THE NATURE OF CRYSTAL IMPERFECTION

III. Epigenetic Dislocations in Metamict Transformations and the Decomposition of Solid Solutions

A. M. KUZMIN

Polymorphic transformations of one modification into another usually develop, in some cases, relatively rapidly—for example, transformations of sulfur; in others, relatively slowly, and the course of the phase transformation requires a long time and is accompanied by the appearance of an intermediate state.

In an article on alkali feldspars, A. M. Kuzmin wrote that in polymorphic transformations a case is possible in which a crystal \((K_1)\), on the path of its transformation into \(K_2\), finds itself in a metamict state \((M)\), representing a fine mixture of chaotically arranged fragments of the crystal lattice with high internal friction, characteristic of amorphous bodies [4]. Such a metamict state is especially characteristic of titanoniobo-tantalates containing uranium, thorium, and rare earths. Usually, polymorphic transformations of phase \(K_1\) into phase \(K_2\) take place, under the appropriate physicochemical conditions, as is believed, without intermediate formation, as, for example, \(\alpha S \leftrightarrow \beta S\). However, in a number of cases the substance of phase \(K_1\) is not immediately and completely replaced by phase \(K_2\); between these extreme members of the polymorphic series there may be intermediate states that are, as it were, a mixture of both phases. If \(\beta S\), located in the interval \(112—100^\circ\), is rapidly cooled to zero degrees, then such sulfur will assume an amorphous character, which will then slowly transform into \(\alpha S\). White tin of the tetragonal syngony begins, at about \(13^\circ\), to transform into gray cubic tin: \(\beta \mathrm{Sn} \to \alpha \mathrm{Sn}\); both of its modifications are present in it and constitute a physical mixture, which, as the temperature is lowered, tends to become single-phase.

However, among minerals there is a number of compounds—for example, titanoniobo-tantalates—whose complex chemical composition includes thorium, uranium, and rare earths. Compounds of this series are not infrequently encountered as fairly well-expressed crystals of cubic, tetragonal, rhombic, and monoclinic syngony; but at the same time such compounds in the solid phase are quite often in a metamict state, which V. Bregger, and after him V. Goldschmidt, regard as glassy, as an amorphous state of the given substance. Owing to this feature, the indicated crystals acquire, in fracture, a smooth and conchoidal surface, a dark-brown and black color, and a resinous luster; anisotropic ones become optically isotropic; transparency is lost; density and hardness decrease; somewhat increas-

the volume increases (for example, in pyrochlores the faces of the octahedron become convex) and cleavage is lost. The just-mentioned types of changes in titano-tantaloniobates, when they pass from the crystalline into metamict states, are accompanied by a series of successive disturbances of the crystalline structure, with the formation of a sufficiently porous mass capable of absorbing water.

Next let us note one more feature of minerals in the metamict state. When heated within the range 500–860°C, as appropriate for each mineral variety in accordance with its characteristic composition, the minerals in general readily return to the initial state, undergoing recrystallization in the process. In this last phenomenon we obtain a clear indication that titano-niobo-tantalates, zircon, thorite, and other minerals, having formed under one set of conditions and then found themselves under another, have a tendency toward a polymorphic transformation, and on the way to this transformation form a metamict body, i.e., a finely dispersed aggregate of microcrystallites of a new phase¹, which, owing to great friction, cannot acquire the corresponding orientation of the crystal of the new phase. Thus, in the metamict ($M$) state in the system of polymorphic transformation $K_1 \to M \to K_2$ there is a case in which $M$ is an intermediate formation, in which the body is retained for an indefinitely long time [4]. However, upon heating, the body from the metamict state readily returns to the initial, already deformed to a certain extent, crystalline state $K_1$, whereas its second polymorphic phase—$K_2$—still remains unknown. The history of the study of the metamict state of a definite group of minerals clearly shows that the homogeneity of the structure of their crystals is disrupted along the entire path of the phase transformation, up to the formation of an amorphous state of the body.

Among endogenous dislocations of crystals associated with epigenetic processes one should also include the decomposition of isomorphous mixtures, a phenomenon that is above all connected with those contradictions which arise in a solid solution, when the temperature is lowered, between the components composing it. Thus, sphalerite ZnS is often described as a mineral containing small emulsion inclusions of chalcopyrite or stannite, or of both minerals together; or it is stated that stannite $\mathrm{Cu_2SnFeS_4}$ is intergrown with chalcopyrite, which apparently represents a product of the decomposition of a solid solution. The following data (Table 1) for sphalerite, chalcopyrite, and stannite show that the lattice constants in $a_0$ for the three named minerals are very close; in the [001] direction the lattice of chalcopyrite is analogous to sphalerite, but is doubled and somewhat compressed in $c_0$. The same remarks also apply to stannite.

Table 1

To the constants given in Table 1, as applicable to 25°C, let us add the indication that the angles of the tetrahedra $(111) : (111)$ of chalcopyrite and stannite,

respectively equal to \(71^\circ 22'\) and \(70^\circ 0.75'\), prove to be close to the angle between the same faces of sphalerite, \(70^\circ 30'\). Within the range \(400\)—\(150^\circ\), under the conditions of a hydrothermal process, the lattices of chalcopyrite and stannine can become not only close to sphalerite, but may even become cubic [9, 10]. Therefore, in the crystal structure they may occur together in the form of isostructural formations.

In the hydrothermal stage, on the surface of the forming sphalerite crystal during its growth, owing to isostructurality, chalcopyrite or chalcopyrite with stannine may be deposited in individual nodes as a continuous or discontinuous zone, if the material for the formation of the indicated minerals is present in the solution that ensures the growth of zinc blende. With a decrease in temperature, contradictory relations arise between the lattices of sphalerite and the chalcopyrite and stannine isostructural with it; as a result of such circumstances, the initially physically homogeneous solid solution becomes inhomogeneous under the new physico-chemical conditions. A similar picture can often be observed in chalcopyrite crystals, in which regularly included grains or strips of sphalerite and stannine are frequently encountered.

In the cubic variety of stannine—isostannine—at high temperatures the substance of sphalerite, or especially chalcopyrite, may be contained from traces up to relatively significant amounts (A. G. Betekhtin); with decreasing temperature the solid solution decomposes with the isolation of chalcopyrite and sphalerite. Therefore it is natural to expect that at the stannine–chalcopyrite, stannine–sphalerite, and chalcopyrite–stannine boundaries, deformations will arise due to compression of the substance of the individual components of the decomposing solid solution into blocks. Within the host and within the guest, and at their boundaries, voids, cracks, and slip planes will arise, leading to the formation of a twinned block structure.

It follows from what has been said that the zones in sphalerite corresponding to chalcopyrite and stannine; in chalcopyrite—the included grains of sphalerite and stannine; and in stannine—of sphalerite and chalcopyrite, were originally, as guests, deposited from time to time, owing to isostructurality, on the surface of the forming crystals of the host and, to some degree, grew. Later, homogeneous crystal structures, because of internal contradictions and decomposition of the solid solution, became inhomogeneous.

Another example illustrating the decomposition of a solid solution, accompanied by the appearance of new linear dislocations, is provided by the titanomagnetites, which are capable of forming a solid solution. According to P. Ramdohr, decomposition proceeds within the range \(700\)—\(450^\circ\); above this temperature the solid solution is homogeneous, while below it its constituent components are readily distinguishable.

Table 2

Ilmenite, depending on its amount, upon decomposition separates out in magnetite in the form of plates, rounded and drop-like, and finely reticulate formations which, as a rule, in their arrangement are oriented symmetrically with respect to the third-order axis, i.e., with respect to the octahedron of magnetite; ilmenite as a decomposition product is oriented by the plane (0001) parallel to the octahedral faces.

As in the case of the “decomposition” of the solid solution sphalerite—chalcopyrite—stannine, so also in the case of magnetite—ilmenite, we are inclined to regard these two minerals as isostructural at temperatures above 450°. In this case, the isostructural ilmenite and magnetite can form a homogeneous solid solution in which, on the growing crystals—for example, of magnetite octahedron—there are from time to time superposed and growing, depending on the amount of titanium in the solution, plates of ilmenite, drop-like grains, and isometric crystals of it. In the case where the mass of ilmenite predominates over magnetite, the reverse picture occurs, since the isostructurality of these two minerals remains valid.

The geometrical constants of ilmenite (Table 2) may be calculated from the lattice constant of magnetite, namely: they will respectively be equal to 5.078 Å and 14.48 Å. These figures are very close to the geometrical constants of ilmenite, i.e., at the stage of formation of the solid solution, magnetite and ilmenite do indeed behave isostructurally, and the structural contradictions in the lattices of these two compounds appear below 450° in the form of regularly constructed poikilitic intergrowths in magnetite crystals.

To this same group of phenomena should be assigned solid solutions of substances corresponding in composition to hematite—Fe₂O₃—and ilmenite—FeTiO₃—which, it is believed, form a continuous isomorphous series; among them, the greatest significance is possessed by those solid solutions in which the ilmenite component predominates over the hematite component. According to P. Ramdohr, at high temperatures FeTiO₃ and Fe₂O₃ can mix in any proportions. With decreasing temperature, miscibility gaps occur, leading to decomposition of the solid solution. In this case the phenomenon of isostructurality occurs. According to P. Ramdohr, ilmenite-hematite solid solutions with a low content of ferric oxide often possess a zonal structure. This latter indicates that those distributions of hematite in ilmenite which can be observed under the microscope were initially formed isostructurally in the form of layers, lenses, spots, and nodes of various shapes on the surface of the ilmenite crystal during its crystallization. At this stage the solid solution is homogeneous; with decreasing temperature, internal structural contradictions lead to the solution becoming heterogeneous: the boundaries between the ilmenite and the hematite included in it become sharply delineated.

Many examples of the decomposition of solid substances can be cited, but there are cases which, although considered to be decomposition, in fact are not such. These should first of all include perthitic formations of orthoclase with a comparatively small content of albite. In this case, most likely, the visible albite is a product of metasomatism. The sodium content in orthoclases, rather, indicates isomorphism of the substance of orthoclase and albite, which is not accompanied by decomposition. The large number of available analyses of orthoclase also argues in favor of the consideration expressed here.

Thus, alongside defects in crystals that arose in them during the period of their formation, the phenomena of polymorphism and the phenomena of decomposition of solid substances contribute to the appearance in them of new defects, which play

play a considerable role in the processes of physical destruction, chemical weathering, and metasomatic transformations and replacement of minerals in their mineral associations.

REFERENCES

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1. In a mass in which fragments of the original phase can and do remain. ↩