On Dislocations in Magnetite Crystals from the Dashkesan Iron-Ore Deposit and Their Role in Martitization

V. I. Sergeev, A. M. Kuzmin

The theory of dislocations in crystals has now gained such broad recognition and application in various fields of modern science and industrial production that in this article the authors do not consider it necessary to dwell at any length on this phenomenon, all the more since a large number of fundamental studies have been devoted to this question [2]. In the present work it is proposed to consider the linear type of dislocations [3, 4, 5, 8], which arise in minerals in the process of crystallization, recrystallization, and deformation in the form of pores, slip, cracks along the boundaries of blocks and grain contacts, and to examine to what extent such types of dislocations contribute to the process of martitization of magnetite. In order to answer the questions just posed, samples of magnetic ironstone from the Dashkesan deposit, obtained from Professor D. P. Grigor’ev, were studied.

The above-mentioned magnetite samples are parallel-columnar aggregates of the first kind (according to D. P. Grigor’ev), which, in the form of veinlets, cut across skarns [1]. The magnetite crystals from these veinlets have the form of a “dog’s tooth”; their length-to-width ratio varies from 2 : 1 to 6 : 1. The dimensions of the crystals studied themselves range from fractions of a millimeter to several centimeters. The surface of individual grains is intricately curved and is covered with contact striation, which, as a rule, arose at the contact of two or several mutually adjacent, jointly growing crystals. Such striation is the result of complex relations in the growth rates of one or another individual and its surrounding neighbors [9]. The columnar magnetite crystals described are terminated by faces of the rhombododecahedron and octahedron; their internal structure is zonal, sectorial-zonal, or nonzonal, since zonality in these crystals, in connection with subsequent processes of metamorphism and deformation, is often obscured or else appears in the form of relics that are not always clearly expressed.

For this reason, as an example, it is necessary to describe the well-expressed zonality of magnetite crystals in the form (110) from the iron ores¹ of the Taitazhirskoe deposit (Gornaya Shoria). In Fig. 1 there is—

¹ In this deposit, magnetite crystals reach 18–20 cm across and are represented in the form of rhombododecahedra and octahedra.

is a somewhat oblique section to \([001]\); the visible traces of the faces\(^2\) correspond to \((10\bar{1})\), \((011)\), and \((\bar{1}01)\). As can be seen from the figure, the width of the zones varies from 0.005 to 0.1 millimeter. The width of one and the same zone during its development over the same growth period did not remain constant; i.e., faces belonging to one form, depending on the influx of material, shifted parallel to themselves at different rates. Within each zone of the crystal it can be clearly seen that its inner part in reflected light appears dense, whereas the outer part of the zone seems darker, and the density of this coloration gradually increases toward its periphery. This feature of the structure of each zone

Fig. 1. Zoned magnetite crystal in section \([001]\): 1 — dense part of the zone; 2 — porous part of the zone; 3 — cracks within the zone parallel to \((\bar{1}11)\) and \((111)\); 4 — cracks parallel to \((\bar{1}01)\) and \((011)\); 5 — acicular hematite. Taigazhir (Mountain Shoria). Magn. 60.

can be explained by porosity, in which light is partly absorbed and partly scattered. The transition from one zone to another is, as a rule, sharp.

Within one and the same zone of a magnetite crystal there occur systems of cracks parallel to the octahedron faces \((\bar{1}11)\) and partly \((111)\). At high magnification the cracks have a punctate character and correspond, like the porosity mentioned above within the zone, to the emergences of the axes of linear or point dislocations \([4, 8]\), partly caused by the inclusion in the crystal of foreign mechanical impurities. In addition, later in the crystal, as a result of deformation, cracks parallel to \((\bar{1}01)\) and \((011)\) appear; along these cracks acicular hematite penetrates into the mass of magnetite.

The described example of zoned magnetite has analogues also among the Dashkesan magnetites, but in the latter, as noted above, the growth zoning was preserved only in the form of isolated relics among grains that had undergone metamorphism and later were subjected to martitization.

First among the Dashkesan ores we shall consider magnetite, a microphotograph of which is presented in Fig. 2. In the latter, a part of the crystal is photographed whose surface in reflected light appears—

\(^2\) In accordance with the position of the figure relative to the first and second coordinate axes lying in the plane of the photograph.

are somewhat oblique to the \([001]\) section; the visible traces of the faces\(^{2}\) correspond to \((10\bar{1})\), \((011)\), and \((\bar{1}01)\). As can be seen from the figure, the width of the zones varies from 0.005 to 0.1 millimeter. The width of one and the same zone, in the course of its development over one and the same period of growth, did not remain constant; i.e., faces belonging to the same form, depending on the influx of material, moved parallel to themselves at different rates. Within each zone of the crystal it can be clearly seen that its inner part in reflected light appears dense, whereas the outer part of the zone seems darker, and the density of such coloration gradually increases toward its periphery. This feature of the structure of each zone

Fig. 1. Zoned magnetite crystal in section \([001]\): 1 — dense part of the zone; 2 — porous part of the zone; 3 — cracks within the zone parallel to \((\bar{1}11)\) and \((111)\); 4 — cracks parallel to \((\bar{1}01)\) and \((011)\); 5 — acicular hematite. Taigazhir (Mountain Shoria). Magn. 60.

can be explained by porosity, in which the light is partly absorbed and partly scattered. The transition from one zone to another is, as a rule, sharp.

Within one and the same zone of a magnetite crystal there occur systems of cracks parallel to the octahedron faces \((\bar{1}11)\) and, in part, \((111)\). Under high magnification the cracks have a point-like character and correspond, like the porosity mentioned above within the zone, to the exits of axes of linear or point dislocations [4, 8], partly caused by the incorporation into the crystal of foreign mechanical impurities. In addition, later in the crystal, as a result of deformation, cracks parallel to \((\bar{1}01)\) and \((011)\) appear, along which acicular hematite penetrates into the mass of magnetite.

The described example of zoned magnetite has analogues among the Dashkesan magnetites as well, but in the latter, as noted above, the growth zoning was preserved only in the form of separate relicts among grains that had undergone metamorphism and were later subjected to martitization.

As the first of the Dashkesan ores, let us consider magnetite, a microphotograph of which is presented in Fig. 2. The latter shows a portion of a crystal whose surface in reflected light appears—

\(^{2}\) In accordance with the position of the figure with respect to the first and second coordinate axes lying in the plane of the photograph.

Further, the magnetites of Dashkesan also exhibit other structural complications. Thus, in Fig. 3a it is evident that magnetite with a block structure, similar to that in Fig. 2, is intersected in the polished section by two systems of fine cracks—slip planes along \(\{111\}\), I and II—which in this case intersect at an oblique angle. In the photograph one system of parallel cracks II is clearly visible, more or less equally spaced from one another. The other system of parallel lines—I—is distributed unevenly,

Fig. 3a. A magnetite crystal, broken into blocks and partially recrystallized, is affected in two directions by a system of slip cracks along \(\{111\}\). The systems of fine lines I and II are distributed unevenly with respect to one another. Dashkesan. Magn. 36.

as if in separate series, which is clearly emphasized by the photograph itself. At the junctions of lines I and II, the areas appear dark. Fig. 3b represents a detail of the picture

Fig. 3b. Detail of Fig. 3a, illustrating the relationships of slip-line systems I and II with respect to one another. Dashkesan. Magn. 120.

shown in Fig. 3a at a magnification of 120 times\(^{4}\), from which it follows that in some cases the slip lines of systems I and II, extending—

\(^{4}\) Fig. 3b should be rotated \(30^\circ\) to the left relative to Fig. 3a.

over a comparatively great distance, intersecting one another; in others, the lines of system I act as though they were an obstacle to the development of individual slip lines of system II and, conversely, lines I readily develop in the narrow intervals bounded by lines II. A similar picture can often be observed in the formation, as a result of deformation, of polysynthetic twins in sphalerites, plagioclases, calcites, rock salt, and other minerals.

As for the thickness of the lines of directions I and II, it is measured within the limits of 0.004—0.002—0.001 mm and less; the spacing of one line from another is rarely 0.1 mm, more often within the limits of 0.05—0.01 mm, and quite often they are located no closer than 0.008—0.005 mm from one another.

It should finally be noted that a series of parallel lines readily affects the dense part of magnetite blocks. The relict part of the block is cut by slip planes less readily. An example of such a fact is the photograph of a detail of a magnetite crystal shown in Fig. 4. In the case of penetration of the slip plane into the relict part of the block, there usually also occurs a densification of the magnetite within narrow limits along the slip plane.

Fig. 4. Magnetite crystal broken into blocks: 1 — relict magnetite; 2 — cracks separating one block from another; 3 — dense magnetite surrounding the relict part as a rim; 4 — slip traces developing in the dense part of adjacent blocks. Dashkesan. Magnification 260.

Further study of magnetite crystals at magnifications of 300—500 times helps reveal new details. Thus, the broad development in the crystal of slips in two directions leads to the fact that its mass in the section of a polished thin section, during etching, is broken up into rectangles (Fig. 5a). In Fig. 5a it is evident that the crystal is divided by frequent parallel lines into narrow strips whose width varies from 0.001 to 0.01—0.03 mm. In the polished thin section, as a result of etching, an oblique-angled network of slip lines is revealed, which divides the crystal into blocks. Within such blocks, relicts of magnetite are preserved, surrounded by a narrow rim of its dense variety. In the narrowest parallelepipeds the relicts of magnetic iron ore, as a result of dynamometamorphism, are completely recrystallized. In the polished thin section of Fig. 5b, relicts of primary magnetite are already encountered much more rarely. In Fig. 5b two systems of parallel lines are also noticeable, which in the ideal

in this case must intersect at a right angle. In some of them, relics of the primary magnetite have still been preserved.

The study of polished sections, individual areas of which are shown in the photographs (Figs. 5a; 5b), makes it possible to reveal a number of details that help in understanding the essence of the observed picture. Thus, Fig. 5b and other photographs clearly indicate that we are dealing with crystals of the cubic system, while the lines themselves, which are clearly visible in Figs. 3 and 4, are slip lines along octahedral planes.

Fig. 5a. Detail. A magnetite crystal is divided by two systems of parallel cracks, cutting out parallelepipedal blocks with remnants of relict magnetite in them. Dashkesan. Magnification 400.

In Fig. 5b it is clearly noticeable that the slips in the micro-areas often have a stepped and ladder-like character.

Fig. 5b. Detail. Parallelepipedal blocks in a magnetite crystal, having a clearly rectangular character. Dashkesan. Magnification 400.

Further, let us emphasize once more that slips develop more readily in the dense part of the magnetite, and the uniformity of the arrangement of the slip lines indicates that we are indeed dealing with a single crystal. Finally, the study of the character

the slip lines themselves at magnifications of 700–800 times indicates that in a number of cases they have a punctate structure (Figs. 5a; 7a; 7b), the individual points of which, 0.002–0.001 mm and less in diameter, when close together appear as continuous lines (Figs. 2 and 3).

We shall conclude the description of magnetites proper from the Dashkesan deposit with a brief consideration of its zoned crystals. Thus Fig. 6a illustrates the character of the zonal structure of magnetite and the relationships of growth structures with dislocations. In Fig. 6a the numbers 1, 2, 3, and 4 denote zones of different widths: the first (1)—0.5 mm wide—has a very fine-banded structure, distinguishable only at high magnifications; the second (2)—0.1 mm—is highly porous, with chaotically arranged black points (voids); the next, third zone (3), is fine-banded, 0.6 mm wide; the fourth (4)—approximately 0.35 mm—is again porous and is partly affected by the development of a later metamorphic blocky structure. During subsequent metamorphism, the zoned crystal breaks up into blocks of the first order, like those described in Fig. 2, within whose mass fragments (relicts) of zoned magnetite are preserved. Then the next stage of metamorphism develops (Fig. 6b), closely connected with plastic deformation. The latter, in the form of slip lines along {111}, breaks the crystal into blocks—parallelepipeds of the second order, within which, at the expense of relicts of magnetic iron ore, the zone of dense magnetite expands. Thus the zonal structure of magnetite grains is destroyed through the subsequent formation of new structures.

Fig. 6a. Relicts of the zonal structure of magnetite. Relicts of growth zones—1, 2, 3, 4, and 5—preserved within the field of the later development of a metamorphic blocky structure. Dashkesan. Magn. 200.

The preceding pages were devoted entirely to consideration of the microstructure of magnetite, which, in the form of veinlets, cuts the skarn in the Dashkesan deposit. Our investigations of this mineral began from a certain moment in its existence, and we attempted, to the extent possible, to reconstruct the history of its life, during which it underwent slowly but inexorably developing transformations. The investigations find the crystal at one of the stages of such transformation: as a result of metamorphism, zoned magnetite crystals break up into blocks of the first order, separated from one another by cracks. Within each block, to one degree or another,

Fig. 6b. Detail along the extent of the relict zone shown in Fig. 6a. Relicts of the zonal structure of a crystal that first acquired a blocky structure and subsequently underwent slipping along lines parallel to {111}. 6—relict magnetite; 7—dense magnetite; 8—hematite. Dashkesan. Magn. 200.

a relict of primary magnetic iron ore is preserved, surrounded by a shell of dense magnetite of varying thickness. Subsequent plastic deformations cause slip along \(\{111\}\) and the formation of second-order blocks in the form of parallelepipeds, in which the dense part continues to develop at the expense of the relicts of primary magnetite until their complete transformation. Subsequent, especially brittle, deformations promote the appearance of variously expressed shattering cracks.

From what has been said above, the question naturally arises of how the process of martitization develops in such formations. Apparently, the dislocations of crystals, which are revealed as a result of etching as pores and cracks of various origin arising in the mineral as a result of metamorphism and deformation, are pathways for the penetration of oxygen into the body of the magnetite grain.

Indeed, hematite and hydrohematite develop first of all, in the form of a fine-grained and dense aggregate, in the intervals separating one magnetite grain from another. Then fine needles of hematite appear along individual small cracks dividing the crystal blocks. In this process, as a rule, acicular crystals of hematite are formed\(^{5}\), which, in their relative position in magnetite, are strictly governed by its structural features. In sections parallel to \(\{100\}\) and \(\{111\}\), the needles are arranged parallel to the cube edges.

At first, individual hematite needles appear in magnetite, their number increasing toward the periphery of the grains or along cracks cutting the crystal. Increasing in number and in size, the hematite needles form elegant framework structures of varying density and complexity.

In Fig. 7a the magnetite crystal consists of relict 1, preserved among its dense variety 2, which is cut by a large number of cracks filled with a dense aggregate of hematite 3. In addition,

Fig. 7a. Martitization of magnetite. Primary zoned crystal. 1 — relicts of primary magnetite; 2 — zone of dense magnetite; 3 — hematite; 4 — distinctly acicular magnetite. Dashkesan. Magnification 400×.

\(^{5}\) In all sections, hematite crystals in magnetite develop only as acicular forms. Such crystals in polarized light are anisotropic and possess a clearly expressed reflection characteristic of hematite. Hematite is not etched by hydrofluoric acid and, in color, differs sharply from magnetite, which is clearly visible in the photographs.

...then, around the relict part of the magnetite, grains of hematite or its granular aggregate appear in the form of a narrow rim, almost entirely replacing individual relicts (Fig. 7a). In the right-hand part, acicular hematite 4 is clearly outlined; in relief, color, and morphology it differs both from dense magnetite 2 and from dense hematite 3. In Fig. 7b, essentially, we have the same picture, with a clearer manifestation of acicular hematite 4. Here it is plainly visible that the acicular hematite, mutually intersecting, is spatially oriented in a definite way, as required by the structural features of magnetite.

In this case (Figs. 7a and 7b), and in a number of other polished sections, at high magnifications one could observe, along the periphery of the relict areas of magnetite, the appearance of parallel-oriented needles or dots¹ of hematite, or of both together. It is also not uncommon to observe the development of hematite along slip planes, where it more often appears in the form of dashed bands; their convergence led to the formation of continuous hematite lines. Dotted forms of hematite appear along the outer edge of relict magnetite or in its place within the aggregate in separate areas along cracks bounding blocks of the second order, and then, as the oxidation process developed, the number of dots and bands increased, up to the formation of individual hematite dotted lines, which later merged into a continuous hematite line.

Fig. 7b. Martitization of magnetite. 1 — relicts of primary magnetite; 2 — dense magnetite; 3 — hematite; 4 — clearly acicular hematite. Dashkesan. Magnification 400.

The examples given of martitization of magnetite from the Dashkesan deposit may suffice.

In accordance with the indications of L. N. Ovchinnikov [6], N. V. Pavlova [7], V. G. Neuhaus [10], and other investigators, an attempt was made to establish the presence of maghemite in the ores studied. However, despite careful searches, we were unable to find the mineral just mentioned. Its presence was also not confirmed by two X-ray analyses. The data of one of them corresponded exactly to magnetite,

whereas the data of another are: 2—3.025; 4—2.965; 1—2.790; 7—2.581; 10—2.522; 1—2.137; 5—2.097; 2—1.709, 2—(1.631); 6—1.614; 8—1.485; 1—1.279; 1—(1.208)⁷; 1—1.122; 3—1.093; 1—1.048; 4—0.969 β, which rather indicates a mixture of magnetite and hematite in the sample, in good agreement with the polished-section data.

In conclusion, we note that the method of etching magnetite ores with hydrofluoric acid gives researchers the possibility of clearly visualizing the internal morphology of the successive development of mineral grains and of solving questions of their metamorphism, both the recrystallization of the magnetite grains themselves and the reconstruction of the sequence of development of the martitization process.

In the cases described, the etching method makes it possible to observe such fine details of grain structure as the zonality of crystals, the inhomogeneity of zones, fine porosity, the block structure of grains, and the extensive development of slip planes parallel to the faces \(\{111\}\). All the features just indicated—dislocations in magnetite crystals, disrupting its homogeneity—are at the same time pathways that favor the development of martitization of magnetite.

REFERENCES

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7. N. V. Pavlov. Magno-magnetite deposits of the Tunguska syneclise region of the Siberian Platform. Proceedings of the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, issue 52, pp. 3–222, 1961.

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10. W. H. Newhouse and P. L. Glass. Some physical properties of certain iron oxides. Economic Geology, vol. 31, no. 7, pp. 699–711, 1936.

1. Sections perpendicular to the needle. ↩

2. In the series of numbers given, the first digit characterizes the intensity \((I)\); the second number, after the dash, in Å determines the position in the powder pattern. The lines given in parentheses correspond to \(\beta\)-radiation lines. ↩