Orthite from Skarns of the Yenisei Ridge

A. D. Nozhkin, V. Z. Mustafin

(Submitted by the Department of Mineral Resources)

Orthite is one of the most widely distributed isomorphous rare-earth minerals. It is known [4, 12, 13] that such minerals are good geochemical indicators of endogenous processes. Therefore, the accumulation of factual material on their physicochemical characteristics in rocks of various genesis is of considerable interest.

Orthites from magmatic rocks of acid composition and from pegmatites have been characterized most fully. For them, a definite regularity has been established in the change of chemical composition and of some other properties depending on the conditions of formation. Information on orthites from rocks of other genesis is insufficient. There are, for example, references to occurrences of orthites in contact-metasomatic deposits by A. G. Betekhtin [2] and E. U. Heinrich [20]. The relative ratios of rare-earth elements of the cerium group in orthites from contact-metasomatic and vein rocks are given in the work of E. I. Vainshtein et al. [4].

This article describes orthite discovered by A. D. Nozhkin in one of the skarn occurrences of the Yenisei Ridge.

Brief characterization of the orthite-bearing skarns

The orthite-bearing skarns are developed at the contact of red porphyritic granites of the Uvolgo-Nagatino type with limestones of the Penchengin Formation of the Proterozoic. The composition of the skarn rocks is variable and changes with distance from the granites. In the immediate vicinity of the granites there are garnet-magnetite skarns. At some distance from the intrusion, garnet-pyroxene and pyroxene rocks begin to predominate, often enclosing smaller bodies of magnetite. With still greater distance from the contact, minerals of the epidote and amphibole groups are present in appreciable amounts in the pyroxene-garnet skarns. In them numerous areas of marbles and weakly skarnified rocks are observed, which gradually pass into marmorized limestones.

Since orthite occurs in the skarns relatively rarely, it is necessary to dwell on their mineralogical characteristics in order to show the relationships of the mineral described with the skarns and, consequently, to determine its place in the overall skarn process.

Garnet-andradite is one of the most widespread minerals of the skarns. The color of the mineral is brown. Hardness is less than 7. Under the microscope it has a light-pink color and is isotropic. The grains are isometric; their dimensions do not exceed 1–1.5 mm. Along with magnetite, it is one of the earliest formations. Judging from the cutting of magnetite bodies by veinlets with garnet, it may be concluded that the process of garnet formation was prolonged.

Pyroxene-diopside is present in variable amounts in all types of skarns. The grain form is irregular, more rarely tabular with serrated edges. Their dimensions vary within the limits of 0.1–1 mm. Relicts of garnet are often observed inside pyroxene grains. The mineral shows noticeable pleochroism from light bluish-green to colorless. Study of several pyroxene grains on a Fedorov stage gave \(2V = +62^\circ\), \(cN_g = 42^\circ\), \(N_g - N_p = 0.029\), which identifies it as diopside containing about 15% of the hedenbergite molecule.

Orthite was encountered in the skarns in association with diopside, clinozoisite, and hornblende. Owing to the almost universal overgrowth of orthite by clinozoisite and epidote, it is difficult to establish its relationships with other minerals. Nevertheless, in several cases it was possible to observe that diopside overgrows grains of the mineral described (Fig. 1), and often penetrates into them. This indicates earlier formation of orthite as compared with diopside.

Fig. 1. Orthite crystal (1) overgrown by diopside (2). Crossed nicols, 40×.

Clinozoisite is observed in a noticeable amount only in association with orthite. The form of the grains is tabular; their maximum dimensions do not exceed 1.5 mm. The mineral has perfect cleavage on (001) and noticeable cleavage on (100). In crossed nicols, clinozoisite has a patchy coloration: grayish-blue to light-yellow and orange. Low interference colors predominate. The reliability of the determination of clinozoisite is confirmed by the large angle \(2V\) \((+96^\circ)\), \(cN = 6^\circ\).

It is highly characteristic that clinozoisite crystals surround orthite and that their optical orientation is close to that of orthite. Slight discrepancies are noted in the extinction angles of the grains. Orthite is often replaced by clinozoisite, as a result of which the former loses its distinct outlines, and the transition from orthite to clinozoisite becomes gradual.

Hornblende is present in the skarns in subordinate amounts. The grain form is irregular; their average dimensions are 0.1–0.5 mm. Pleochroism is sharply expressed: \(N_g\)—light grayish-brown coloration, \(N_m\)—brown-green, \(N_p\)—dark dirty-green with a brownish tint. The maximum extinction angle relative to the cleavage is 23°.

In hornblende crystals situated near orthite, substantial changes in the character of pleochroism are observed. They are manifested in an intensification of coloration to dark, almost black tones at the periphery

of that part of the crystal which faces the orthite. Such changes were probably caused by the action of radioactive radiation and, in character, are analogous to those observed in typical pleochroic halos.

Hornblende intensely replaces all the minerals described above. Within its grains one can often see relics of pyroxene, clinozoisite, and other minerals.

As follows from the description given, in mineralogical composition the skarns belong to the light, low-iron varieties according to the classification of F. N. Shakhov [21].

The intrusive rocks with which the orthite-bearing skarns are genetically associated are represented by red and pinkish-gray medium- and coarse-grained porphyritic granites. Under the microscope it has been established that they consist of 25–30% albite-oligoclase No. 12, orthoclase (up to 30%), microcline (10–15%), quartz (20–25%), and biotite (up to 5%). Among the accessory minerals present are epidote, tourmaline, monazite, and apatite. The texture of the rock is granitic.

Physical properties and optical characteristics of orthite

The mineral described occurs in the rock in the form of crystals, the average size of which varies within the range 0.2–1.5 mm.

The color of the mineral is honey-yellow to brown, sometimes almost black. The fracture is conchoidal, the luster resinous, hardness about 5.5. The specific gravity, determined by the method of hydrostatic weighing, is 3.71.

Microscopic examination of transparent thin sections reveals grains of the mineral with rather distinct crystallographic outlines. In the observed sections of the crystals one may infer the presence of faces of the following forms: (100), (001), more rarely (201).

The plane of the optical axes is parallel to (010); the angle \(cN^d\) varies from 24 to 34°, which corresponds to the data given for orthite by Winchell [5]. Twins on the plane (100) occur rather often (Fig. 2). The mineral is biaxial, negative. The angle of the optical axes is equal to 66°. The refractive indices are \(N_g = 1.738\), \(N_p = 1.716\). The value of \(N_m\) was calculated from the graph of A. I. Boldyrev [3] and the Mertie nomogram [19]:

\[ N_m = 1.730,\quad N_g - N_p = 0.022. \]

Fig. 2. Aggregate of zonal twinned crystals of orthite. Crossed nicols, 50×.

Zoning of the crystals is very characteristic of the orthite described. It is manifested in the alternation of differently colored zones, each of

which possess qualitatively distinct pleochroism. The zones are, as a rule, parallel to the outlines of the crystal. Most often four clearly expressed zones are observed in a crystal. The central one usually has a rather variable mottled coloration and more often pleochroizes from light yellow-brown to dark greenish-brown. The next—broad—zone pleochroizes from almost colorless to golden yellow. Toward the periphery it is replaced by a brown zone 0.1–0.05 mm wide. The outer narrow zone is also yellow; in the character of its pleochroism it is analogous to the already described zone of similar coloration. The noted zones, in turn, are characterized by a heterogeneous structure; each of them represents a series of much thinner zones that differ somewhat in color intensity. A noticeable decrease in the width of the zones across the elongation of the crystals, as compared with their width at the ends, is striking. The absorption scheme along the indicatrix axes is \(N_g > N_m > N_p\). The optical orientation of the individual zones differs somewhat, which causes their alternating extinction. Crystals are observed in which the outer and central zones first become extinct; then, upon rotation of the grain in the same direction through 15–25°, the intermediate zones become extinct. In the broad zones, fine cracks of radial orientation can often be observed (Fig. 3). They can be traced

Fig. 3. A distinctly zoned crystal of orthite is replaced by clinozoisite (light); fracturing is noticeable in the zones. Crossed nicols, 40×.

only within a single zone, in this respect resembling dehydration cracks in colloform formations.

X-ray studies and chemical composition of orthite

For complete identification of orthite from the skarns, X-ray studies were carried out in the laboratory of the Department of Physics of the Tomsk Polytechnic Institute by L. K. Gultyaeva; their results are given in Table 1.

The X-ray photograph was taken in a camera 57.3 mm in diameter using iron and molybdenum radiation. Shooting conditions: iron anticathode \(V = 35\) kV, \(J = 14\) mA; molybdenum anticathode \(V = 50\) kV, \(J = 15\) mA.

The small number of lines and their reduced intensity on the Debyegrams are probably the result of disturbance of the crystal structure of the mineral under the action of the radioactive radiation of uranium and thorium. It is known that minerals containing radioactive elements are more often found in an altered state [5, 20]. E. U. Heinrich [20]

asserts that for orthite three lines are obtained most distinctly: 2.95; 3.51; 2.68. According to O. N. Lozhnikova and S. Ya. Yakovleva [7], for many orthites it is impossible to obtain X-ray patterns even after ignition. The orthite described was also subjected by V. A. Ermolaev to repeated X-ray goniometric studies after ignition at a temperature of 800° for 8 hours. However, satisfactory results were not obtained.

Table 1

The chemical composition of the orthite was determined by I. P. Gorelov in the laboratory of the Department of Inorganic Chemistry of Tomsk State University. The results of the analysis and their recalculation to the crystal-chemical formula according to V. S. Sobolev [17] are summarized in Table 2. For comparison of the chemical composition of orthite from skarns with the composition of orthites from other deposits, Table 3 gives data taken from certain published works [1, 6, 9, 10, 14, 18, 22, 23].

As follows from Table 3, orthites from different rocks are characterized by a rather variable chemical composition, which is in full agreement with the opinions available in the literature.

The orthite described has a high content of such oxides as Al₂O₃, CaO, TiO₂, as well as the alkalis Na₂O and K₂O. The mineral is characterized by a somewhat reduced amount of rare earths. The total content of the latter in orthites from granites and pegmatites is usually co-

varies within the range 14–22%. True, V. V. Lyakhovich [8] indicates broader limits of fluctuation in the content of rare earths in this mineral (2.44–19.22%). In his opinion, the lowest contents of TR₂O₃ determine that the mineral belongs not to orthite proper, but to a transitional variety of the epidote–orthite type. A. I. Bazhenov [1] had arrived at a similar conclusion somewhat earlier.

Table 2

Common divisor: 2546 : 26 = 97,9

Crystallochemical formula:

\[ (\mathrm{Ca}_{3,07}\mathrm{Fe}^{2+}_{0,47}\mathrm{TR}_{0,85}\mathrm{Mn}_{0,02}\mathrm{Mg}_{0,46}\mathrm{Li}_{0,04}\mathrm{Sr}_{0,01}\mathrm{K}_{0,45}\mathrm{Na}_{0,20}) \]

\[ (\mathrm{Fe}_{0,84}\mathrm{Al}_{3,85})[\mathrm{OH}]_{4,33}\mathrm{O}_{0,67}[\mathrm{Ti}_{0,36}\mathrm{Si}_{5,27}\mathrm{O}_{1}]. \]

In terms of Al₂O₃ and CaO content, the Yenisei orthite approaches rare-earth epidotes. At the same time, it differs sharply from the latter in the increased amount of other components, in particular rare earths, uranium, and thorium, as well as in its optical and other properties. The elevated contents of Al₂O₃ and CaO can be explained by the fact that the orthite is in close intergrowth with clinozoisite and epidote and is replaced by them. For this reason, contamination of the orthite sample by the named minerals cannot be excluded.

The contents of SiO₂, Fe₂O₃, MgO, and H₂O do not exceed the limits of their average values in orthites from other rocks. It is quite indicative that the water content correlates well with the graph of the dependence of the specific gravity of orthites on water content. The amount of water, 1.9%, in our orthite corresponds to a mineral variety with a specific gravity of 3.68 from the graph of H. G. Templ [18]. The specific gravity of the orthite described is 3.71. This

Table 3

Chemical analyses of orthites and epidotes from rocks of various composition

Continuation of Table 3

this figure fits entirely within the range of specific-gravity values for orthites, depending on the sum of the rare earths present in them [5, 18]. Worthy of attention is also the fact that strontium was found; in the analyses cited, it is noted only by V. V. Ploshko [14] and D. A. Mineev [10], as well as lithium, an element comparatively rare for orthites.

Orthite, as is known, is a radioactive accessory mineral. Clear radiographs of skarn hand specimens containing orthite were obtained on X-ray film with an exposure of 5–6 days. Microradiographic study of transparent thin sections with orthite, at an exposure of 25 days, reveals a uniform, moderately dense distribution of alpha-particle tracks, indicating that the radioactive element enters isomorphically into the crystal lattice and, consequently, is not a constituent part of a mechanical impurity. Data from colorimetric and pearl-luminescence analyses showed that the radioactivity of our orthite is explained not only by thorium, but also by the presence of uranium. The need to determine uranium in orthites, along with thorium, in geochemical studies has been indicated, in particular, by D. S. Mineev [11]. In his opinion, orthite is a complex mineral with respect to the content not only of rare earths, but also of uranium and thorium. Indeed, analyzing the individual composition of the rare earths (Table 4), it is not difficult to note that, in the mineral described, along with the cerium earths, elements of the yttrium group also play a noticeable role. At the same time, the curve of quantitative ratios of rare-earth elements in orthite (Fig. 4) is characterized by two distinct

Fig. 4. Graph of quantitative ratios of rare-earth elements in orthites from rocks of different genesis:
1 — orthite from skarns, 2 — orthites from granites, 3 — orthites from pegmatites.

maxima, for cerium as well as for erbium (in the yttrium group). It is appropriate to note that, in general, rare-earth minerals having a distinct maximum at erbium are rare. D. A. Mineev, describing rare-earth epidote [10], was the first to point out a distinct erbium maximum.

It is very characteristic that orthites from granites contain a vanishingly low amount of rare earths of the yttrium group (Table 4). In pegmatites their content increases appreciably, and it was precisely in these rocks that דער

Table 4

Composition and content of rare earths in orthites from rocks of different genesis

yttrium-orthite has been found [8]. In orthite from the skarns of the Yenisei Ridge, however, the yttrium group of rare earths plays an even greater role. The absence of analyses of rare earths in orthites from skarns of other regions and from hydrothermal veins does not permit any definite statement on the content of elements of the yttrium group in orthites from these rocks. However, the relatively elevated contents of elements of the yttrium group in pegmatites and in the orthite described here are fully consistent with ideas concerning the great mobility of the elements of the yttrium group and their tendency to accumulate in minerals of the final stage of magmatic evolution. In particular, A. S. Pavlenko et al. [13] point out that in pegmatites and metasomatic formations the yttrium rare earths also enter, in elevated amounts, such minerals specialized in the cerium group as orthite and thorite.

At present, studies by Soviet scientists have established rather clearly the geochemical association of the cerium group of rare earths with thorium, and of the yttrium group with uranium. In minerals possessing a large isomorphic capacity, a closeness of the Ce/Y and Th/U ratios is observed. Using orthite as an example, this was shown quite convincingly by D. A. Mineev [12]. The results of our investigations are in full agreement with these propositions. In the cerium–yttrium orthite from the skarns, the indicated ratios are quite comparable (Table 5).

Table 5

Ratio of cerium rare earths to yttrium rare earths and of thorium to uranium in orthites

NOTE: Data on orthites 2–6 are taken from D. A. Mineev [12].

Yttrium and scandium are in close geochemical kinship with the rare-earth elements. In the present orthite their content is very modest: $Y_2O_3$ — 0.91%; $Sc_2O_3$ less than 1% (of the sum of rare earths).

It is necessary to dwell somewhat more fully on the characterization of the cerium group of rare earths in orthite, since it is, after all, a sharply selective cerium mineral. In particular, it follows from Table 4 that, for orthites of different genesis, the total content of elements of the cerium group, as well as the content of its individual members, undergoes substantial changes. Thus, lanthanum contents range from 13 to 20%, cerium from 32 to 54%, praseodymium from 3.4 to 8%, etc. Neodymium is distinguished by a constant and relatively high content, which was known earlier as well. Therefore A. S. Pavlenko et al. [13], in order to characterize the geochemical features of the rare-earth elements of the cerium group in minerals from rocks of different composition, proposed expressing them in the form of ratios to neodymium. It turned out that these ratios reveal—

Table 6

Relative contents of rare-earth elements in orthites of different genesis

Continuation of Table 6

show a clear dependence on the genetic type of the rock. The ratios of cerium to neodymium and lanthanum to neodymium are especially indicative.

Table 6 gives the relative (to neodymium) contents of elements of the cerium group for 20 samples of orthites of magmatic, pegmatitic, skarn, and hydrothermal genesis, as well as their mean values by groups. From the graph constructed from these data (Fig. 5), the following conclusions may be drawn concerning the distribution of rare earths in orthites of different origin.

1. The Ce/Nd and La/Nd ratios in orthites of magmatic genesis prove to be higher than in orthites from pegmatites; in skarn orthites these ratios increase and exceed those for pegmatitic and even magmatic orthites. In hydrothermal orthites a further significant increase in the ratios occurs.

2. The distinct similarity of the Ce/Nd and La/Nd curves once again confirms the paired character of cerium and lanthanum [15] and, consequently, the geochemical closeness of their migration paths in endogenous processes.

3. The relative contents of praseodymium and samarium in orthites from rocks of different genesis undergo slight fluctuations; moreover, for the former, a noticeable maximum is observed in orthites from pegmatites, whereas the latter under these conditions is characterized by a minimum.

Fig. 5. Character of variation in the contents of rare-earth elements relative to neodymium in orthites of different genesis. I — granites, II — pegmatites, III — skarns, IV — veins of hydrothermal origin.

Conclusion

The study of orthite from the skarns of the Yenisei Ridge makes it possible to draw the following conclusions regarding the conditions of occurrence and the physicochemical characteristics of this mineral.

1. Orthite-bearing skarns are characterized by a garnet–pyroxene composition. Orthite, along with garnet, is one of the earliest minerals in the skarns and is encountered, as a rule, in the endoskarn zone. It occurs in close association with clinozoisite, epidote, diopside, and hornblende.

2. The mineral is characterized by an elevated content of potassium and sodium, as well as by the presence of such elements rare for orthite as lithium, strontium, and scandium. Probably, with an increase in the content of alkali elements—potassium and sodium—the possibility arises for isomorphic entry of lithium and alkaline-earth strontium into the crystal lattice of the mineral described. As for scandium, it is in close geochemical association with the lanthanoids.

3. The rare earths in the orthite described are distinguished by a somewhat reduced content in comparison with their average values for orthi-

from granites and pegmatites. In the composition of the rare earths, along with elements of the cerium group, a noticeable role belongs to elements of the yttrium group. At the same time, the comparability of \(\Sigma TR_{\mathrm{Ce}}/Th\) and \(\Sigma TR_{\mathrm{Ce}}/U\) is confirmed (Table 5).

1. With respect to Ce/Nd and La/Nd, the studied orthites from the skarns of the Yenisei Ridge occupy an intermediate position between orthites of pegmatites and orthites of hydrothermal formations.

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