FORMATION OF PORES IN CRYSTALS

A. M. Kuz’min

Whatever methods are used to obtain crystals, however slowly one tries to grow them in the hope of obtaining the most perfect polyhedra, the crystals obtained are, to one degree or another, porous. Natural crystals—whether their origin is due to magmatic melts, gaseous and hydrothermal processes, or whether they are formed under conditions in the near-surface part of the earth’s crust—and in our laboratories, always contain some quantity of pores.

The pores present in crystals always prove to be filled with gaseous, gas-water, and aqueous products. Such inclusions in crystals are partly preserved and partly evaporate, leaving voids in their place. In some cases pores arise simultaneously with the nascent crystal—syngenetic (primary) pores; in others, pores are created in an already finished crystal some time after its formation, as epigenetic (secondary) formations.

The methods of decrepitation and homogenization are based on Sorby’s proposition that gaseous, gas-like, and liquid inclusions are captured by a crystal during its growth under definite physicochemical conditions of temperature and pressure, and that the chambers are impermeable to gaseous and liquid inclusions. These methods, in essence, make it possible to solve to some extent the inverse problem: to determine those physicochemical conditions under which the crystal captured gas-water products. Hence, naturally, the investigator cannot be indifferent to the nature of pore formation and to their position in the crystal; this should accordingly help the investigator determine whether the pores are, by origin, syngenetic (primary) or epigenetic (secondary).

The doctrine of the porosity of crystals should be based on Academician A. V. Shubnikov’s statement (1935) that the degree of perfection of crystals is determined entirely by the size of the aggregates—molecules (submicrons, crystallites)—which, differing from one another in their dimensions by even a very small amount and being deposited not especially strictly parallel, cannot form an ideally even layer even on the initial flat face and cannot pack densely upon it. The next layer, likewise composed of “submicrons” not strictly identical in size, is formed on the preceding layer, which bears micro-roughnesses, and itself

turns out to be still more uneven. Defects in the packing of such submicrons on the face will be projections and depressions of various sizes, cracks and labyrinths of channels and voids on the forming face, not occupied by crystallites, equal to 1–2–4 and more molecules—submicrons. What has been said is confirmed quite well (Fig. 1).

In connection with what has just been said, one may cite examples from experimental work on the crystallization of alums and of sodium and potassium chloride crystals. In the first case, prof. A. V. Lavrskii and I obtained crystals of potassium and ammonium alum in the presence of a 5% solution of borax in the form of cubes, up to one cubic centimeter in size.

Fig. 1. Pores and channels visible under a microscope on the surface of a sodium chloride crystal. Magnification 120X

A feature of these crystals was that they were built up of cubical crystallites 0.05 to 0.1 mm in size. The character of the packing of the cubical crystallites in the crystals obtained vividly confirms A. V. Shubnikov’s idea of the structure and growth of crystals (Kuz’min, 1969). The block (mosaic) structure of such alum crystals, the absence of close and strict parallelism in the packing of the crystallites, the presence of depressions, cracks, channels, and volumetric voids in every deposited layer—all this is so obvious that it requires no more detailed description of the fact itself (Kuz’min, 1969).

As for sodium chloride and potassium chloride crystals, they, it must be said, provide abundant material for judging the nature of porosity. The faces of prismatic crystals¹ and the surfaces of their end parts have a complex sculptural structure. The latter is due to the deposition on their surface of a large number of flattened cubic crystallites, in the packing of which there is no strict parallelism, and the crystals themselves are of unequal size and height. Moreover, relatively large crystals, when examined under a microscope, turn out to be intergrowths of 2, 3, 4, and more very small crystallites, which, when they are packed onto a new surface, project in the intergrowth sometimes with their vertices, sometimes with their edges; sometimes one or another cubical crystal-crystallite rises somewhat above its neighbors, or, in comparison with them,

lags behind in growth and in this case forms a depression equal to 1–2–3 crystallites. The nonuniform and discontinuous coating of the crystal surface of the halides studied leads to the appearance on their surface of a large number, varying in size and shape, of protrusions and depressions, fissures and a labyrinth of channels.

As can be seen under the microscope, oval, round, and square voids—depressions—arise at the boundary of two, three, or four intergrown crystallites, filled with gaseous products from the mother liquor. At the point of contact of 4 crystallites one can often encounter a void which, at high magnifications, has the character of a four-sided pyramid with its apex directed downward. Oval and round voids often constitute a channel of square cross section going into the depth of the crystal. Channels of rectangular cross section, going into the depth, either taper to a wedge or widen and branch, forming a complex labyrinth with blind alleys and intricate passages.

In studying pores that appear rounded, it is clearly seen that pores on the surface of a face behave in two ways with depth: some of them taper with depth or end bluntly, while others extend inward, but at the same time at various levels there appear their own pores, not reaching the surface, characteristic of one or another level.

At the next stage of growth the just-described features of the face, with its complex sculpturing, will be covered by the next new layer of salt. In this process the defects are partially healed, and at the same time the preceding pores, together with the mother liquor contained in them, are trapped; the newly formed faces will bear their own sculptural features and their own defects.

As an illustration of the role of the porosity of a crystal during its weathering, the following example may be given. In the museum of the Department of Mineralogy of TPI there was a large crystal of chrome alum, \(KCr(SO_4)_2 \times 12H_2O\), in the form of an octahedron with an edge of 12 cm. On the faces of the crystal there were quite many defects in the form of very narrow short fissures, visible in a \(10\times\) magnifier, and pores; the octahedral faces themselves were not ideally smooth. With the passage of time, in the places of the defects there appeared a small number (about 30) of efflorescences 2–5 mm in size, with a clearly concentric-radiating structure, light violet in color, corresponding to alum with a smaller amount of crystallization water. The efflorescences on the surface of the crystal slowly increased in size. New efflorescences appeared. However, the surface of the crystal as a whole remained the same as it had been for 40 years. When it was being demonstrated to students, with a light blow it crumbled into a powder of light violet color, in which there were fragments of black-violet alum. It is clear that the evaporation of part of the crystallization water occurred through the channels present in the crystal and communicating with the surface.

In addition to what has just been said, let us note that in cleavage pieces of rock-salt crystals, pores and channels that might be filled with liquid products are not visible under the microscope in transmitted light. But if such a piece is placed with one face on a filter-paper sheet moistened with water and then the wetted surface is again examined under the microscope, a comparatively large number of pores and channels parallel to \(\{100\}\), filled with liquid, will be visible. By raising and lowering the microscope tube, one can clearly convince oneself that the pores—negative crystals—are located at different levels. This fact may indicate that the pores present in crystals can communicate with one another through a system of very fine channels. In deformed crystals of sodium chloride, the loss of gaseous inclusions could be facilitated by the cleavage system itself along \(\{100\}\).

Pores in crystals are arranged regularly in connection with their growth. Thus, crystals of sodium and potassium halides often turn out to be zonal. The number of zones in them can be large. In crystals 5–4 mm thick, sometimes up to 40 zones were counted, spaced from one another by 0.01–0.001 mm. The zones themselves are emphasized by the presence in the crystal of pores arranged parallel to the growing faces. The pores have the character of negative crystals. Their dimensions vary over wide limits, from 0.008–0.0005 mm and smaller in cross section and from 0.001–0.004 to 0.008–0.02 mm in length, sometimes even up to 0.5–1 mm.

Only the pores indicated, initially filled with mother liquor, are those sculptural defects of the face that are represented in Fig. 1. The number of pores along the zones is determined by the dimensions of the negative crystals. In their arrangement along the plane of a zone one may distinguish: 1) closely spaced point-like ones, which at low magnifications appear to be a continuous line; at magnifications of 200× the line breaks up into separate scattered point defects separated from one another by an interval; 2) point-like ones, distinguishable at magnifications of 50–60×; 3) closely dashed, dashed, and sparsely dashed; 4) “tubular” ones. In addition, parallel to the faces of the crystal there are cavities in it that are described as swirls, lenses, and loops. Many similar examples could be cited from the mineral kingdom.

The porosity of minerals can to a certain extent be judged from their specific gravity. This is especially characteristic of experimentally obtained halide crystals. Thus, for sodium chloride, instead of 2.168 g/cm³, the density decreased to 2.0–1.89 g/cm³; for potassium chloride, instead of 1.99, it was determined as 1.90–1.8 g/cm³; and for potassium bromide, 2.5 g/cm³ instead of 2.75. Halides with reduced specific gravity contained abundant liquid inclusions. After some time had elapsed, the liquid evaporated and the crystals acquired a clearly expressed pearly luster, indicating the porosity of the crystals.

Up to this point the discussion has concerned the porosity of crystals that forms in them during the period of their growth, i.e., pores that the author assigns to syngenetic ones. Another group of pores, in relation to crystals, is later—epigenetic—arising in them as a result of various causes that bring about the appearance of deformations in the crystal and various defects in them. Epigenetic (secondary) defects arise as a result of polymorphic transformations, plastic deformations in minerals and rocks with the participation of tectonic processes as a result of compression and tension. Pores in minerals may arise as a result of chemical and physicochemical processes.

Defects of crystals arising as a result of polymorphic transformations are closely connected with the rearrangement of their internal structure. This character of polymorphic transformations is correspondingly reflected in the external form of the crystals, in its primary integrity, in the internal structure of the crystal, in the geometrical constants of the lattice, in its density, and in a number of other physical properties.

As examples of the formation of defects as a result of polymorphic transformations, let us cite the polymorphic series of silica; from it we shall consider the phase transformation of β-quartz (870–573°) into α-quartz (below 573°) with an inversion temperature of 573°. In this transformation the volume of quartz decreases by a minimum of 5.5%; the ratio of its axes $c_0 : a_0$ for the former is 1.0934, and for the latter 1.100. The compression of the crystal during this phase transition is accompanied by compression along the $a_0$ axis by 0.0854 Å and by shortening of the $c_0 = 0.0517$ Å axis, i.e., the compression of the volume occurs nonuniformly. The density characteristic of β-quartz, equal to 2.51, increases for α-quartz to 2.655 g/cm³.

From the example given, it is clear that during phase transformations from one modification to another a crystal undergoes, to varying degrees, deformation due to its volumetric expansion or contraction. In the rate of their development, these latter phenomena greatly outpace the rate of the process of lattice rearrangement, which in crystals begins at many points and most readily at points of various kinds of defects. This nonuniform compression of quartz during its polymorphic transformation from the $\beta$ modification to the $\alpha$ variety gives the primary quartz a microblock structure, which under crossed nicols causes the appearance in it of wavy extinction. The boundaries of the blocks and the healed cracks in quartz are the sites where micropores appear, with gas–liquid inclusions in them. Under these conditions the primary gas–liquid inclusions are scarcely preserved. Therefore, decrepitation of magmatic quartz with wavy extinction shows that the bursting of gas–liquid inclusions occurs within the range of $200$–$580^\circ\mathrm{C}$.

Phase transformations of perovskites ($\mathrm{CaTiO_3}$), titanobariums ($\mathrm{BaTiO_3}$), and leucite ($\mathrm{KAlSi_2O_6}$) first lead to a block structure, and within each block a domain structure develops, as well as the formation of a large number of pores, barely visible at a magnification of $200\times$.

Plastic deformations also favor the formation in crystals of pores in any quantity. For example, minerals possessing cleavage deform plastically along the cleavage. When crystals are deformed along the plane of distinct and especially imperfect cleavage, pores readily develop in the plane of these directions. In crystals of rock salt, slips in the direction of one of the $\{110\}$ planes develop with greater friction than slips along the plane of perfect cleavage $\{100\}$. Therefore, in rock salt syngenetic liquid inclusions are oriented in the plane of the cube faces, whereas in the $\{110\}$ plane epigenetic pores predominantly develop. Numerous observations indicate that waters penetrate along the slip plane $\{110\}$, producing partial leaching of halite with the formation of cubic, rectangular parallelepipeds with sides parallel to the faces of the cube. The fact that these pores in the $\{110\}$ plane are epigenetic is also indicated by the fact that in some of them there are precipitates of iron oxide hydrate and, in the majority of cases, with rare exceptions, they prove to be empty.

Abundant pores can be encountered in micas subjected to deformation by bending. In this case, parallel to the cleavage along $\{001\}$, the mineral splits into very thin plates. Under nonuniform compression, lenticular voids appear in them, with clearly expressed interference colors around them of the Newton’s-rings type. Such voids, as a rule, contain gases.

There are a number of crystalline hydrates, for example $\mathrm{CuSO_4\cdot 5H_2O}$, $\mathrm{CaSO_4\cdot 2H_2O}$, $\mathrm{Na_2CO_3\cdot 10H_2O}$, $\mathrm{Na_2SO_4\cdot 10H_2O}$, $\mathrm{MgSO_4\cdot 7H_2O}$, and others, which contain water of crystallization. In addition, inclusions of the mother solution are often present in the crystals of these salts. Naturally, or forcibly, they lose water, which leads to the destruction of the crystals. Crystals of halite and barite containing liquid inclusions crack upon heating predominantly along the cleavage plane. Study of the crystals just mentioned clearly shows that weathering and the loss of part of the water begin at individual points; then evaporation spreads through the mass. Initially transparent crystals thereby begin to grow cloudy and become opaque, and upon further weathering they readily crumble into powder.

Thus, the crystals with which we have to deal are far from perfect. They contain pores of various shapes and sizes. These pores contain gases and liquid, or only liquid; such inclusions may have been contemporaneous with the formation of the crystal. In this latter case they could have been preserved in the crystal if the negative crystal proved impermeable to them. Alongside these there are pores that arose in crystals later than their formation, as a result of various causes. Such pores constitute part of those pores, channels, which may be filled with gaseous products. The paths by which gaseous or liquid products penetrate into the pores of crystals are the paths of their evaporation, if the physicochemical environment is unfavorable for their preservation in the form of inclusions.

1. Prismatic and platy crystals of halides obtained from aqueous saturated solutions in the presence of alcohol. ↩