THE NATURE OF CRYSTAL IMPERFECTION

IV. Plastic Deformations

A. M. KUZMIN

Exogenous phenomena that disturb the primary homogeneity of crystals include plastic and brittle deformations of mineral formations, which in their manifestation under natural conditions are, as a rule, closely connected with slowly and continuously developing tectonic movements that have taken place in the history of the development of the Earth’s crust. Plastic deformations, widely distributed in the realm of crystals, especially attract our attention. They appear when external mechanical forces are applied to them in the form of compression (pressure), tension, and torsion.

The principal form of motion in this case is slip. The latter may be simple slip or hinged slip. The simple type of slip under a given force includes deformations in which one part of the crystal is plastically displaced (slips) relative to another. Such displacements (Fig. 1), as a rule, must be multiples of \(n_1 a_0\), \(n_2 b_0\), \(n_3 c_0\) (\(a_0\), \(b_0\), \(c_0\) are lattice parameters; \(n_1\), \(n_2\), \(n_3\) are integers), and they develop in such a way that physical homogeneity is preserved, i.e., in the deformed crystalline structure near and long-range order is preserved. Thus, in Fig. 1, A, in the direction of the parameter \(b_0\), the displacement of one point of the crystal lattices relative to another by \(n_1 a_0\) parameters is shown; the homogeneity of the crystals in this case remains to a certain extent as before. In the same Fig. 1, B, an example is presented of the slip of one packet of lattices—

Fig. 1. Example of possible slip of blocks in the direction of the parameters \(a_0\), \(b_0\)

...one with respect to the other in the direction \(a_0\) by \(n_2 b_0\) parameters. In any other direction (Fig. 1, B), the applied force simultaneously produces sliding of one part of the crystal relative to the other both in the direction \(a_0\) and in the direction \(b_0\), by \(n_1 a_0\) and \(n_2 b_0\) parameters, without disturbing the overall homogeneity of the deformed crystalline body.

In support of what has just been said one may cite the investigations of I. Ya. Dyadkina [2], from which it follows that large muscovite crystals of the Slyudyanogorsk deposit undergo plastic deformation in the plane of schistosity of the enclosing rocks, as a result of which individual packets, layers, and sheets of this mineral are displaced relative to their neighbors by sliding in the cleavage plane \((001)\) in the direction \([100]\). The very fact of such sliding is established from the presence of relative displacement of individual growth zones in one and the same deformed crystal, when mica plates are split into sheets [2].

In studying the morphology of mica crystals, one can often observe on faces \(\{010\}\), \(\{221\}\), \(\{113\}\), and others sharply expressed steps, protruding thin packets and edges of mica sheets, while measurements of interfacial angles, for example \((001):(010)\), are not equal to \((001):(\bar{0}10)\). Figure 2 shows three crystals on which the angles \(A\) and \(B\), \(V\) and \(G\), \(D\) and \(E\), \(Zh\) and \(Z\), measured with an applied goniometer, are not equal and give full grounds for saying that, in addition to sliding in the plane \((001)\), a certain rotation about \([001]\) also takes place (Fig. 2).

As a result of deformation, the faces \(\{001\}\), \(\{010\}\), \(\{221\}\), \(\{113\}\) prove to be, to varying degrees, wavy and crumpled. On the face \(\{001\}\) one can see a series of deformation lines which, by their position, correspond sometimes to percussion figures, sometimes to pressure figures, and sometimes to both together.

Fig. 2. Sliding phenomena in micas:
\(a\) and \(b\)—microsliding in the plane \((001)\); \(v\)—projection of a crystal onto the plane \((010)\); \(g\)—slidings parallel to the edge \((001)(113)\); \(d\)—slidings and flexural crumpling perpendicular to \(\{221\}\), banding perpendicular to \((010)\).

On the plane \(\{010\}\), fine parallel bands are quite noticeable—microtraces of sliding. In a number of cases, parallel to this direction, mica plates are split by cleavage along \(\{010\}\) into bands of various widths (down to hairlike dimensions). Under deformation in the direction \([010]\), the bands are deformed longitudinally and in the indicated direction, while the plane \((010)\) itself, as a result of sliding, experiences various inclinations to \((001)\). In the direction \([001]\) ...

individual strips elongated along [100] and parallel to (010), relative to their neighbors undergo a flexural bending which, toward the edge of the crystal plate, passes into a narrow slit filled with deformed mica flakes; in the opposite direction it wedges out.

Further, during deformation of mica crystals, cracks and flexure bends, ridges, grooves, and general waviness arise both parallel to {221} [6, 7] and {113} (Fig. 2, c), and perpendicular to {221} (Fig. 2, d).

All the indicated types of deformation usually and predominantly develop in micas perpendicular to the corresponding faces, and the pressure at the same time causes the appearance of new slip planes, which develop now perpendicular to (001), now inclined to it at various angles.

Deformations that are accompanied by slip along 001 are necessarily accompanied by separation of packets and layers from one another and by the appearance, in the cleavage plane, of abundant rounded lenses—voids 5–0.01 mm in diameter. Delamination voids of larger diameter also appear, depending on the character of the deformation. A deformation of similar character in the cleavage plane (001), leading to splitting of the crystal along the cleavage, has been described by the author for gypsum crystals [4, 3]. Under tangential pressure parallel to (001) in the direction [100] and [010], a sound in the form of a creak is clearly heard.

In conclusion concerning the plastic deformations of mica, it should be noted that the published works of N. M. Uspenskii, V. D. Nikitin, I. Ya. Dyadkina, A. S. Nikanorov [2, 7, 5], and the investigations of the author of the present article, testify that physically micas, with respect to their mechanical properties and their ability to exhibit slip in many directions, are still far from being studied. For example, the phenomenon of elasticity of micas is always accompanied by slips along the cleavage plane, but this slip is necessarily accompanied by at least a point delamination of microdeformations in the cleavage plane, which, of course, is intensified during repeated elasticity tests. However, for example, muscovite thereby loses some transparency, and the intensity of the pearly luster on (001) increases.

In particular, for NaCl, KCl, CaCO₃, and many other minerals, slips along {100} and especially along {110} do not occur without trace [3]. At least in the region of slip there appear clearly distinguishable, for example, optical anomalies caused by friction of a large number of thin plates moving relative to one another [3]. The stresses arising in this case can be removed as a result of annealing or as a result of prolonged rest. Optical anomalies in NaCl crystals proved to be removed after two years: deformed cleavage pieces of rock salt became completely isotropic. The same samples of rock salt with anomalous birefringence equal to 0.005–0.007, after two years, showed in an isotropic field the presence of separate regions with weak birefringence no higher than 0.002–0.001.

Simple slips under mechanical force are readily manifested, for example, in graphite along {0001}, hematite {0001}, gypsum along {010}, disthene {100} and {001}, rock salt {110}, {100}, galena {100} and {110}, antimonite {010} and {001}, and in many other minerals.

Talc, when rubbed, breaks up into a large number of parallel {001} plates that easily slide one over another. Graphite and hematite behave in exactly the same way. Crystals of the minerals just mentioned, under directed tangential pressure, resemble a deck of cards.

mica are capable of forming a strip, a ribbon of the corresponding substance, consisting of scales, plates overlapping one another; i.e., plastically, these minerals behave similarly to zinc crystals under deformation, sliding along \(\{0001\}\). In gypsum, slips [4] are distinctly manifested in the cleavage plane \((010)\) in the direction \([001]\) or in \([100]\). In the first case, slip leads to the formation of a finely fibrous structure of the face \((111)\); in the second, on the surface \((100)\), the results of slip in the direction \([100]\) are usually represented by an alternation of narrow, thin, fibrous strips with strips bearing very fine, clear, conchoidal fractures. In disthene, slip occurs along the plane of very perfect cleavage \(\{100\}\); the displacement axis corresponds to \([001]\). During deformation, on the cleavage plane \((001)\) ledges and depressions of various sizes are clearly visible, corresponding to the displacement of individual plates with different amplitudes. On crystals of disthene, displacements of individual thin plates also occur along \(\{001\}\), which leads to differential slips along this plane; as a result, the crystallographic axis \([001]\) appears curved both in the plane parallel to the face \((100)\) and in the face \((010)\).

Exactly the same picture of plastic deformation can be observed on crystals of antimonite \((\mathrm{Sb}_2\mathrm{S}_3)\), which, when subjected to pressure in the direction \([001]\), plastically undergo slip in the cleavage plane \(\{010\}\). On the plane of imperfect cleavage \(\{001\}\), the protruding—shifted—thin plates are separated by shallow

Fig. 3. Photograph of a curved antimonite crystal

gaps. On the surface \(\{010\}\), especially on the cleavage parallel to it, transverse striation is clearly visible, sometimes very fine. In a number of cases this striation has a two-slope character and is more like twinning. Slips parallel to \(\{001\}\) favor bending of the \([001]\) axis and the formation of complexly deformed antimonite crystals (Fig. 3).

Plastically deformed crystals of galena are encountered more often than undeformed ones. Crystals of lead glance are then subjected to bending and twisting, as a result of which the surface of perfect cleavage is deformed to varying degrees, the crystal blocks are plastically deformed, and slips along {100} and {110} develop in the crystal, and deformation twins are formed (Fig. 4).

Thus, in some cases, waves—ridges of small amplitude—are present on the cleavage surface. In other, more frequent cases, galena crystals prove to be more coarsely deformed. Their cleavage planes are bent, complexly curved, without obvious traces of the manifestation of brittle deformation. Examples of such deformation may be found in D. P. Grigor’ev [1]. Here an example of a complexly deformed crystal is given (Fig. 4). In fact, such a picture, proceeding from a study of the nature of such a formation, is explained more simply and more naturally. In the appended Fig. 4 it is clearly seen that the cube surface is cut, parallel to two other cubic faces, by narrow band-lines of varying width, which represent either projections or depressions on the cleavage surface of the crystal. The lines themselves are thin plates of galena which, as a result of pressure, have shifted in one direction or another by \(n \cdot a_0\) (\(a_0 = 5.924\ \text{Å}\) for PbS) lattice parameters.

Fig. 4. Slips in galena along {100} and formation of deformation twins

Study of a fresh cleavage surface with the MIM-8 showed that a galena crystal consists of blocks. The blocks are elongated in one direction, and in each block slips along the cube and bundles of slips along the rhombic dodecahedron are distinctly manifested. The boundaries between the blocks are parallel to cube faces or, more rarely, to rhombic-dodecahedron faces. Relative to one another the blocks are somewhat disoriented and deformed, which is clearly visible when light is reflected from the cleavage plane; such a picture can be seen still better in a goniometer, when a point light signal gives a scattering of light points over the whole field.

Fig. 5. Scheme of slip in rock salt: \(a\)—slips parallel to the face (110); \(b\)—slips parallel to (100); \(v\)—scheme of the sculpture of a cube face formed by slip; \(g\)—scheme of lamellar slip parallel to the face (100), explaining the formation of the sculpture of the face (001) [3]

view. In another plane, parallel to the cleavage along (001), perpendicular to the drawing, the deformed crystal likewise consists of blocks bearing slip traces along the cube and the rhombic dodecahedron. The long axes of the blocks are elongated in one direction.

If such crystals are subjected to deformation, then with the participation of slips the crystallite blocks, initially isometric, are elongated in the direction perpendicular to the pressure. When such block structures are examined in reflected light, it is clearly seen that each galena block has undergone slip along (100) and (110). At the same time, no less clearly, the blocks are squeezed out from the narrow part into the widened part. However, the parallelism of the slip lamellae within a block is well maintained. Individual lamellae, protruding beyond the limits of the block, form its serrated outlines.

The phenomena of slip in crystals of rock salt have been examined in sufficient detail [3]; it has been convincingly shown that slips in crystals develop only along the cleavage plane—a conclusion that can also be drawn from consulting handbooks of mineralogy and crystallochemistry and from the works of E. Schmid and W. Boas [9], and D. P. Grigor’ev [1], which give information on the presence in minerals of slip planes—translations—which, as a rule, coincide with the position in the crystal of a cleavage plane of one quality or another. A. M. Kuz’min, studying the mechanism by which optical anomalies appear in rock salt and observing the manifestation of slips along the cleavage plane, came to the conclusion [3] that slip in NaCl appears more often along the cleavage {110} and more rarely along {100}. This latter feature is explained by the fact that cleavage along {110} is imperfect, and displacements along it by \(na_0\) lattice parameters are accompanied by greater friction; at the same time, the motions occur not only along {110}, but also simultaneously along {100}. Slips along {100} develop more easily, and displacement in this plane by \(na_0\) parameters disturbs the visible homogeneity of the crystal to a lesser degree. In the same work it is indicated that deformation in crystals of rock salt is, in general, of two orders. Traces of first-order deformation are plainly visible to the naked eye (Fig. 6); they appear in the form of wedging block lamellae arranged parallel to the faces of the cube or to one of the planes of the rhombic dodecahedron. Traces of second-order deformation are distinctly manifested in optically anomalous areas of cleaved pieces of salt in the form of thin lamellae \(0.01\)—\(0.001\) mm thick. Systems of such lamellae are arranged parallel to {110} and {100} and often intersect one another (Figs. 5, 6, 7). In rock salt, by deforming it, patterns were obtained analogous to the structure of microcline and to the structure of polysynthetic twins of plagioclases [3].

It was said above that the stress created by slip can be removed in one way or another, whereas the mechanical disruption of the integrity of the NaCl crystal thereby produced by second-order slips remains, as do the first-order disturbances.

Plastic deformations according to the principle of slip along the cleavage surface are expressed quite clearly in mica-like minerals; for example, in the micas themselves, slip in the directions [100] and [010] leads to sufficiently clearly expressed ledges and ribbing on the faces {110}, {221}, and {010}, parallel to the edge (110):(010); separate projections and ribbing are present on (010). The same pattern can be observed on the faces of talc and chlorite crystals.

As for slip according to the hinge principle, it consists in the following. Figure 7 shows a series of schemes of hinge slip arising as a result of pressure. Figure 7,a presents a detail

Fig. 6. Schematic representation of the plastic structure of rock salt as a result of deformation: a—specimen split along cleavage, slip bands along {110}; b—specimen with slip traces both along {110} and along {100}; c, d—deformation by impact on the faces of a cube

Fig. 7. Scheme of hinged slip in minerals of the calcite type

Fig. 8. Scheme of hinged slip in the formation of twins in plagioclases according to the law [001], (010), (001) and (100)

hinge glide. At point \(O\) lies the axis of the hinge, marked by a circle; \(AO\) is the radius of the hinge glide, the projection of the plane of the hinge with individual points of the spatial lattice \(A_1, A_2\ldots\). When the hinge is rotated from position \(OA\) through a certain angle into position \(OA'\), the points \(A_1, A_2\ldots\) will be found at the points \(A'_1, A'_2\ldots\). Since the lattice constants \(OA_1, OA_2,\ldots\) are to a certain extent rigid under the same physical conditions, then when point \(A_1\) is displaced to \(A'_1\) by one parameter, point \(A_2\) is displaced to \(A'_2\) by two parameters, and so on; that is, each point of the hinge describes an arc of the corresponding radius and corresponding length. Naturally, such a displacement of the points of the lattice, which as a whole constitutes a rigid system, leads to its rearrangement as a whole, and the new lattice that forms thereby is a symmetrical construction with respect to the initial one. Such a rearrangement, however, cannot but cause a number of plastic deformations, whose magnitude increases with the increase in the size of the hinge radius, counted from point \(O\) (Fig. 7, \(a\)), and with the length of the path of displacement of the individual lattice nodes. Noticeable plastic deformations usually occur in the boundary region of two adjacent lamellae of a mechanical twin.

Figure 7, \(b\) shows a scheme of single-row glide of molecules; Fig. 7, \(v\) gives a scheme of two-row glide of molecules. In Fig. 7, \(g\), a multi-row type of glide from right to left is presented. The points circled denote the positions of the glide hinges. Figure 7, \(b\) illustrates the formation of mechanical twinning by hinge glide of the calcite type [3]. If the odd parts of the twin \(1, 3, 5, 7\ldots\) are spatially parallel to the initial position of the crystal, then the even parts of the twin \(2, 4, 6, 8\ldots\) have undergone hinge glides, and in the boundary region of the even and odd parts they bear clear traces of the manifestation of pearly luster, indicating the appearance, in the glide plane, of delamination and the formation of voids. The same glide planes with the formation of flat void lenses, as planes of reflection, can be observed within the even parts of mechanical twinning. Taken all together, this indicates that in this type of glide, deformations in the plane \(\{01\overline{1}2\}\) are inevitable.

A fine example of the manifestation of such mechanical twinning is calcite. The most widespread, often polysynthetic, twins for it prove to be those along the plane \((01\overline{1}2)\), less often \((01\overline{1}1)\). Parallel to these directions, even cleavage surfaces can be obtained; a series of mirror-smooth surfaces of varying extent is clearly observed, parallel to \(\{01\overline{1}2\}\) or \(\{01\overline{1}1\}\) in the deformed part of the crystal, indicating the presence in their plane of a certain empty space in the form of very flat lenses and slits [4], which cause the appearance of pearly luster inside the crystal. Such surfaces are essentially nothing other than a manifestation of cleavage or, as it is often called, parting along \(\{01\overline{1}2\}\).

Thus, the hinge method of formation of polysynthetic calcite twins along \(\{01\overline{1}2\}\), arising in its grains during plastic deformations, is in full agreement with Baumhauer’s and Mügge’s experiments on obtaining artificial glide twins. As for the usual interpretations of the formation of calcite twins by rotation of one part of the crystal relative to another through \(180^\circ\), this may be regarded as one of the ways of describing the formation of twins, having no real significance.

The following example of mechanical twinning may be considered the formation of polysynthetic twins of plagioclases—\(\{n\mathrm{CaAl}_2\mathrm{Si}_2\mathrm{O}_8\cdot(100-n)\mathrm{NaAlSi}_3\mathrm{O}_8\}\). The latter are among the principal minerals occurring in igneous rocks. For them, as for feldspars, two series of twins are characteristic. Growth twins, formed by the intergrowth of two individuals or by the fusion into lava of bars or plates of lime-sodium feldspars, are, as a rule, simple and are usually represented by twins on \((001)\), \((010)\), \((021)\), \((02\bar{1})\), and \((100)\). Plagioclases, both in microcrystals and in simple twins, occur extremely rarely. For them, polysynthetic twins are most typical. Polysynthetic twinning is often not only very fine, but also very complex, and its formation cannot be explained by ordinary crystallization. The exceptional occurrence of plagioclases in the form of polysynthetic twins indicates that they are relatively easily deformed under the influence of tectonic pressure, of compression of the rock during cooling, with the formation in them of hinge slip. The polysynthetic twinning that arises in this process is similar in character to twinning in calcite.

1. In fact, the formation of feldspar twins according to the law \([001]\), \((001)\), \((010)\) by hinge slip may be represented in the following way. Imagine a crystal in the form of a bar \(ACDF\), elongated along the axis \([100]\) (Fig. 8-2). Pressure is exerted on the upper half of the crystal. In this case the part \(BCDE\) undergoes hinge slip and takes the position \(BC_1DE\), which, with respect to the part \(ABEF\) that has not undergone slip, will be symmetrical. The hinge plane \(BE\), parallel to \((001)\), is the twin plane*) of the Manebach twin.

2. In the case of an acicular plagioclase crystal \(ACDF\) (Fig. 8-1), pressure applied essentially to its right-hand part—\(BCDE\)—as a result of hinge slip will merely bring the indicated right-hand part into the position \(BC_1D_1E\), which will be the symmetrical reflection of the left-hand part of the twin \(ABEF\). The twin plane is the plane of hinge slip \(BE\), parallel to \((010)\) of plagioclase. In this case we obtain the twin known as the albite twin.

3. Next let us consider the formation of twins according to the Carlsbad law \([001]\). There are two cases. In the plagioclase crystal of Fig. 8-4, let us draw the section \(abcdef\), parallel to \((010)\). Forces acting on the right-hand part of this crystal (Fig. 8-3) are oppositely directed, as shown in the figure; by causing slip in the crystal structure, they will bring this part into the position \(A_1C_1D_1F_1\); the latter will be symmetrical if it is rotated by \(180^\circ\) about the left-hand part of the crystal, and it will take the initial position. The twinning plane is the plane of slip parallel to \((010)\).

In the other case, if the deforming force exerts pressure on the left-hand part \(ABEF\) (Fig. 8-5) of the same crystal \(ACDF\), then the twin plane will prove to be the hinge plane of slip \(BE\), parallel to \((100)\).

A series of the following schematic, specially selected drawings (Fig. 10) of grains of polysynthetic twins of plagioclases gives a clear

*) This plane, as applied to polysynthetic twins, is often called the plane of intergrowth of the twin, thereby including in this concept the possible genesis of polysynthetic twins. The mechanical mode of formation of polysynthetic twins is closely connected with deformation of plagioclases and the phenomenon of hinge slip. Therefore, in the present case the plane separating two neighboring parts of the twin should be regarded, in polysynthetic twin formations of the calcite type, as the plane of hinge slip, or simply as the twin plane.

the notion that they owe their origin to deformation of rocks. Zoned crystals of labradorite, andesine, and oligoclase in the corresponding rocks sometimes do not show polysynthetic twinning. Fragments of zoned plagioclase in tuffs and tuff lavas often prove to lack a polysynthetic structure. Conversely, plagioclases of igneous rocks that have undergone deformation to varying degrees are always polysynthetic. Thus, Fig. 9-1 shows a crystal that has experienced bending, as a result of which a system of glides of twin lamellae parallel to (010) develops in it. The following Fig. 9-2 depicts a plagioclase crystal that was subjected to crushing. In the places where the fragments rubbed against one another, wedging-out polysynthetic twinning appears. In Fig. 9-3 it is clearly seen that a zoned crystal of oligoclase in the upper part was broken out, and its wedge-shaped fragment, as a result of deformation, became finely polysynthetic and, sliding along the fracture surface, caused the appearance of small wedge-shaped bands of twinning in another part of the same fragment. The same picture is shown in Fig. 9-4. Polysynthetic twinning readily develops in plagioclase grains on their projecting parts within individual deformed areas (Fig. 9-5), especially in places where harder parts of the rock exert pressure, for example, quartz grains (Fig. 9-6).

In Fig. 9-7, for a Carlsbad twin with twin plane (010), it is shown that, as a result of pressure, polysynthetic twinning develops with a hinge plane parallel to (010). Fig. 9-11 shows a zoned crystal of andesine which, as a result of deformation, forms a Carlsbad twin with a hinge plane parallel to (100), while in each of its halves, owing to mutually directed stresses, polysynthetic twins with hinge planes parallel to (100) form as wedges interpenetrating one another. In Fig. 9-8 an andesine Manebach twin (010) is presented, in which, as a result of deformation, a polysynthetic structure develops in patches both parallel to (001) and to (100). Finally, Fig. 9-9 depicts a case in which an isometric grain of labradorite plagioclase No. 52, as a result of deformation with bending and twisting, acquires the typical character of polysynthetic twinning with gliding along a plane parallel to (001) and (010), and with the formation of a neutral diagonal band.

Concluding the description of polysynthetic twins as mechanical ones, arising during deformation of rocks, one should dwell on one more interesting phenomenon, which may be regarded as a certain rotational motion of one part of a polysynthetic twin relative to another, in the so-called plane of intergrowth of the twin. Similar cases are often encountered in the study of plagioclase on the Fedorov stage. These phenomena, however, lead to scatter in the measurement data, or to the impossibility of obtaining symmetrical behavior of the twin when the plane of the twin seam is brought into coincidence with the plane of the microscope.

Thus, in the case of the Carlsbad twin in Fig. 10, A, the crystallographic axes $c_1$ and $c_2$ of two adjacent plates of plagioclase are strictly parallel; in Fig. 10, $A_1$ and 10, $A_2$, the axis $c_2$ relative to the axis $c_1$ may be rotated to the right or to the left by some angle. For the Manebach twin in Fig. 10, Б, the crystallographic axes $c_1$ and $c_2$ in two adjacent individuals are arranged strictly parallel, but cases often occur in which the axis $c_2$ relative to $c_1$ may be rotated in the plane 001 to the right or to the left by some angle.

In Fig. 10, В, an albite twin intergrowth is shown in its normal-

position; in Figs. 10, B₁ and 10, B₂ it is shown that one individual is rotated relative to another by a certain angle in the plane (010). In the first case (Fig. 10, B) the optical indicatrices of two adjacent crystals are arranged symmetrically relative to one another, whereas

Fig. 9. Schematic sketches of grains of polysynthetic twins of plagioclases. 1 — andesine-oligoclase, zonal, twinning according to the albite law; 2 — andesine-oligoclase; 3 — andesine-oligoclase; 4 — oligoclase; 5 — andesine; 6 — andesine, oligoclase, almost parallel (100); 8 — andesine, parallel (010); 9 — labradorite No. 52, twins parallel to (001) and (010); 10 — superposition of new, distinct twinning on the former, which is expressed quite noticeably, the boundaries are diffuse; 11 — twin according to (100) of andesine-labradorite; 12 — platy type of polysynthetic albite.

Fig. 10. Examples of possible slip, with rotation in the plane of the twin seam of one individual relative to another, similar to it, through a certain angle. A — a series of Carlsbad twins. Б — Manebach twins and B — albite twins.

in the second the optical indicatrices will be arranged asymmetrically.

Figures 9–12 show a small part of metasomatic albite from pegmatites that underwent deformation and constitute a tectonic breccia cemented by quartz. The large albite crystals also underwent deformation in the form of twisting and compression along (001), acting simultaneously and leading in general to polysynthetic twinning. In addition, the compression was expressed in the formation of small folded forms analogous to those shown in the figure. The anticlinal parts themselves are sometimes smooth, sometimes sharp, with different amplitudes of curvature. Twisting, intensifying slip along the twinning plane, leads to discontinuous dislocations in the form of faults and shifts with an amplitude of no more than 2–5 twin lamellae. Discontinuous dislocations affect the direction transverse and diagonal to the twin banding, or the discontinuous disturbances do not extend beyond the boundaries of the grain and develop within it. Polysyntheti-

plastic twins, judged by the thickness of the twin lamellae, in some cases are well preserved in a thin section throughout the entire grain. In others, polysynthetic lamellae in the “anticlinal” areas become thinner. On passing from one anticline to another, the twin lamella gradually narrows and becomes thinner. In this latter case, the twin lamellae in the anticlines, as shown in Figs. 9–12, sometimes split into 2–3 narrow twin lamellae. In places of displacements one may encounter pinched narrow wedges. Some of them are then subjected to greater deformation, as a result of which twinning appears in them more frequently than in the neighboring blocks. Other wedges, pinched between polysynthetic blocks, show weak twinning.

Thus, the scatter and certain discrepancies obtained as a result of studying polysynthetic twins of plagioclases on the Fedorov stage are a consequence of the deformations to which the plagioclases are subjected. The magnitude of the scatter is, to a certain extent, a measure of the deformation.

Examples of exogenous plastic deformations, accompanied both by simple slip and by slip with a hinge, may be cited from any class of minerals and in large numbers. The examples considered above clearly show that such deformation phenomena are widespread in nature and are extremely varied in their form of manifestation. It is clear to us that the character of deformation of rocks depends on the capacity for plastic deformation of the mineral aggregate itself composing them. It is clear, for example, that the plastic properties of limestone-marble, composed of an aggregate of calcite, are much greater than those of granite or sandstones. It is also clear that, under plastic deformations, crystals lose the primary homogeneity of their crystalline structure; new dislocated areas appear in them. Dislocations, arising in the form of fine disturbances of homogeneity in crystals, especially during metasomatic processes, are pathways for the penetration of solutions into crystals: between the crystal and the solution, mutual reactions of dissolution and replacement arise, with removal of part of the old material and deposition of new material in its place. Plastic deformations of minerals thus create pathways for the penetration of mineralized solutions. For example, fine-grained and dense marbles are not favorable for metasomatic replacement. If, however, marbles, owing to recrystallization, are transformed into coarse- and giant-grained varieties, then such rocks, being plastically deformed, are readily subject to replacement.

In granitoids, orthoclases, plagioclases, hornblendes, micas, and other minerals undergo plastic deformations; along the corresponding slip pathways they readily undergo sericitization, epidotization, muscovitization, biotitization, and chloritization. These metasomatic processes in granitoids are in a number of cases intensified, since plastic deformations are partly replaced by, or accompanied by, variously expressed brittle deformation, which facilitates easier penetration by metamorphosing solutions.

LITERATURE

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1. A. M. Kuzmin. Cleavage and slip planes in rock salt. Geology and Geophysics, No. 6, 1960.

2. A. M. Kuzmin. Dislocations of gypsum. Proceedings of TPI, vol. 127, issue 1, 1964.

3. A. S. Nikanorov. On repeated defects in muscovite crystals. Transactions of the All-Union Mineralogical Society, XC, issue 4, 1961.

4. V. D. Nikitin. Features of the formation of muscovite crystals in pegmatite veins. Transactions of the Leningrad Mining Institute, vol. XXIX, issue 2.

5. V. D. Nikitin. Zoning and microzoning in muscovite. Transactions of the All-Union Mineralogical Society, XXXI, No. 2, 1950.

6. N. M. Uspenskii. On pseudo-cleavage of mica. Transactions of the All-Union Mineralogical Society, XXIX, 1950.

7. E. Schmid, W. Boas. Plasticity of Crystals, Especially Metallic Crystals. Moscow, 1938.