [116]. Thus, for example, E. McKie in 1917 expressed the consideration that orthoclase and microcline are polymorphous modifications and, on the basis of paragenesis, established that one modification passes into the other at approximately about 600–700° [120]. However, E. McKie himself was unable to transform microcline into orthoclase even at temperatures close to the melting points.

How old judgments about microcline continue to remain unchanged may be shown by several examples. Thus, E. Bayer regards the monoclinic character of feldspars as an apparent phenomenon, since in his opinion orthoclases are dispersion-twin formations composed of submicroscopically small triclinic unit lamellae [98].

P. N. Chirvinsky believes that perthitization and microclinization appear in mixed crystals of alkali feldspars in connection with their transition into more stable forms under the influence of various factors, chiefly pressure, then cooling, etc. [92]. He closely links the occurrence of microcline in rocks with the manifestation of dynamometamorphism, citing data from a large number of Russian and foreign scholars.

M. A. Usov, taking into account the occurrence of microcline in hypabyssal and vein rocks, supposes that the transition of the primary modification of potassium feldspar into the stable form “takes place at the critical temperature, if not as a result of slow cooling, as occurs in a typically abyssal setting, then under the action of an excess of mineralizers” [84]. Further, M. A. Usov, on the question of microclinization, repeats what was said by P. N. Chirvinsky and says that this process of transition of potassium feldspar into microcline, occurring at high temperature, “may take place in any rock that has undergone mechanical deformation” [84].

A. E. Barlow, considering the formation of microcline at the expense of orthoclase possible, draws attention to the necessity of taking into account the relation between the amount of microcline formed at the expense of orthoclase and the intensity of the pressure to which the rock is subjected, i.e., this scholar wished to see in the process of microclinization of orthoclases a method for estimating the magnitude of stress during deformation.

G. L. Oling is also inclined to believe that the microcline twinning structure in orthoclase arises as a result of pressure. He sees confirmation of this idea in the practice of preparing thin sections. At least, by avoiding, in preparing a thin section, excessive stresses arising from hammer blows, grinding, and heating, one can significantly reduce the number of cases of the possible formation in orthoclase of microcline twinning [63; 97].

M. A. Usov admits that the molecules $\mathrm{KAlSi_3O_8}$ and $\mathrm{NaAlSi_3O_8}$, each separately, are characterized by polymorphism, which for $\mathrm{KAlSi_3O_8}$ appears in the form of monoclinic orthoclase and triclinic microcline; for $\mathrm{NaAlSi_3O_8}$ the polymorphous modifications are monoclinic sodium orthoclase (barberite) and triclinic anorthoclase (analbite) and albite. M. A. Usov considered orthoclase and microcline dimorphous on the grounds that the optical properties of the lamellae of microcline and of non-cross-hatched microcline differ sharply from the optical properties of orthoclase. These named polymorphous varieties may form the following isomorphous series [83]^1)

^1) The concept of isomorphous series in the group of alkali feldspars was given by M. A. Usov 11 years earlier than by Oling [63].

orthoclase — natronorthoclase,
microcline — anorthoclase,
microcline — albite.

D. S. Belyankin considers polymorphic relations between orthoclase and microcline to be entirely possible [14]. However, he believes that potassium orthoclase, like sodium orthoclase, is equally unstable below the melting point. As for the question of the temperature of the transformation of orthoclase into microcline, this question the scholar left open. However, D. S. Belyankin regards it as too great a luxury for the alkali feldspars to appear, “besides the monoclinic, in two independent triclinic modifications” [14].

G. L. Oelling distinguishes three polymorphic modifications among the alkali feldspars: sanidine, orthoclase, and microcline. Each of the indicated modifications, possessing definite properties characteristic of it, initiates a continuous isomorphous series:

sanidine — barbierite,
orthoclase — albite,
microcline — analbite,

and since the minerals of these isomorphous series are in essence formed under definite conditions and at the same time possess definite qualitative features, naturally they are also polymorphic series [97].

The sanidine series, represented by sanidine, sodium sanidine, and hypothetical barbierite, is characteristic chiefly of effusive rocks. The minerals of this series form at temperatures above 900° and, upon rapid cooling, find themselves in the quenched stage [97]. Sanidines transform into orthoclase, natronorthoclase, anorthoclase, and albite.

The orthoclase series, represented by orthoclase, natronorthoclase, and albite, is characteristic both of effusive rocks and of intrusive formations and pegmatites. The minerals of this series, as is generally thought, form at temperatures somewhat below 900°. However, there are indications that orthoclases may form as high-temperature minerals. The position in this series of albite as a triclinic feldspar, alongside monoclinic orthoclase, is unclear.

Finally, the third polymorphic microcline series: microcline—anorthoclase—analbite. Microcline and anorthoclase are most characteristic of intrusive rocks, although anorthoclase is rather often encountered also in effusive formations. The minerals of this series are formed, as is generally believed, at temperatures below 900°.

In this classification of the alkali feldspars [97], which represents isomorphous-polymorphic series, the very principle by which the principal polymorphic modifications are distinguished remains unclear, as do the serious reasons that led the author to set microcline apart as an independent series of minerals.

A similar scheme of polymorphic series of the alkali feldspars is also proposed by A. N. Winchell, who admits the possibility of polymorphism for the substances $\mathrm{KAlSi_3O_8}$ and $\mathrm{NaAlSi_3O_8}$ in the form of three principal modification states, constituting respectively three series of isomorphous formations [135]:

monoclinic: sanidine — barbierite
monoclinic and triclinic: adularia — albite
triclinic: microcline—analbite.

Sanidine is regarded in this scheme as a stable modification at high temperatures, while adularia is considered to form under low-temperature conditions. The author indicates that adularia passes into sanidine at 900°, and that the transition itself takes place very slowly.

E. Spencer, in considering the alkali feldspars, does not speak of their polymorphism, but at the same time points out that there are substantial differences between orthoclase and microcline. He does not agree with the ideas concerning the pseudomonoclinic nature of orthoclase and indicates that “the continuous orthoclase–microperthite series, on heating, passes into a sanidine–anorthoclase series of the same composition. The solubility of the sodium component in microcline is less than in orthoclase, and the transition of microcline into sanidine on heating takes place considerably more difficultly than that of orthoclase–microperthite” [78]. But from his judgments on potassium–sodium feldspars it follows that there exist the four following polymorphic series of feldspars, each of which possesses its own special features:

1. Sanidine–anorthoclase
2. Orthoclase–microperthite
3. Microcline
4. Adularia

A. E. Fersman, noting the complex relationships between orthoclase and microcline, says that “at high magmatic temperatures the stable form is orthoclase (or sanidine); upon further cooling the equilibrium shifts into the field of high-temperature pegmatites, where microcline is stable, so that below 550° it is replaced by adularia-like orthoclase.” Orthoclase in the form of adularia appears below the critical temperature of water (400–350°); within the limits 400–300° apparently only monoclinic feldspar is stable [86].

More developed ideas about polymorphism among crystals, and in particular with respect to potassium–sodium feldspars, are found in T. Barth [101] and D. S. Belyankin [14], and, conversely, we find a denial of polymorphism in orthoclase and microcline in P. Groth [33], owing to his peculiar views on polymorphic phenomena.

T. Barth, concerning the concept of polymorphism, found it necessary to distinguish three main types: polytypy, polysingony, and polytropy. Polytypy is a case of polymorphism in which the transition from one state to another is accomplished with a complete transformation of the space lattice, for example, quartz and tridymite. Polysingony is a polymorphic case that comprises such polymorphic states of a substance which, in passing from one modification to another, show certain changes in the space lattice. The physical properties, however, change only slightly; for example, α-quartz and β-quartz. Finally, the third type of polymorphic transformations, called polytropy, comprises cases in which, in passing from one modification to another, changes in geometrical symmetry and structure do not occur, or almost do not occur, but the physical properties thereby manifested make it possible to distinguish the corresponding modifications. To this type of polymorphic states belong, for example, orthoclase and microcline [101].

From what has been said it is clear that it is proposed to classify all polymorphic modifications according to the degree of expression of polymorphism, on the basis of their clearly and unclearly distinguishable structural and physical features. If the separation of polytypic and even polysingonic polymorphic varieties will not encounter great difficulties, then the assignment to the polytropic type of polymorphic states may to a considerable degree have a subjective character. For example, in accordance with T. Barth’s principles, the polymorphism of ortho-

…oclase, especially sanidine and microcline, can with equal success be assigned to the polymorphic type of polysingony.

D. S. Belyankin, in defining the concept of a “mineral,” also touches on the question of polymorphic modifications as independent mineral species, and at the same time gives a classification of polymorphic varieties as crystal-chemical phases differing from one another in the character of the manifestation of geometrical and physical properties [14]. He divides polymorphism, according to the character of its manifestation, into orders. To the first order he assigns polymorphic phases having a sharply expressed opposition in their physical and physico-chemical properties: solubility, hardness, density, birefringence, the character of the optical indicatrix, its orientation, etc. As an example of mineral formations that may be assigned to first-order modifications, the author singles out the mineral species: quartz, tridymite, cristobalite [14]. The mineral varieties—modifications of $\alpha$- and $\beta$-quartz, $\alpha$-, $\beta$- and $\gamma$-tridymite, $\alpha$- and $\beta$-cristobalite—D. S. Belyankin assigns to the category of the second order of polymorphism [14].

Further, this scholar points out that, in addition to polymorphic phases of the first and second orders, polymorphic phases of the third and fourth orders are also “conceivable.” D. S. Belyankin assigned “orthoclase” and sanidine to a polymorphic phase of the third order, while he regards “orthoclase” and microcline as polymorphic phases of the fourth order, differing from one another in the character of the optical indicatrix and in its position in the lattice of these minerals [14].

From the outside, the classification just mentioned seems rational, but it cannot be considered fully developed, since there are not yet sufficiently precise formulations as to which polymorphic formations should be assigned to one or another order. As regards the polymorphism of the potassium-sodium feldspars, in this respect D. S. Belyankin is right that, in the character of their polymorphism, orthoclase, sanidine, and microcline differ among themselves; but why he assigns orthoclase to two different polymorphic orders remains unclear.

The tendency of scholars to distinguish several degrees of polymorphic modifications should be welcomed as a necessity in finding new ways toward a more perfect understanding of the diversity of manifestations of a substance of one and the same composition. However, it must also be acknowledged that these tendencies, without having worked out new concepts in the theory of polymorphism, have so far amounted only to a classification of polymorphic modifications according to the degree of their distinctness.

In order to conclude the summing up of whether orthoclase and microcline should be regarded as polymorphic modifications, I consider it necessary to dwell on one more point of view, of which P. Groth was an advocate. He defines polymorphism, or physical isomerism, as the property of bodies to assume, under different circumstances, different crystalline structures and, at the same time, different crystalline forms. As a characteristic property of polymorphic varieties he considered the ability of solid bodies to transform into one another merely by a change in temperature; in this process the properties in the body being changed also change continuously¹) [33; 115].

The same investigator proposes that the phenomena of polysymmetry be distinguished from the phenomena of polymorphism. This latter is characteristic of bodies that acquire a higher symmetry, not inherent to them, by complex twinning, owing to the ability to form mimetic formations. On this question P. Groth adheres entirely to the views of Mallard and sees a manifestation of polysymmetry in the potassium feldspars, which assume the properties of crystals of the monoclinic system, composed of triclinic lamellae according to two twin laws, having submicroscopic dimensions. This

¹) The transition from one modification to another always takes place abruptly (A. K.)

the latter feature, taken together, accounts for the appearance of optically homogeneous orthoclase crystals, which should be regarded as submicroscopically twinned, as pseudomonoclinic crystals [33]. P. Groth expresses this same point of view on orthoclase and microcline in his latest works [115].

However, in this solution of the question with respect to orthoclase and microcline, P. Groth is not consistent. For example, boracite, leucite, potassium sulfate (\(K_2SO_4\)), like many others similar to them, are built of a larger number of lamellae of rhombic syngony, which are in polysynthetic twin intergrowth [33]. Crystals of boracite, often well crystallized and bearing the faces \(\{100\}\), \(\{110\}\), and \(\{111\}\), under ordinary thermodynamic conditions represent pseudocubic crystals composed of thin, microscopically optically anisotropic lamellae that differ in magnitude. However, on heating at about \(265^\circ\), boracite crystals transform into cubic, isotropic crystals, while displaying contraction and evolution of heat. Potassium sulfate, on heating in the interval \(600\)—\(650^\circ\), suddenly changes from a biaxial into an optically uniaxial crystal; at the same time the birefringence increases considerably and the optical sign changes. Pseudocubic leucite becomes cubic at about \(560\)—\(600^\circ\).

The examples just cited, although they pertain to pseudosymmetric and mimetic crystals, at the same time also reveal polymorphic transformations “in the proper sense of the word” [33].

The essence of the contradictions in the attribution to polymorphic and polysymmetric formations, as follows from P. Groth’s examples, consists in the following. If crystals of higher symmetry take on the character of crystals of lower symmetries, then in this case one speaks of polymorphic transformations. Conversely, if crystals of lower symmetry acquire the character of crystals of higher symmetry by twinning, then such formations are regarded as polysymmetric.

Therefore crystals of that polysymmetric form of a substance in which the twin structure can no longer be distinguished are only apparently homogeneous bodies; they differ from simple or twinned crystals of lower symmetry only in those properties which are conditioned by the manner in which they are built from these crystals; continuous transitions with intermediate properties may exist between both forms [33].

In contrast to polysymmetry, “polymorphic modifications are truly different modifications (phases). In the transition from one to another, there generally occur abrupt changes of all properties, both scalar and vectorial, and the transformation takes place at a definite transformation temperature” [33].

If, in the primary state, perovskite, boracite, leucite, and other minerals similar to them are single crystals and physically homogeneous, and then, as a result of a change in physicochemical conditions, the same substances become heterogeneous and acquire a complex polysynthetic-twin structure, then the question arises: how, in comparison with what has just been said, should the relations between orthoclase and microcline be regarded? One may, for example, with equal success accept orthoclase as primarily homogeneous; then latticed microcline will not appear to us, but will in fact be, a polysynthetically complex formation. In that case, following P. Groth’s examples, we shall be compelled to regard microcline as a polymorphic variety of orthoclase and to consider his train of thought on this question incorrect.

From the data just presented it is clear that the tendency to distinguish polymorphic varieties in the manifestations of potassium-sodium feldspars, it seems to me, is supported by an ever greater and greater number of adherents.

X-ray studies, as noted in their place, make it possible to regard as proven the existence of monoclinic and triclinic alkali feldspars; the same studies prove that orthoclase cannot be identified with microcline, and that one may confidently speak of the dimorphism of these two minerals of interest to us. The difficulties encountered in distinguishing polymorphic modifications compelled serious attention to be paid to the degree of expression of polymorphism, as was done by T. Barth and D. S. Belyankin; but their attempt to differentiate the degree of expression of polymorphic properties is not very satisfactory and requires further development, with the use of more subtle and objective methods for distinguishing polymorphic varieties from one another. In the following chapter I consider it necessary to dwell on certain features of polymorphism which, for us geologists and mineralogists, could have great practical significance for elucidating many aspects both of the origin of mineral associations and, especially, of the assessment of changes in the physicochemical conditions under which the mineral complex was formed.

General views on polymorphism and the polymorphism of minerals

The data cited above, dealing with the question of the polymorphism of orthoclase (adularia), microcline, and sanidine, usually do not touch upon the fundamental aspects of polymorphism. In order to clarify the question of this phenomenon, we must pause somewhat over modern conceptions of polymorphism and over tendencies to broaden this concept. First of all, let us say that polymorphism is not among rare phenomena; on the contrary, it may be asserted that, as a rule, every substance in the solid state is characterized by several polymorphic modifications, and that the number of these modifications may be large [48; 60].

V. I. Vernadsky specifies this latter idea in the following way. Proceeding from the theory of phases, he concludes that “there may be observed as many solid phases of one and the same chemical composition as there exist symmetry classes” [24], and that polymorphic varieties are related to one another as different phases. Hence it naturally follows that, for one and the same substance, there can be no more than 32¹ polymorphic modifications, i.e., “each polymorphic variety of one and the same compound belongs to different crystal classes, and within a crystal class (structure) the phenomenon of polymorphism does not exist” [24].

Polymorphism in itself is at present understood as the property of many substances to give different crystalline structures under different thermodynamic conditions and, consequently, to assume different forms [86a]. A change in thermodynamic conditions—temperature and pressure—entails a change in the absolute constants of the crystalline structural arrangement. Therefore, the polymorphism of transformation is closely connected with the discontinuous change in the state of the units composing the crystal, caused by the interaction of these units [86].

According to A. E. Fersman, “in the new formulation of crystal chemistry, the phenomenon of polymorphism should be understood as a morphotropic change at constant chemical composition, caused by the influence of thermodynamic factors on the properties and interaction of the component substances” [86a].

Thus, the phenomenon of polymorphism in its entire scope is considered only in relation to pressure and temperature. At least, V. D. Kuznetsov, in his survey on questions of the polymorphism of crystalline

states that each crystalline modification is stable within a definite temperature interval, which depends on the external pressure [46]. Similar assertions, incidentally, are also found in works on physical chemistry.

However, in all discussions of polymorphism, it seems to me, the following points are overlooked, although they can often have a corresponding effect on the manifestation of polymorphism. One of the factors influencing the appearance of polymorphic modifications is, for example, the very medium in which crystallization occurs. For example, $\alpha$-S—rhombic—separates from aqueous solutions and from melts, whereas another modification, $\gamma$-S—the second monoclinic modification of sulfur—is obtained from a solution of sulfur in hot alcohol or benzene.

Another important factor that, other conditions being equal, determines the appearance of polymorphic modifications is the concentration of the solution itself ($C$), from which crystallization of the salt takes place, and the substance present in comparatively small amounts as so-called impurities. As an example one may cite investigations to determine the conditions of formation of zinc sulfide (ZnS) in the form of wurtzite and sphalerite as a function of temperature conditions and the concentration of sulfuric acid, i.e., of the pH value. It was found that, at one and the same temperature, the precipitation of sphalerite and wurtzite crystals depends on the concentration in the solution of sulfuric acid present in amounts within the range from 0 to 6%, i.e., at one and the same temperature wurtzite will preferentially precipitate from more acid solutions, while at low sulfuric-acid concentrations sphalerite will preferentially precipitate.

A similar dependence between the medium and polymorphic modifications also occurs, for example, in the case of crystallization of pyrite and marcasite, the latter, as a rule, separating from acid solutions.

A third example of the occurrence of one or another modification from solution as a function of the concentration of the crystallizing substance itself may be taken from experiments on the spontaneous crystallization of monochloroacetic acid at different contents of it in solution and under different temperature conditions [125]. Thus, for example, at a temperature of 40° the $\alpha$-modification of monochloroacetic acid crystallizes from solutions with a concentration not lower than 90.5%, whereas for the separation of the $\beta$-modification at the same temperature the content in the solution must be not less than 92.5%, and for precipitation in crystals of the $\gamma$-modification of monochloroacetic acid under the same temperature conditions the concentration of the solution must be not lower than 96%.

In the experiments just mentioned, temperature plays a role in the formation of the modification not in itself; its influence is expressed only in the magnitude of the concentration, i.e., as a rule, with increasing temperature the solubility of the substance also increases. Therefore it may easily turn out that, if, for example, the $\alpha$-modification in the case of 90% content of monochloroacetic acid in the solution will crystallize at a temperature below 38.9°, then this solution under these conditions will still be unsaturated with respect to the $\beta$-modification, whose crystallization at the same concentration is possible only when the solution is cooled below 35°.

The influence of the character of the solution—for example, of admixtures of co-dissolved substances—on the character of crystals formed earlier in the same solution is manifested mainly in the alteration and appearance of new forms. We are so accustomed to this fact that we direct our attention only to its external manifestation, without considering its internal content. Thus, for example, in the case of NaCl or $K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$, under appropriate crystallization conditions, new forms appear, which then become predominant. We are accustomed to suppose that, with preservation of the type

the lattice are preserved, as are the lattice constants, although this latter point may, to a certain extent, be doubted. The latter follows from the fact that when the conditions of crystallization change, the energetic relations between the crystal and the medium from which the crystal separates change accordingly. A change in the energetic conditions of the medium must correspondingly affect changes in the values of the constants, even if only by a very small amount that is difficult to detect in our investigations.

N. I. Khitarov, dealing with the state of the residual magmatic solution, expresses the view that a rearrangement of the solvent molecules under the influence of the molecules of the dissolved substance may be the cause of possible abrupt changes in the course of the development of critical phenomena [91]. This must be understood to mean that such mutual relations are established between the molecules of the solvent and the molecules of the dissolved substance as must correspondingly affect the structure of the crystal and the character of the manifestation of new forms on the crystal.

The influence of the character of the medium on the form of crystals crystallizing in it, other conditions being equal, also follows from Le Chatelier’s principle. This relation between the medium and the form of crystals was expressed by V. I. Vernadsky in the following way [25]. If by \(e_1\) we denote the internal energy of crystals, by \(e_2\) its surface energy, and by \(e_3\) the energy of the medium in which the crystal grows, then in the case \(e_2 = e_3\) equilibrium is established between the form of the crystal and the solution feeding it. New forms will not appear on the crystal so long as this equilibrium is maintained. If, however, \(e_3 \gtrless e_2\), then, naturally, the equilibrium between the crystal and the solution is disturbed. The influence of the medium in the case \(e_3 > e_2\) will lead to the appearance on the crystal of such forms as would increase the surface energy of the crystal and make it equal to \(e_3\). Conversely, in the case \(e_3 < e_2\),¹ such forms would begin to appear on the crystal as would ultimately lower the surface energy of the crystal to \(e_3\), which would lead to equilibrium between the energy of the medium and the surface energy of the crystal: \(e_3 = e_2\), and hence to equilibrium of the system.

In confirmation of what has just been said I shall cite the first example that came to hand. Thus, O. G. Padurova gives for wulfenite from various deposits, studied at different times, the ratio \(a : c\) [64].

\[ \begin{aligned} a : c &= 1.57793 &&(\text{N. Koksharov})\ [42] \\ a : c &= 1.5776 &&(\text{S. Koch})\ [117] \\ a : c &= 1.5773 &&(\text{O. G. Padurova})\ [64] \\ a : c &= 1.5710 &&(\text{O. G. Padurova})\ [64] \\ a : c &= 1.5734 &&(\text{S. Koch})\ [117] \\ a : c &= 1.5746 &&(\text{E. Flint})\ [89]. \end{aligned} \]

This list of constants for wulfenites could be made quite large. For example, S. Koch gives geometrical constants, in addition to the above-mentioned cases, for 4 more typical crystals from different deposits. According to his data, the ratio \(a : c\) for wulfenite ranges from 1.5636 to 1.58446, and the average for 6 typical crystals is 1.5776.

¹ V. I. Vernadsky, in his reasoning on the influence of the medium on the crystal in the case \(e_3 < e_2\), made, it seems to me, an error. He believed that if \(e_3 < e_2\), i.e., if the energy of the medium in which crystal growth takes place is less than the surface energy of the crystal, then under these conditions the medium does not exert its influence on the crystal and thus does not cause the appearance of new polyhedral forms on the crystal. This assertion contradicts facts and experiments.

O. G. Padurova, for wulfenites from Almalyk having \(a : c = 1.5773\) and identified by him as the first (I) type of crystals, gives a chemical composition determined only by the content of molybdenum and lead, whereas in wulfenite, as in the second (II) type of crystals from the same deposit, with constants \(a : c = 1.5710\), besides lead and molybdenum there is an admixture of barium, the presence of which, in the author’s opinion, explains the deviation in the angles from the mean value [64].

Taking into account the data of O. G. Padurova, one may imagine the following case. During crystallization, crystals of wulfenite with \(a : c = 1.5773\) are first formed, and then barium appears in the solution, as a result of which the subsequently growing zones of wulfenite, being barium-bearing, will have somewhat different constants: \(a : c = 1.5710\). Since the difference in the constants of these two mineral varieties is small, naturally the outer zone of crystals of wulfenite of the second type (II), according to O. G. Padurova, differing in color from the inner part of the wulfenite, is deposited during crystallization on crystals of the first type (I). However, the geometrical superposition of wulfenite-II on wulfenite-I was not a simple continuation of the development of the previously existing crystal forms of the first type. Changes in the character of the medium, in particular the appearance of barium in the solution, must have been reflected in a corresponding manner in a change in the original character of the crystals and in a change in their geometrical constant \(a : c\). Wulfenite-I is characterized by the presence of simple forms, of which the predominantly developed faces of the form \(\{001\}\) determine the tabular habit of the crystals. Wulfenite-II in the crystals has the faces \(\{111\}\), \(\{011\}\), \(\{012\}\), \(\{113\}\), which determine its bipyramidal habit, in contrast to the tabular habit of wulfenite-I.

S. Koch writes that, in order to understand the above-noted fluctuations of \(a : c\) for wulfenites from different deposits, it is necessary to know their chemical composition. For example, the red coloration is explained by the presence either of chromium or of vanadium, while the appearance of yellow coloration is connected with the presence of up to 6.88% CaO. V. C̆epharovich connects the yellowish, brownish, and greenish colors of wulfenites, from light to dark tones, with the presence in lead molybdate of calcium, iron, and copper.

From the example considered it follows that, in accordance with a change in the composition of wulfenites, their geometrical constants also change correspondingly. Therefore, for wulfenite and other analogous cases we have the right to speak of the presence of polymorphous varieties even within the limits of one and the same symmetry class, and this phenomenon in the present case must be regarded as a necessity, not a luxury, for each given modification, differing from another with respect to the geometrical constants \(a : c\) by a relatively small amount, is characterized by definite typomorphic properties and is formed as a phase state under strictly definite physico-chemical conditions \((T, P,\ \text{and}\ C)\).

We shall supplement what has been said with the following. E. S. Fedorov, on the basis of a study of the geometrical structure of polymorphous modifications of many mineral substances, comes to the conclusion that, despite the essential difference in the geometrical structure of crystals of different polymorphous varieties, extending to a difference in the type of crystal, nevertheless very often in the complexes of polyhedra of polymorphous bodies there is observed a great similarity, “and only the significance of the faces in the complex changes” [88]. On this basis E. S. Fedorov believes that “one can hardly doubt the correctness of Pasteur’s general proposition, according to which polymorphous varieties exhibit closeness in their complexes.” At the same time E. S. Fedorov notes that in a number of cases this closeness “is obscured to varying degrees by secondary changes during the process of transformation,” which in some cases may be sufficiently great [88]. We can only add to what has been said that, in polymorphous transformations, changes in the complex

faces of a polyhedron, but has come to be “in its structural construction,” may be so slight that they are not revealed upon sufficiently clear and easy study.

All that has been said above gives us full grounds for saying that one and the same substance, manifesting itself independently in crystals or occurring as isostructural components in isomorphous formations [48], depending on T, P, and C (understanding C broadly as the concentration of the solution, as a medium containing, along with the principal components, co-dissolved substances and impurities, and also as a physically and chemically complex and variable medium), is capable of forming, out of the possible 230¹, a rather large number of polymorphous modifications that are stable to varying degrees and expressed in different ways. At the same time, polymorphous modifications of another order are clearly indicated, for which the habit, or otherwise typomorphic, forms, even within one and the same kind of symmetry, are formed only under definite physicochemical conditions of the medium and, consequently, are a very sensitive instrument for recognizing the character of changes in general, and in particular the energetic state, of the surrounding environment. The correctness of this conclusion is confirmed by numerous works on the typomorphism and topomineralogy of individual minerals².

Consideration of the typomorphism of alkali feldspars in terms of their genesis also clearly indicates that the energetic conditions of their formation in each individual case are different and that these conditions must in one way or another be reflected in the structural features of their space lattice and in the morphology of the crystals themselves. This conclusion is also confirmed by the few X-ray metric data of E. Schibold, according to which the geometrical constants of the unit cell of adularia, sanidine, and microcline, calculated with an accuracy of up to 0.25%, are clearly different from one another [93]. The transition of sanidine into orthoclase and of the latter into microcline is also closely connected with the conditions of change in the energy of the medium. Therefore we are entitled to regard such varieties as α- and β-sanidines, orthoclases, atiroclases, anorthoclases, and α- and β-adularias as polymorphous varieties which, by their appearance, indicate definite relations of T, P, and C of the medium, while the transition of the just-named modifications from one into another testifies to particular changes in their surrounding environment.

In conclusion, we may say that the ideas set forth above concerning the connection of the typomorphism (habit) of crystals with definite energetic conditions of the medium in which these crystals are formed, and the idea that each typomorphic form of a crystal of one and the same substance possesses its own geometrical constants, allow us to understand the entire diversity of polyhedral forms of a crystalline substance and to regard them as polymorphous formations, as indicators of a definite physicochemical state of the medium, and, in the appearance of new forms developing in place of earlier ones, to see changes in the qualitative and quantitative characteristics of the medium surrounding the crystal.

On the Possible Metamict State of Microcline

In addition to the polymorphous state characteristic of feldspars, another state is also possible for them, which is called metamict. The concept of the metamict state of minerals was first given by V. Brögge—

...by which this type of state of matter was represented as the transformation of originally crystalline minerals into bodies that appear amorphous, optically isotropic, and he regarded metamictness of a substance as inherent in minerals containing rare earths, or attributed this tendency toward isotropization to certain minerals having a complex crystalline molecule. A similar state was first established also for gadolinite, fergusonite, samarskite, zircon, and other minerals which, while birefringent and sometimes retaining an external crystalline form, behave as optically isotropic media [31; 62].

It was then shown that fergusonite, gadolinite, samarskite, and zircon, containing rare earths and radioactive elements, not only lose their characteristic crystal structures, but, in the course of their transition into the metamict state, change their color and specific gravity. It was also demonstrated that such substances, when heated, are capable of restoring their crystal structure and again acquiring the optical and other physical properties characteristic of them. The transition of metamict minerals from the amorphous to the crystalline state upon heating is accompanied by the liberation of heat, sometimes in such large quantities that the mineral itself, having been heated, for example, to \(80^\circ\mathrm{C}\), as has been well established for fergusonite, then spontaneously becomes incandescent to white heat.

In these studies of metamict substances it became quite clear that, in the transition from the crystalline to the metamict (amorphous or glassy) state, the entire crystal structure characteristic of the given substance is disrupted. According to O. Mügge, this latter state has the character of a disperse medium, in the mass of which, in the case of an incomplete transition, one may encounter individual included portions, preserved from alteration, characteristic of the substance in its primary crystalline state [62].

The nature of the transition of crystalline bodies into the metamict state still remains unresolved. Metamict changes in minerals were explained exclusively by the content of rare earths, while the transition itself into the metamict state was explained by the addition of water to the substance. O. Mügge considered the transition of crystalline bodies into the metamict state possible only under the influence of radiation from \(\alpha\)-particles of radioactive substances, which are present in one quantity or another in such minerals. To this one may only add that at the present time it can be considered fully evident that the rare earths too, although to a slight degree, possess radioactive properties.

V. M. Goldschmidt, in opposition to O. Mügge, gives a large list of minerals containing radium, thorium, and uranium for which the metamict state is unknown; he notes, for example, the cases of the minerals bröggerite, thorium monazite, and thorianite, distinguished by a very high content of radioactive substances, but occurring only in the crystalline state. He also points out that gadolinite, in contrast to the minerals just mentioned, contains a very small amount of radioactive substances and is for the most part known as a metamict mineral [31]. On the basis of what has just been said, the conclusion is drawn that the transition into the metamict state cannot be regarded as taking place exclusively under the influence of radioactivity. In his opinion, the metamict state is characteristic of crystalline bodies which, as for example in silicates, tantaloniobates, and titanoniobates, contain rare earths and are essentially compounds of weak bases with weak acids. In contrast to them, compounds of rare earths, as weak bases, with strong acids have not been found in an amorphous (glassy) state. Nevertheless, he was inclined to think that the metamict state belongs among widely distributed phenomena. At least he...

cites as examples \(\mathrm{Ag_2S}\), \(\mathrm{Ag_2Se}\), \(\mathrm{Ag_2Te}\), for which the metamict state is characteristic, and in this case they do not reveal a crystalline structure even when the X-ray method of investigation is applied [31].

V. M. Goldschmidt himself explained the transformation of the crystalline state into the glassy state by the phenomenon of rearrangement of ions of weak acids and weak bases, which may occur when a crystal passes from one thermodynamic field into another, in which the thermal bonds are different from those under the conditions of formation of the crystal. As for the presence of water and radioactive substances in metamict substances, in this respect he believed that water does not enter into the stoichiometric formula of the substance, but enters the metamict body after its formation; however, at the same time it was allowed that water under such conditions may promote, first, the movement of ions and, second, produce hydrolytic cleavage. The role of radioactive substances in metamict transformations is reduced only to accelerating the rearrangement of ions, which, however, proceeds independently of radioactivity [31].

For us, in the conclusions just set forth, what is important is that the transition into the metamict state for one substance or another is very closely connected with a change in the thermodynamic field, and this condition, in essence, as we know, may cause the appearance of a new polymorphic modification. Therefore we are inclined to regard the metamict state of a substance as an intermediate one in a series of polymorphic transformations during the transition of a body from one crystalline state to another. If we designate the metamict state by \(M\), and the polymorphic crystalline modifications by \(K_{\alpha}\) and \(K_{\beta}\), then the ideas set forth may be expressed in the form of the following symbolic formulas: \(^1\)

\[ K_{\alpha} \rightleftarrows M \to K_{\beta} \quad \text{or} \quad K_{\alpha} \begin{matrix} \nearrow & K_{\beta}\\ \searrow & M \end{matrix} \]

from which it is clear that the crystalline body \(K_{\alpha}\), in its transition into the modification \(K_{\beta}\), first passes into the metamict state \(M\), in which it is retained or may be retained for an indefinitely long time. That the glassy state may be relatively stable for a comparatively long time, without undergoing any changes, was also stated by Berger, who recognized the glass-like state in the general system of the universe as the fourth state of matter [16]. On the question of the glass-like state, E. E. Egert notes that an unstable form may persist for a long time if the rate of transformation is very small, or if embryos of the stable form are absent [95].

It also follows from our formula that the metamict body, under suitable conditions, for example upon heating, readily transforms into \(K_{\alpha}\), but the transition of \(M\) into the modification \(K_{\beta}\) has not yet been established by anyone for known metamict minerals, either under natural or under experimental conditions.

If our considerations of the metamict state as an intermediate one in the transformations \(K_{\alpha} \to K_{\beta}\) are correct, then we, naturally, must consider that

\(^1\) The formula of polymorphic transformations \(K_{\alpha} \to K_{\beta}\) is hardly correct for substances for which an explicitly metamict state has not been established. It may be assumed that the state \(M\), as an intermediate one in this case of transformations, is rapidly passing and is not detected by us in the course of such transformations. Therefore it may be assumed that in the series of polymorphic changes of monoclinic sulfur into rhombic sulfur, rubber-like sulfur is an intermediate metamict state, from which it emerges by rapid crystallization into the rhombic modification.

upon the transition \(K_{\alpha} \to M\), the metamict state of the substance is one of the limiting and complete cases of the transition of a crystalline body into the amorphous (glassy) state. However, in these transformations \(K_{\alpha}\to M\) we may, and must, have cases of individual intermediate polymorphic stages of change \(K_{\gamma}, K_{\delta}\ldots\).

\[ K_{\alpha} \rightleftarrows K_{\gamma} \rightleftarrows K_{\delta} \rightleftarrows \ldots\ldots\ldots \rightleftarrows M. \]

As an example one may cite zircon, for which we know crystalline forms, including uniaxial ones, and also know well its metamict states. Thus, according to A. O. Rosenzvit, in studying certain properties of metamict zircon in a heating microscope and in studying its birefringence, it was found that when this mineral is heated to \(500^\circ\), zircon changes from isotropic to biaxial with low interference colors; when heated within the range from \(500\) to \(700^\circ\), a still weak increase in the magnitude of birefringence is observed. With further heating within \(820\)—\(1100^\circ\), the birefringence increases rapidly and reaches the normal value for ordinary zircon [70]. Most probably these changes, observed by A. O. Rosenzvit, proceed by jumps, and not smoothly. It must be thought that, in the transformation of zircon into the metamict state, it undergoes the same changes of its lattice in the reverse order to the experiments of O. A. Rosenzvit. In addition to this, let us note that, as intermediate crystalline states \(K_{\alpha}\to M\), cases are recorded in which zircon reveals biaxial optical properties [23]. G. Rosenbusch gives a number of examples of anomalies of zircon both with respect to specific gravity and with respect to optical properties, which change upon heating toward those of normal zircon [127].

Proceeding from what has just been said, we can broaden the concept of metamictness somewhat and regard it as a series of polymorphic, not always distinctly discernible transformations, in which one of the special cases of its aggregate changes may be the glassy state. As special cases in the series of polymorphic changes we may also consider cases in which, during the transition from one modification to another, no complete reconstruction of the crystal occurs, and in this transformation it does not become homogeneous or completely glassy. For example, for microcline we know fairly frequent cases when it appears homogeneous, devoid of the microcline lattice. This is that variety of potassium-sodium feldspars which we above singled out as agiroclase. Therefore, in the scheme of transformations

\[ \text{Orthoclase} \rightleftarrows \begin{matrix} \text{Latticed}\\ \text{microcline} \end{matrix} \to \text{agiroclase} \]

latticed or patchily extinguishing microcline\(^1\), which reveals structurally different fabrics in its various parts, must be regarded as metamict; and this state for microcline is stable and very widespread.

In exactly the same way, for example, the lamellar or any other structure that has to be observed in leucites, perovskites, boracites, analcimes, and other minerals analogous to them should also be regarded as their metamict state. It is also curious to note that, upon heating, the given metamict bodies, under the corresponding conditions, chiefly

\(^1\) It has long been known in physics that thin plates of one and the same isotropic substance arranged in parallel, and bodies consisting of parallel alternating thin plates of two media differing in composition but optically likewise isotropic, reveal distinct properties of birefringence in polarizing apparatus [14; 90]. Therefore the birefringence of ordinary microclines (metamictoclases) cannot be regarded as accidental anisotropy, nor the microcline lamellae as isotropic.

thus, in laboratory experiments, they pass into a homogeneous crystalline state.

Exactly as in the series of transformations of quartz,

\[ \mathrm{Qu}^{>575}\longrightarrow \begin{matrix} \text{wavelike}\\ \text{extinguishing} \end{matrix} \mathrm{Qu}\longrightarrow \mathrm{Qu}^{<575} \]

wavelike-extinguishing quartz should also be regarded as a metamict substance. Incidentally, we shall return once more, in another place, to the question of wavelike-extinguishing quartz.

Thus, on the basis of the data cited above, we must assign to the metamict state of a substance those cases which, in a series of polymorphic transformations, not only can assume a glassy state, but can at the same time acquire such intermediate structural features as are determined by the necessary distribution of crystallites of some form in the mass of the visible crystal. We have such examples in microclines, leucites, boracites, quartz, and other minerals, for which a clearly expressed heterogeneity is characteristic. To designate only the state just noted, characteristic of microcline in its usual sense, we do not propose a new term, on the grounds that between the heterogeneous structure of microcline, quartz, leucite, and other minerals and the glassy state of clearly metamict minerals there is something in common, which is determined by one or another degree of their aggregateness, or, in other words, by the dispersity of the crystallites. If we understand the amorphous or glassy state as one in which very small crystallites of the substance of the new modification \(K_\beta\) are distributed in the mass of this substance in an arbitrarily disordered way, then in microcline, leucite, boracite, and other analogous minerals the crystallites of the modification \(K_\beta\), or of some other intermediate variety \(K_\gamma, K_\delta\), being more mobile, manage to distribute themselves into lamellae, bars, needles, and fibers distinguishable under the microscope, which, within the limits of the former modification, are regularly distributed relative to one another in the form of fine-polysynthetic twin formations.

Further, it is necessary to introduce a certain fundamental clarity into the concepts of the “metamict” and “amorphous” states. What content should be put into these terms? On the basis of the available data we may say that the metamict state, like the glassy state, will be characteristic of substances having, as a whole, a chemical composition corresponding to definite stoichiometric formulas and to the crystalline structures of the corresponding mineral body. We regard metamict states as intermediate ones, in which a body may find itself in a series of its polymorphic transformations. For metamict substances it is characteristic that, under suitable physicochemical conditions, they return either to their original polymorphic modification \(K_\alpha\), or else transform into \(K_\beta\). Hence it is clear that, for one and the same substance in the series of its polymorphic transformations, there may be several such metamict states:

\[ K_\alpha \longrightarrow K_\gamma \longrightarrow M_1 \longrightarrow K_\delta \longrightarrow M_2 \longrightarrow K_\beta. \]

In contrast to the metamict state, the properly amorphous-glassy state, or glass, chemically and mineralogically represents complex substances, the chemical composition of which corresponds to a series of substances of different composition. On crystallization, such formations prove to consist of an aggregate of different minerals. An example of such amorphous glassy formations is volcanic glass.

Thus, lattice, fibrous, patchy-fibrous, and patchy-extinguishing microclines must be regarded as mineral forma-

—formations in a metamict state. The frequently observed inhomogeneity of extinction and the difference in optical properties within one and the same grain, observed in sanidines, orthoclases, anorthoclases, and adularias, indicate their transition into the metamict state, into microcline—metamictoclase. This metamict state for metamictoclase may be prolonged, may persist for a very long time, and, under the appropriate conditions, may pass into a polymorphous variety, thus becoming a physically homogeneous modification—agiroclase. Finally, in confirmation of the fact that grid and fibrous microclines may represent a parallel and grid-parallel intergrowth of a larger number of lamellae of orthoclase, anorthoclase, and agiroclase, it may be pointed out that there often occur in nature close and regularly grid-like mutual intergrowths of polymorphous minerals. Mutual intergrowths of andalusite and sillimanite, enstatite and clinoenstatite in meteorites are examples analogous to microcline. In this connection Van Hise also points out that in nature the processes of polymorphous transformations often take place under such conditions that this limit remains unattained and, as a result, precisely those intergrowth phenomena are obtained of which mention was made above [96].

Topomineralogy of the Alkali Feldspars

Among the very interesting and practically important questions is the question of the topomineralogy of the alkali feldspars, i.e., under what geological conditions which kinds of feldspars occur in accordance with their typomorphism. It became well known very early that the alkali feldspars are characteristic minerals of massive rocks and that adularias occur in Alpine mineral veins, while sanidine occurs in effusive rocks.

As soon as the fundamental distinction between orthoclase and microcline had been established, it was recognized that microclines have a wide distribution and are characteristic of rocks that have undergone dynamometamorphism; or else opinions were expressed that microclines are characteristic of rocks in which orthoclases undergo alteration, and especially albitization.

Despite the fact that the question posed here is in part clearly statistical in character, nevertheless it is not at present possible to answer it fully, for the reason that in descriptions of feldspars clear indications are not always given both with respect to the species of feldspar and, especially, to its typomorphism. General information on the distribution of feldspars may be found in large handbooks of mineralogy and petrography, but concerning the quantitative aspect of their distribution there is far from complete information only in E. Mackinen [120]. The latter, on the basis of the materials at his disposal, gives statistical data from which we may draw a number of the following conclusions.

1. Phenocrysts of effusive rocks, apparently predominantly kainotype, according to E. Mackinen, are represented substantially by sanidine, which makes up 70% of the total number of cases recorded by him; 26% falls to anorthoclases, and only 4% to albite. This scholar does not mention porphyritic segregations of orthoclases, the presence of which for many extrusive rocks has been indisputably established [85], both in kainotype and in paleotype formations. With respect to albite we may note that, according to the latest data, it enjoys a relatively wider distribution than was formerly believed [85].

With regard to the typomorphism of feldspars, it may be noted that in effusive rocks orthoclase occurs in types I and II of our classification. For anorthoclase this question is not entirely clear. Sanidine occurs predominantly in tabular crystals of type III. However, sanidine, like orthoclase, is also characterized by typomorphic forms I and II. Crystals of types I and II form twins, and crystals of type I are manifested chiefly in twin intergrowths according to the Carlsbad law. Crystals of type II form twins according to the Baveno and Manebach laws and occur relatively rarely.

In addition, we shall note the case of the occurrence not of microcline, but of orthoclase, in basalts, andesites, and dacites in forms resembling phenocrysts. Orthoclase as such, in separate large crystals and fragments, sometimes together with large grains of quartz, was more than once encountered by me in the olivine basalts of eastern Transbaikalia, which cut porphyritic granites. In this case I consider it possible to explain such occurrences of orthoclase and quartz as foreign bodies, torn from the walls during the passage of basaltic magma through fractures across the granites. The porphyritic crystals of alkali feldspar of the granites in this case originally corresponded to microcline, which, during prolonged heating in the basalts, lost its characteristic microcline structure and assumed the character of orthoclase.

1. In plutonic rocks, according to E. McKieen, among the alkali feldspars microcline and microcline-perthite enjoy exceptional distribution. In contrast to it, orthoclase occurs extremely rarely [120]. At least this assertion may be quite valid for the Finnish intrusive rocks. Yu. I. Polovinkina regards microcline for the granites of the middle part of the Ingulo-Ingulets watershed as the sole representative of the alkali feldspars [65]. In the literature we find many indications of the preferential development of microcline in intrusive rocks instead of orthoclase [38, 84, et al.].

However, somewhat later, on the basis of statistical data, E. McKieen comes to the conclusion that orthoclase also occurs in intrusive rocks and, of the total number of cases he recorded, accounts for no more than 20.8%, whereas microcline accounts for 33.3%; anorthoclase in plutonic rocks belongs to comparatively rare minerals and amounts to only 4.3%. Albite, however, accounts for a very high percentage, equal to 42.5%.

Looking closely at the figures just cited, one may doubt that these figures correspond to reality. This applies especially to albite, for the total number of cases taken by E. McKieen is small and is determined by the number 24. Moreover, the method of determining the feldspars scarcely corresponded to the task posed and was beyond dispute. In any case, the study of the intrusive rocks of the Caucasus and of the alkaline intrusive formations of the Kola Peninsula shows with complete obviousness that anorthoclase in intrusive rocks is a common mineral. And with respect to its ratios with microcline, investigators of the Caucasus are inclined to the view that in the younger intrusive formations containing alkali feldspars, anorthoclase enjoys preferential distribution, whereas in the more ancient Precambrian potassic-sodic feldspars are represented exclusively by microcline [15].

As for orthoclase, it must be admitted that in intrusive rocks it is less widespread and yields place to microcline and anorthoclase. This is true not only for the plutonic rocks of Finland, but also for the intrusive rocks of Gornaya Shoria, the Altai, Transbaikalia, the Kalba Range [35], the South-Yenisei Ridge [47], the northern coast of the Kola Peninsula [17], and many other regions of the USSR. But at the same time there are not infrequent indications that, alongside microcline in plutonic

rocks, orthoclases also occur in relatively smaller quantities. These cases were especially recorded by A. N. Zavaritskii in the intrusive alkaline rocks of the Berdyush pluton [38]. O. A. Vorob’eva and V. G. Makhlaev, in considering the character of feldspars in the Khibiny intrusive rocks, indicate that microclines occur together with anorthoclases [30; 58]. Similar frequent cases of the joint occurrence of microcline and anorthoclase in the non-intrusive rocks of the Caucasus are noted by D. S. Belyankin, L. A. Vardanyants, and others [21; 15; 22; 36]. At the same time, L. A. Vardanyants, in some anorthoclases having an optical-axes angle from \((-)\,35^\circ\) to \((-)\,60^\circ\), notes a barely perceptible manifestation of traces of the microcline lattice and uneven extinction within one and the same grain. This latter fact illustrates as well as possible all conceivable relations in the transition of anorthoclase into another, agpaitic modification with \(2V\), fluctuating from \((-)\,65^\circ\) to \((-)\,89^\circ\), and often lacking the microcline lattice [21], i.e., through microcline or, bypassing it, anorthoclase passes into agiroclase.

Finally, let us note that cases have more than once been recorded of the occurrence specifically of orthoclase in anorthosite, gabbro, diorite, and monzonite rocks.

Explanations that microclines are formed in the presence of deformation of intrusive rocks were advanced by a number of persons and later supported by P. Chervinskii and other scholars [49; 67; 97].

A. K. Boldyrev, studying microclines of granite and granite-gneisses of the Kola Peninsula, characterized by freshness, tentatively explained the appearance of such microclines by recrystallization of destroyed and turbid alkaline feldspars as a result of dynamometamorphism and hydrochemical processes. Judgments on the formation of microcline under complex combined phenomena of deformation and the action of emanations, expressed by M. A. Usov [84], and the opinion of Yu. A. Kuznetsov that microcline develops only under abundant emanation activity [47], may be, to a certain degree, correct, since processes of pressure, the prolonged preservation of high temperature during the cooling of intrusive masses, and, finally, the influence of emanations increasing the sodium content in orthoclases, as H. Sh. Wang believes [132], may promote the acceleration of the transition of orthoclase into microcline through lattice microcline or sanidine into orthoclase and into anorthoclase, and then the transition of both minerals into microcline, if the latter is regarded as the principal product of all possible transformations of alkaline feldspars. The joint occurrence in rocks of microcline with orthoclase [38; 65; 67; 51] or of anorthoclase with microcline may indicate an actual connection of these minerals in their primary formation and subsequent transformations; moreover, there may be cases in which, in lattice microcline, individual “spindles” and lamellae correspond in their optical properties to orthoclase [128], which facts speak only of processes of transformation of orthoclase into microcline, while remnants of orthoclase in one form or another indicate that this process did not reach completion and was arrested at a certain stage of its formation.

As for anorthoclase, as a mineral typical of young intrusive rocks, this fact requires further verification. At least L. A. Vardanyants considers it necessary to acknowledge that, in the Caucasus, the presence of anorthoclase cannot be regarded as an unconditional indicator of the young age of intrusive rocks.

And it seems to me that anorthoclase may likewise be no less typical a mineral for more ancient intrusive formations as well. This follows from data from the study of the Khibiny alkaline rocks, where anorthoclase, together with microcline and agiroclase, is a common mineral. M. A. Usov established the presence of anorthoclase and microcline-anorthoclase in many Domesozoic (? A. K.) intrusive rocks of Dzungaria [82]. D. S. Korzhinskii in the Stanovoi

on the ridge he studied young granodiorite intrusions, in which he describes anorthoclase, orthoclase, and notes that sometimes microcline also occurs together with them [45].

And finally, with respect to the character of typomorphism, alkali feldspars of intrusive rocks are characterized by forms of types I and II. Flattened type forms [1], compressed along the second axis \((b)\) and resembling the tabular forms of sanidine, occur predominantly in Carlsbad twins. Crystals of type II, forming twins according to the Manebach and Baveno laws, usually retain the tetragonal appearance characteristic of them.

As regards the predominant development in rocks of one or another typomorphic variety of alkali feldspars, it may be noted that in the porphyritic granites—adamellites of the right bank of the Cherni River south of the Oli River, the porphyritic segregations of microcline are represented exclusively by crystals of square-prismatic habit of type II.

However, alongside the habit of feldspar crystals just described, microcline crystals are encountered rather frequently which, while predominantly elongated along the first axis, are also somewhat elongated along the third crystallographic axis, and at the same time the crystals of the Drachenfels type are somewhat flattened in the direction of the second crystallographic axis. As a result, such crystals acquire the appearance shown in Fig. 6. Orthoclase crystals of analogous appearance were described by C. W. Wang [33]. In the Aturkol rapakivi, developed in the basin of the Bashkaus River in Altai, porphyritic segregations of microcline occur in Carlsbad twins and have a vertically prismatic hexagonal habit characteristic of type I crystals of kali-sodic feldspars.

Fig. 6

Crystals of type II, of square-prismatic habit, occur more rarely in the same Aturkol massif and form small continuous fields, within which alkali feldspars of another habit are not encountered. In the porphyritic intrusions of the Charysh River basin in Western Altai, the porphyritic segregations of microcline are tabular and form Carlsbad intergrowth twins. If definite physicochemical conditions of crystallization must correspond to each type of crystal, as polymorphic varieties, then, naturally, due attention should be paid to this typomorphic feature, which may help us to understand certain questions of magma formation.

Alongside the typomorphism of alkali feldspars, the sizes of the crystals in which they occur should also be noted. Their sizes, as a rule, depend on the character of the rocks containing them. Therefore we shall narrow this question and confine ourselves to consideration of the sizes of kali-sodic feldspars—

those which occur predominantly in porphyritic and porphyroid igneous rocks. At the same time, the porphyroid and porphyritic segregations, in their relative dimensions, vary within wide limits: from small ones, scarcely differing in size from the grains of the groundmass, to gigantic dimensions. Thus, for example, in trachyte rocks the tablets of sanidine not infrequently reach \(3—3.5 \times 1.5—2 \times 0.5\) cm, but occasionally such tablets have dimensions up to \(6—7 \times 3—4\) cm. Orthoclases and especially microclines in Eastern Transbaikalia, in type-II crystals in porphyroid granites and adamellites, occur in three principal sizes: \(1—1.5 \times 1—1.5 \times 2.5\) cm, \(2—2.5 \times 1.8—2.5 \times 3\) cm, and \(2.5—3 \times 2.5—3 \times 4—5\) cm. However, here too smaller and larger ones occur, which in individual cases attain gigantic dimensions. Thus, in porphyroid granites of the Kiya River, a left tributary of the Shilka River, in one of the gullies I found microcline crystals in the form of a square bar with a cross section of \(4—5.5 \times 5—6.5\) cm, while their length was \(8—10\) cm. Orthoclases of similarly very large size (probably anorthoclases, \(Np : (010) = 2.5—3^\circ\)) were encountered by the geologist D. M. Korytova in porphyroid granites in the Klyuchevsk and Davendinsk regions of the Shilka Range. In the Sretensk porphyroid granites, thick-tabular alkali feldspars reach dimensions of \(3—5\) cm in length, \(1.5—3.5\) cm in width, and up to \(1—1.5\) cm in thickness. It is interesting to note that such tabular crystals bear zones of overgrowth with subsequent intergrowth of tabular grains of alkali feldspar according to the Carlsbad law. In the Attyrkol porphyroid rapakivi in the Southwestern Altai, porphyroid microcline in the form of type-I crystals forms Carlsbad penetration twins, in which individuals along the third axis often reach \(5—7\) cm in length, along the first axis \(2.5—3.5\) cm, and the thickness of the twin up to 3 cm; the faces \(\{\bar{2}01\}\), in comparison with \(\{001\}\), are often expressed very sharply.

Sometimes porphyroid segregations in porphyroid granites attain very large dimensions. Thus, in the granites of Corsica there occur porphyroid crystals of orthoclase with well-developed forms \(\{010\}\), \(\{001\}\), \(\{110\}\), and \(\{\bar{2}01\}\), which in twins according to the Carlsbad law reach up to 14 cm in length and up to 7 cm in thickness. A. V. Moskvin describes microclines from nepheline syenites of the central part of the Alai Range, from the banks of the Dzhury-sai River, which are represented by well-developed crystals, up to \(20—25\) cm in length. Porphyritic segregations of microcline in these syenites, up to \(5—6\) cm in length, are rare [61].

In conclusion, let us dwell on the consideration of one case in the study of alkali feldspars, by the example of which we may try to introduce some of our topomineralogical corrections. Thus, C. V. Wang studied more than 140 well-expressed crystals of orthoclase. The author reports that the crystals studied come from pegmatites associated with an aegirine lakkolith. The country rock was deeply weathered and had turned into a clayey mass, from which the studied feldspar crystals, called by him orthoclase, were extracted [133]. Three features are remarkable in these crystals studied, and they should be mentioned here. Morphologically the crystals are elongated, chiefly in the direction of the first crystallographic axis and somewhat less along the third axis. Of the simple forms that determine the habit of the crystals, \(\{001\}\), \(\{010\}\), \(\{110\}\), \(\{\bar{2}01\}\) are developed. In addition, faces of the forms \(\{130\}\) and \(\{\bar{1}11\}\) occur, which have no special significance for the habit. Such crystals, in their character, are completely identical with porphyroid segregations of microcline crystals in granites, widely developed in the Shilka-Nerchinsk region and in the Shilka Range. The form \(\{\bar{1}01\}\), characteristic of feldspar crystals from the pegmatites of Eastern Transbaikalia, was not recorded at all. Therefore, taking into account the typomorphism of feldspar crystals, we must

we shall regard C. V. Wang’s orthoclases as originating from intrusive rocks, and not from pegmatites. This conclusion, of course, requires verification.

The second peculiarity of the orthoclases under consideration is the following. In transverse sections, orthoclase crystals prove to consist of two sharply different parts. The larger inner part of the crystal is glassy-transparent and has the properties of sanidine¹. The outer part, in the form of a narrow rim, borders the sanidine crystals on all sides; this part is usually opaque, colored flesh-red, and under the microscope bears the characteristic features of a microcline structure. The boundary between the microcline rim and the sanidine core is usually irregular. The occurrence of sanidine in intrusive rocks is a very curious fact and likewise requires verification.

The third peculiarity of C. V. Wang’s orthoclases is that study of them in twins according to the Carlsbad and Manebach twinning laws shows that the rotation of one individual with respect to the other in the formation of simple twins does not always correspond exactly to \(180^\circ\); namely, there are deviations to one side or the other by various angular amounts from 0 to \(40^\circ\) and more.

And, finally, concluding our consideration of the behavior of alkali feldspars in massive rocks, let us note for anorthoclases, in their topomineralogy, the following extremely interesting feature. Anorthoclases, as minerals, chemically form a continuous isomorphous series of composition \((\mathrm{Na}, \mathrm{K}) \mathrm{AlSi}_3 \mathrm{O}_8\); nevertheless, over the areas of some territories, as was noted above, potassium anorthoclases are predominantly widespread, whereas in other territories sodium anorthoclases become widespread. The potassium mineralogical province is made up of the anorthoclases of the Caucasus and the Kola Peninsula, whereas the Mediterranean region and, in part, the eastern coast of the Atlantic Ocean constitute a province of distribution of sodium anorthoclases, which includes the islands of Pantelleria, Sardinia, Corsica, France, Norway, and other localities.

As an example of how the topomineralogy of alkali feldspars makes it possible to draw a number of theoretical conclusions, requiring verification, on questions of magma formation, we may turn to the work of L. A. Vardanyants. Studying the granites of the Malka and Mushta in the Central Caucasus, which form part of one and the same laccolith, he found in them anorthoclase, grid microcline, and non-grid microcline, taken to be orthoclase. He established that the granites of the Malka River are typical microcline varieties, while in the granites of the Mushta River, along with ordinary microclines, there are microclines with \(2V = (-) 60^\circ\), which are presumably transitional varieties to anorthoclases (the author has no basis for this conclusion. A. K.). In addition, in the same laccolith there are areas of granites with anorthoclase typical for the Caucasus. On the basis of the distribution of alkali feldspars in the laccolith, L. A. Vardanyants concludes that in that part of the granitic body where the magma had a higher temperature and where its crystallization apparently proceeded more slowly, we have microcline varieties of granite with a very stable optical characteristic of microcline itself. In the more peripheral parts of the intrusion, for example, in the basin of the Mushta River, where the phenomenon of quenching could have been manifested to a greater or lesser degree, the optical characteristic of the potassium-sodium feldspar becomes less stable, and here, along with microcline, there also appears typical anorthoclase, which in some cases even becomes predominant.

1. In pegmatites feldspars are always encountered in relatively large and even gigantic crystals, quite often in well-formed individuals. According to E. Mackinen’s statistical data, alkali feldspars in pegmatites are represented chiefly by microcline, which

in the total mass of feldspars is 51.5%, orthoclase accounts for 13.7%, and albite for 34.8% [120]. For Finnish pegmatites, only microcline is most typical [121]. L. L. Solodovnikova, with respect to the pegmatites of northern Karelia, asserts that “orthoclase is never encountered in them”; the potassium feldspar is represented only by microcline [75]. However, according to P. K. Grigor’ev, orthoclase is also present in the pegmatites of Karelia alongside microcline [32].

N. I. Bezborod’ko, studying feldspars from the pegmatites of Volhynia, found that of the 138 specimens he studied, 82.61% were represented by microclines and microperthites, and only 17.3% was accounted for by orthoclase and orthoclase-perthite [2].

The feldspars of the miaskitic pegmatites of the Ilmen Mountains are represented by perthitic, lattice microcline, and, more rarely, by non-lattice microcline (agiroclase). Cryptoperthites form completely independent crystals or develop in the form of a border around microcline-perthites. The ratio of the sodium molecule to the potassium molecule in the microclines ranges from 1:2 to 1:3, while L. L. Solodovnikova notes that in the pegmatites there is more potassium than in the miaskites themselves. A geochemical peculiarity of the feldspars is the considerable content of the celsian molecule (1–2%). Chemically, the feldspars of the miaskitic pegmatites in general show complete analogy with the feldspars of granite pegmatites [77].

According to A. E. Fersman, microcline is common for geophases C and D (700–550°). In geophases E (about 500°), there are crystals of orthoclase or orthoclase-perthite; the latter, at about 400–350°, approach adularia in their character, which, as is known, forms from solutions at temperatures not below 300–320° [86]. At temperatures above 800°, apparently, only orthoclase forms.

As for information on the distribution of anorthoclases in pegmatites, no corresponding indications are found in A. E. Fersman on this matter. However, information on the occurrence of this variety of potassium feldspars in pegmatites is available from M. M. Al’tgauzen for the Kan deposit of pegmatites described by him [1].

N. N. Gudkova notes the presence of anorthoclase in the aegirine-feldspar and pectolite-yuksporite types of pegmatites of Mount Yukspor on the Kola Peninsula [34].

The typomorphism of feldspars is determined mainly by the habit of the crystals, for which the predominant development of the faces {110} and {010} is characteristic, as defining a hexagonaloid type of crystals, on which the faces of the form {201} are absent, or, compared with {010}, have subordinate significance. On crystals of microcline from the pegmatites of northern Karelia the following forms are most highly developed: P {001}, M {010}, T {110}, l {1̄10}, and x {1̄01}; to a lesser degree, z {130}, y {2̄01}, and o {1̄11} are developed, and only rarely n {021} [75]. Such crystals are most often somewhat elongated along the vertical axis; apparently, they are also quite often somewhat elongated in the direction of the first crystallographic axis; more rarely they are somewhat compressed along the third axis. In all cases, the crystals of alkali feldspars of pegmatites appear short-prismatic. For pegmatitic feldspars, as noted above, a relatively large and even enormous size of the crystals is characteristic, both in their individual occurrence and in the pegmatitic aggregate.

1. In the fourth group of feldspars, as a special genetic group, E. Mackinen includes drusy formations characteristic of intrusive rocks and pegmatites. From the statistical data it follows that, in 52% of all recorded occurrences, the alkali feldspars in druses are represented by microcline, in 8% by orthoclase, and in 40% by albite. Ne-

A shortcoming of this statistics is, as a rule, the absence of indications as to the form in which albite occurs. The latter, as observations show, is present chiefly in the form of platy aggregates of cleavelandite, coating drusy crystals of orthoclase, microcline, quartz, and other minerals.

The typomorphism of the feldspars in the present case is determined as the final phase of pneumatolytic or pegmatitic processes. Despite the fact that the general conditions of formation of drusy alkali feldspars of miarolitic cavities in intrusive and pegmatitic rocks bear several distinctive features, E. McKie considered it necessary to combine them, while at the same time separating them from the drusy formations of adularias, which are closely connected with hydrothermal processes. According to G. Kalb, in the described cases of drusy formations, microcline and orthoclase occur in crystals in whose morphology the faces \(\{010\}\), \(\{001\}\), and \(\{110\}\) take part, as well as the faces \(\{\overline{1}01\}\), \(\{\overline{2}01\}\). In a few cases it was also necessary to observe such crystal forms in whose structure the faces \(\{0\overline{1}0\}\) and \(\{\overline{1}01\}\) played an essential role, giving the crystals a square prismatic negative type (type V of our classification of typomorphism).

V. Brögger, from a series of drusy formations in the augite syenites of Norway, describes crystals of microcline which have sharply expressed tabular forms elongated along the \(c\) axis, whose habit is determined predominantly by the development of \(M\) \((010)\). The face \(P\) \((001)\) is of relatively small size. The remaining faces \(z\) \((130)\), \((1\overline{3}0)\), \(y\) \((\overline{2}01)\), \(o\) \((\overline{1}11)\), \(e\) \((1\overline{1}1)\), \(T\) \((110)\), \(l\) \((1\overline{1}0)\), and \((100)\) are usually very narrow and have subordinate significance.

1. Finally, the last representatives of the topomineralogy of the potassium–sodium feldspars considered by E. McKie are the hydrothermal formations, often represented by adularias. E. McKie and others assign them to orthoclases; E. Spencer—to microclines. Of all the cases of adularia occurrence cited by E. McKie, numbering no more than 12, the share of orthoclase accounts for no less than 83.4%, and that of albite—16.6%. However, it is now reliably known that among adularias there are representatives of both monoclinal and triclinic feldspars, and that among adularias, alongside members rich in potassium, there are varieties in which sodium plays a noticeably greater role in comparison with potassium. This circumstance prompted K. I. Viyka to distinguish sodium adularia as kalinatron-microclase [134].

In some quartz veins, closely connected in their development with pegmatites, both microclines and orthoclases occur. As for the typomorphism of adularias, it has been studied in the best manner, since adularias often occur in transparent and well-formed crystals. In our view, among adularia crystals one may distinguish two principal types of crystals: adularia rhombohedral (VII-A) and adularia prismatic (VII-B). The first, according to G. Kalb, will be characteristic of hydrothermal ore veins; the second habit of adularia crystals, on the contrary, is most typical of mineral veins of Alpine character.

In Khakassia E. N. Zykov¹ found, in large quantity, crystals of adularia (Fig. 7), often associated with chalcopyrite. Crystals up to 0.5 cm in cross-section are represented by both adularia types. Crystals of type VII-A have a rhombohedral habit, but the face \(\{\overline{1}01\}\), in comparison with \(\{110\}\), receives predominant development. Adularia VII-B has a typically prismatic habit. Crystals of this habit are somewhat compressed along the vertical axis and in a number of cases assume a tabular character along \(\{101\}\). Glafira adularias are also characterized by complex twins according to

(001), (0$\bar{2}$1), and (0$\bar{2}$1). The adularia crystals described by E. N. Zykov are, in their main features, similar to the typical ones, but at the same time differ sharply from them; these adularia are typomorphic for Khakassia.

1. For contact-metamorphic conditions of mineral formation, when in the contact zone profound changes of rocks take place with the formation of hornfelses, skarns, and migmatites, there are no precise indications as to which alkali feldspars form under these conditions and what their typomorphic features are. A. Laitakari indicates that microcline occurs among the contact minerals in the limestones of the island of Pargas. V. M. Gold-

Fig. 7. Typomorphism of the adularia of Khakassia

T-(110) P-(001) M-(010) x-(101) z-(130) O-(111)

schmidt, in weakly metamorphosed rocks of the Christiania region, encountered a feldspar with a microcline lattice. In hornfelses, however, there is apparently a predominant development of orthoclase as compared with microcline. With regard to other feldspars that might occur in contact-metamorphic rocks, for example anorthoclase or sanidine, there are no indications. In the skarns of Norway, occurrences of orthoclase are noted. However, there are grounds to doubt the correct-

…of this argument, since E. N. Zykov found orthoclases in adularia crystals of types VII-A and VII-B in the skarns of Khakassia in association with chalcopyrite, pyrite, quartz, and calcite.

1. In migmatites, microcline is encountered more often than orthoclase; but if migmatization is accompanied by albitization, then one may observe the replacement of potash-soda feldspars by albite, with the formation of albitic rocks and albitite migmatites and albite-quartz veins, in which albite is often represented by cleavelandite.

2. As for the alkali feldspars in metamorphic rocks which, by their nature, are regarded as regionally metamorphic, in this case microcline is predominantly widespread [104]. Orthoclase apparently occurs rarely. Anorthoclase, according to F. Becke, is not known in crystalline schists. G. Rosenbusch likewise indicates that anorthoclase is not found in crystalline schists¹, and feldspars with the properties of sanidine are equally absent.

In the metamorphic rocks of the Southern Yenisei Ridge, Yu. A. Kuznetsov demonstrated the predominant distribution of orthoclase and orthoclase-perthite [47]. He found, for example, a predominant development of orthoclase in garnet-orthoclase gneisses and in the charnockitic group of rocks—bogunites and kuzeevites—which are part of the Kansk metamorphic complex of Lower Archean age. P. I. Lebedev also notes that in the charnockites of Volhynia the alkali feldspar is represented chiefly by orthoclase, which, in his opinion, has more the character of sodic orthoclase [51].

In the alkali gneisses of the Binnental in the Alps, a case is noted of the simultaneous presence of orthoclase and microcline, without grid twinning. Both minerals constitute a substantial part of the gneisses; moreover, in the gneisses of Binnental microcline sharply predominates over orthoclase, whereas in the gneisses of Simono, Lucomagno, and other localities of the Binnental, microcline and orthoclase occur in more or less equal quantities, and in the gneisses of Leventina orthoclase predominates over microcline [99]. The data just cited on orthoclase and microcline in gneisses provide grounds for saying that here, too, microcline is in close association with orthoclase and is represented by the polymorphic variety agiroclase.

The typomorphism of microcline and orthoclase in metamorphic rocks still remains unresolved.

1. Crystals of alkali feldspars are also known in sedimentary rocks. In clastic formations, for example in sandstones, alkali feldspars enter as a result of the destruction of the corresponding rocks; they also enter sedimentary rocks of any composition as a result of effusive activity. In both cases, fragments of potash-soda feldspars, if they are not altered as a result of diagenesis, preserve their characteristic features.

Furthermore, cases of the formation of alkali feldspars in sedimentary rocks, especially in limestones, are not infrequently reported. Such indications are found in F. Grandjean, R. A. Daly, L. Cayeux, P. A. Zemyatchensky, and others [68, 69]. Alkali feldspars, as a rule, occur rarely in sedimentary rocks and behave now as orthoclase, now as microcline. Most investigators agree that authigenic feldspars formed within the mass of the sediment are always euhedral [81].

Conclusions

Summing up all that has been said above on the question of the relationship between microcline and orthoclase, one may emphatically stress once again that on the basis—

in the interpretation of X-ray studies, the view must be abandoned that orthoclase is submicroscopically twinned microcline.

It is also proved by X-ray methods that there are no grounds for speaking of the identity of microcline and orthoclase.

Our views on polymorphism make it possible to pose more broadly the question of the interrelations of the alkali feldspars and to give an idea of their systematics, in which microcline must occupy an appropriate place in the schemes below.

Feldspars, as aluminosilicates, are divided by chemical composition into alkali and alkaline-earth:

                         FELDSPARS
            ┌────────────────┴────────────────┐
     ALKALI FSP.                     ALKALINE-EARTH FSP.
          │                       ┌────────────┴────────────┐
   POTASSIUM-SODIUM          BARIUM-POTASSIUM       LIME-SODIUM
          │                       │                     │
     ORTHOCLASES              CELSIANS              PLAGIOCLASES

The composition of the alkali feldspars is determined by their content of potassium and sodium; the alkaline-earth feldspars, by chemical composition, are divided into barium-potassium and lime-sodium feldspars. We may unite the potassium-sodium feldspars under the general name orthoclases¹.

Alkali feldspars—orthoclases, in turn, according to their morphological, physical, and genetic features, are divided into sanidines and adularias, as characteristic representatives of the extreme genetic associations of magmatic rocks. Feldspar formations that owe their origin to pegmatitic and pneumatolytic processes, by their morphological properties, are intermediate between feldspars of magmatic proper and hydrothermal origin. Therefore the classification of alkali feldspars and their nomenclature may be represented as follows.

                           ORTHOCLASES
                  ┌─────────────┴─────────────┐
              sanidines                    adularias
          ┌───────┴───────┐                    │
     α-sanidine        orthoclase              │
      ┌────┴────┐       ┌────┴────┐      ┌────┴──────────┐
anorthoclase  β-sanidine monorthoclase agiroclase  agiroadularia monadularia
        \        \          \        \        \          \        /
                         METAMICTOCLASE
                          (MICROCLINE)

From the scheme presented it is evident that orthoclases, as a family, break down into sanidines and adularias.

Sanidines, as high-temperature formations, in the stage of formation—depending on the conditions and duration of cooling—give rise to true sanidines, or, more precisely, α-sanidines, which in effusive rocks pass into β-sanidines and are partly preserved in the form of the α-modification. Under intrusive conditions of rock formation, α-sanidines are transformed into orthoclases. However, the latter, as phase formations, themselves for the most part undergo changes which in the main do not reach completion and remain for an indefinitely long time in the form of uneven and patchily extinguishing and fibrous-lattice formations, well known to us as microclines. To these latter, as being in a metamict state, we propose to assign the name metamictoclases, which usually possess a microcline structure.

Comparatively more rarely in intrusive rocks, and often in effusive formations, orthoclase occurs as such, as if in a quenched phase, to which the name monorthoclase should be assigned. It bears within itself, to varying degrees, a pronounced tendency toward transition both into the metamict phase—metamictoclase—and into the adularian phase with the properties of “microcline deprived of a microcline lattice,” which we have already called agiroclase. Agiroclases, as non-lattice, homogeneous formations, together with lattice-bearing metamictoclases, have usually been united under the general name microclines, which we must now abandon.

Anorthoclase represents one of the principal polymorphic phases of the transformation of β-sanidine and partly γ-sanidine (orthoclase), to which it is close in its optical properties and from which it differs by its agiry. Anorthoclases themselves are not sufficiently stable and reveal a tendency toward transition into the metamict state, as is indicated by cases of the appearance in them of elements of inhomogeneity in the form of patchy extinction and a fibrous-lattice structure, often expressed as a very fine fibrous structure.

Adularias, in turn, according to their morphological features, are represented chiefly by monoclinic varieties, which we therefore designated as monadularias, and by the comparatively rarer adularias with an agiry structure, as agiroadularias. Among forms similar in appearance to adularias one may distinguish clevelandite as an adularia-albite, sometimes containing an increased amount of potassium. There are isolated indications of the occurrence of cases of microclinization of adularias. Study of opaque varieties among adularias, which show uneven extinction and the appearance of a lattice, gives clear indications that in this variety of feldspars as well there is a transition of adularias into the metamict state.

Finally, the presence in mineral vein formations of zeolites indicates the occurrence of adularias which, in their chemical composition, should correspond to plagioclases. Such formations should be regarded as plagiadularias.

Thus, the group of alkali feldspars, with clearly expressed polymorphic properties, breaks down into a monogyric and an agiry series. As is evident from the presented Table 4, the monogyric series of potassium-sodium feldspars comprises α- and β-sanidines, orthoclase (γ-sanidine), monorthoclase, and monadularia (α-adularia). To the agiry series of alkali feldspars belong β-anorthoclase, agiroclase, and agiroadularia¹. Each distinguished polymorphic modification possesses its own optical properties.

Table 4. Classification of the alkali potassium-sodium feldspars

Monoclinic series	Agiric series
Sanidine: $\alpha$-sanidine $\beta$-sanidine Sanidine-orthoclase (orthoclase, $\gamma$-sanidine) Monorthoclase ($\delta$-sanidine) Monadular ($\alpha$-adularia)	Anorthoclase: $\alpha$-anorthoclase $\beta$-anorthoclase Agiroclase Agiroadular ($\beta$-adularia)

Sanidines, as polymorphic modifications, are observed in two phases. For one of them, $\alpha$-sanidine, the characteristic feature is the position of $N_p N_g$, parallel to $(010)$; for the other modification, $\beta$-sanidine, the plane of the optical axes is arranged perpendicular to $(010)$.

Sanidine orthoclases ($\gamma$-sanidines) are those varieties whose formation, according to A. E. Fersman, is possible at high temperatures. The optical properties of this orthoclase are unknown to us. They are ascribed relatively large angles of the optical axes; the plane of the optical axes in them is arranged, as in $\beta$-sanidines, perpendicular to $(010)$; the angle between the plane $N_p N_g$ and the axis $a$ fluctuates for the majority between 0 and 6°. Sanidine orthoclases, or simply orthoclases, as a rule, during prolonged processes of their formation by crystallization and during their long existence under slowly changing physico-chemical conditions, undergo polymorphic changes, and only comparatively rarely can their monoclinic modifications be preserved in a different (below 600–300°) thermodynamic setting. This type of orthoclase we shall designate as $\delta$-sanidines, or as monorthoclases, for which the optical properties cited above are in fact characteristic.

As was noted, $\beta$-anorthoclases in their optical properties—for example, in the angle of the optical axes—stand closest to $\beta$-sanidines, while the increase of $2V$ to the dimensions characteristic of most monorthoclases perhaps indicates that either the $2V$ of anorthoclases increases with time, or $\beta$-anorthoclases represent polymorphic phases of orthoclase which, partly changing into metamikto-clase and agiroclase, partly changing the position of the plane $N_p N_m$ with respect to $(010)$, acquire the character of anorthoclases.

$\alpha$-anorthoclases are also quite possible; in them the plane $N_p N_g$ will be arranged almost parallel to the plane $(010)$ and will make the small oblique angles characteristic of anorthoclases. Such $\alpha$-anorthoclases must represent products of alteration of $\alpha$-sanidine and must themselves be transformed into $\beta$-anorthoclase.

Thus, a systematic consideration of the position of microclines among the alkali feldspars has made it possible to draw a number of conclusions and to arrive at the conclusion that, during crystallization under magmatic conditions, depending on the relative rate of cooling and the thermodynamic setting, one of the above-mentioned polymorphic modifications of the feldspars will be encountered; while microcline, as an intermediate formation in the series of polymorphic transformations, is found in rocks often together with residual and final phases of transformations.

The position of metamikto-clase (microcline) in the series of alkali feldspars may be illustrated by the following scheme.

α-sanidine
↓
β-sanidine → β-anorthoclase
orthoclase (γ-sanidine) → metamiktochlase → agirolase
orthoclase (γ-sanidine) → monorthochlase → metamiktochlase
β-anorthoclase → metamiktochlase
β-anorthoclase → agirolase
metamiktochlase → agirolase

In pegmatitic and pneumatolytic conditions, the formation of potash-soda feldspars most probably proceeds from orthoclase (γ-sanidine), which also subsequently more often passes into metamiktochlase.

Metamiktochlase, so well known to us from its structural features as microcline, we now conceive as a mineral formation that is in a metamict state. In this latter state the substance of the feldspars, during its polymorphic transformations, is delayed for a comparatively long time. The transition of alkali feldspars into metamiktochlase may be promoted by temperature fluctuations during the cooling period, and by emanational and dynamometamorphic processes, which merely accelerate the delayed processes of polymorphic transformations of potash-soda feldspars into agirolase, but at the same time often prove insufficient to bring these transformations to completion.

LITERATURE

1. Altgauzen M. N.—The Kansk pegmatite deposit. Collection “Pegmatites of the USSR,” 1, 1936.

2. Bezborodko I. I.—The principal types of feldspar deposits of Volhynia. Materials of the Conference on Feldspar. Publ. of the Geological Committee, 1928.

3. Belyankin D. S.—Syenite porphyry from the Chu River. Proceedings of the Petrograd Polytechnic Institute, vol. XXVII, 1918.

4. Belyankin D. S.—On the question of the interrelations of feldspars. Concerning their phase diagram proposed by I. Foote. Proceedings of the Petrograd Polytechnic Institute, vol. 28, 1919.

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1. To the monogyric series of alkali feldspars one must assign barbierite (sodium orthoclase), and to the agiry series—albite and clevelandite; we did not find it necessary to dwell on their consideration on the preceding pages. ↩↩↩↩↩↩↩↩↩↩↩↩

2. Such a formulation of the question puts forward, as a necessity, the carrying out of special X-ray and crystallographic investigations of typomorphic crystals for the purpose of calculating their geometrical constants and confirming the conclusions made above. ↩↩↩