ON THE COLORATION OF SYNTHETIC FLUORITE AND THE GRAPHITE ENCLOSED IN IT

A. M. Kuz’min

In one of the problem laboratories of the Tomsk Polytechnic Institute, graduate student N. M. Timoshenko obtained, by the Stockbarger method, synthetic fluorite colored throughout the mass green. A small number of fragments of this mineral, knocked out along the cleavage \(\{111\}\), were placed at my disposal in order to determine the nature of the coloration. It was assumed that its origin was due to the presence of rare earths, in particular samarium, europium, and ytterbium [17,5]. However, carefully performed spectral analyses by A. D. Glazunova showed the complete absence in the samples of the elements just indicated.

The material of synthetic fluorite given to the author for study consisted of several cleavage plates \(2\)—\(3\) mm thick, knocked out from one and the same specimen. The surface of the fragments corresponded to perfect cleavage along the octahedron \((111)\) and was smooth, while a barely noticeable fine stepping on it was seen in places only under the microscope. The block structure of synthetic fluorite is not visible on the surface of its cleavage, whereas on the cleavage planes of natural crystals a block structure* is often observed even macroscopically. Therefore, on the goniometer the reflected ray from the cleavage face \(\{111\}\) of the synthetic fluorspar did not show any appreciable scatter of signals: they were sharp and single. By contrast, the scatter of signals from \(\{111\}\) of natural crystals is clearly expressed [11].

The coloration of the fragments of synthetic fluorite is heterogeneous, unevenly zonal and mottled. Thus, along the edge of the cylindrical rod the coloration within a narrow \(2\)—\(3\) mm border is colorless or slightly bluish, pale violet, and even slightly pinkish. Then the coloration of the border toward the center of the rod gradually, but at the same time rapidly, changes to a blue, grayish-blue color of varying intensity, which also gradually and rapidly changes to green. The green part of the fluorite, from greenish and yellow-greenish tones, gradually became deep green, so that over a certain extent it also gradually and relatively rapidly changed to a blue and blue-violet color of varying intensity. In general, the variegation of the fluorite coloration indicates that the color pigment in the crystals studied is distributed extremely unevenly.

Study of fragments of synthetic fluorite under the microscope showed that they contain rare, small-sized (0.5—2 mm)

* Within an individual block of a crystal, the crystallites composing it are spatially oriented, but some blocks of the crystal are noticeably misoriented relative to others. Such misorientation can easily be detected in goniometry of the crystal in the form of a scatter of signals by a face [11].

“debris” and unevenly distributed individual plates and scales of graphite crystals. Irregularly shaped “debris” consists of a scaly aggregate of graphite, torn by the fluorite melt from the walls of the graphite crucible. It should be noted that around such “pieces” the intensity of the green and bluish coloration of the synthetic fluorite mass noticeably increases.

Most of the graphite in fluorite is represented by crystalline individuals measuring from 0.01 to 0.0001 mm across and smaller, of hexagonal and trigonal habit (Fig. 1). Some of them are represented by regular hexagonal thin plates with faces \(\{0001\}\), \(\{10\overline{1}0\}\) (Fig. 1, a). Some of the platy crystals bear faces \(\{0001\}\), \(\{10\overline{1}1\}\) or \(\{0001\}\), \(\{10\overline{1}1\}\), \(\{10\overline{1}0\}\) (Fig. 1, b, c), and in this latter case the graphite crystals appear as a shuttle or lentil-shaped grains. Crystals of this mineral are rarely encountered in the form of combinations of two very compressed \([0001]\) rhombohedra with faces \(\{10\overline{1}3\}\), \(\{01\overline{1}3\}\) (Fig. 1, g, d). Sometimes platy graphite crystals have a clearly trigonal habit (Fig. 1, g, d, e, zh). Very rarely, on platy crystals, one can observe glide striations along \(\{11\overline{2}1\}\) (Fig. 1, e). Fig. 1, zh shows how, in individual cases, platy crystals intergrow, forming penetration twins of graphite. Sometimes graphite plates in the mass of fluorite form a kind of chain, the members of which lie in the host crystal at different levels parallel to \(\{111\}\) (Fig. 1, i).

Fig. 1. Morphology of graphite crystallites included in synthetic fluorite

Cases are not uncommon when the edges of tabular and scaly crystals along the periphery prove to be uneven, as if torn, crumpled, and split along cleavage. Platy crystals sometimes turn out to be six-rayed formations and assume the character of diverse petal-like or rosette forms (Fig. 1, z; Fig. 3), or form flat twinned crystals of the aragonite type. On the surface \((0001)\) of platy crystals, the appearance of knobby growths or ridges that do not, in general, disturb the symmetry characteristic of them can often be observed (Fig. 3).

Proceeding to specify the position of graphite in the body of fluorspar, let us note the following feature in the structure of the fluorite itself, discovered by the author of this article. Synthetic fluorite, etched with hydrochloric acid, proved to be composed entirely of separate microcrystals (crystallites, according to Kuz’min [10], or submicrons, according to A. V. Shubnikov [25]) in the form of cubes ranging in size from 0.015 to 0.0001 mm. The cleavage planes along \((111)\) were etched for 3–4 hours, after which the surface of the octahedron face was completely covered with triangular projections and pits (Fig. 2), the faces of which corresponded

Fig. 2a. Relief of the etched (111) surface of green fluorite, cut by triangular projections and pits corresponding to the triangular apices of feldspar cubes

Fig. 2b. Relief of the etched (111) surface of natural colorless fluorite, cut by a large number of protruding triangular cube apices measuring 0.005—0.001 mm

Fig. 3. Morphology of graphite grains included in the mass of a fluorite crystal. Photograph parallel to the (111) face of fluorite. Magnification 600×

faces of the cube, and the vertices formed by them were vertices of the cube. Taken together, what has just been said is a real expression of Haüy’s idea that crystals are composed of “bricks” or “corpuscles,” according to Huygens and M. V. Lomonosov.

Incidentally, this feature of the composition of fluorite crystals from crystallites is also characteristic of natural crystals, in which the crystallites are smaller in size (0.005—0.001 mm), and the crystals themselves are deformed and often have a complex block structure.

All graphite crystals in the mass of the cylindrical rod of synthetic fluorite single crystal are arranged parallel to the faces of the octahedron (Fig. 1, к), and by their regular arrangement in it they indicate a sequence in the formation of a very fine zonality of growth (Fig. 4). The regularity of the structure of both minerals is epitaxial in character.

Fig. 4. Dense inclusion of graphite crystallites in violet-blue synthetic fluorite. The visible crystallites, 0.008—0.002 mm in size, are arranged parallel to the face of the octahedron. Magnification 300 X

That is, in the general plan the direction \([0001]\) of graphite coincides with the direction \([111]\) of fluorite. The length of the diagonal \([111]\) of the structural cell of \(\mathrm{CaF}_2\) is 9.45 Å, whereas \(C_0\) for the rhombohedral \(\beta\)-modification of graphite is 10.04 Å [2]. Thus, a correction of 6% is quite sufficient for the epitaxial character of the intergrowth of graphite with fluorite [8; 24].

Optically, the majority of platy and scaly crystals of graphite in synthetic fluorite are transparent, colorless, or colored light bluish or grayish-blue, green of various shades. Rare translucent scales appear whitish. Opaque in the fluorite described were only rare pieces of graphite torn from the walls of the graphite crucible. The outlines of the graphite plates and scales, in comparison with fluorite, are very sharp (Fig. 3), indicating that the refractive index of graphite is not less than 2.0. In transmitted light \([0001]\) they are, as a rule, colorless or bluish; in reflected light—metallically lustrous; in sections parallel to \((0001)\), isotropic. In sections perpendicular to \((0.0001)\), graphite is anisotropic: \(Np(Ne)\) parallel to \([0001]\), \(Nm(No)\) parallel to \((0001)\). Pleochroism is distinct: along \(Np\)—light blue, along \(Nm\)—blue. In synthetic fluorites colored green, pleochroism along \(Np\)—colorless or weakly yellowish, along \(Nm\)—green, greenish-blue. From the data just presented it follows that the bluish-violet and bluish and greenish colors of fluorite owe their origin to finely dispersed graphite. It is therefore quite natural that

the intensity of coloration of fluorites in the samples studied depends on the amount of graphite included in the fluorite: the denser the coloration of the fluorite, the higher the graphite content.

In the Russian literature the first information on transparent platy graphites belongs to P. P. Pilipenko and I. I. Oreshkin, who studied this mineral from the talc schists of Transbaikalia [14]. The graphite studied by them was represented by platy and scaly varieties 0.5—0.0002 mm thick. The thinnest of them, 0.2—0.0002 mm, are transparent. Their refractive indices were determined by the authors as 1.98—2.03 for red rays. In thin plates and scales the color of Transbaikal graphite is dark blue, bluish, greenish-gray to dark green. The coloration of the graphite is sometimes uniform, sometimes mottled. According to Haber, transparent graphite scales 0.002—0.0002 mm thick are greenish-gray in transmitted light and \(N = 2.00 \pm 0.04\) [29]. A. P. Ubbelohde and F. A. Lewis give more refined refractive indices for graphite, namely \(N_m = 2.15 \pm 0.04\) and \(N_p = 2.04 \pm 0.04\) [19].

The color of colored fluorite in recent years has become a special subject of investigation. However, the problem of the coloration of fluorite is still considered unresolved by most investigators [15, 12, 7, etc.]. Nevertheless, investigators are making one attempt after another to find partial solutions to this problem, considering the color of fluorites from various standpoints. The literature on the coloration of fluorites is extensive. We shall confine ourselves only to a brief consideration of certain main trends that explain, in one way or another, the color of \(\mathrm{CaF}_2\).

Thus, one group of investigators explains the color of fluorites as a consequence of the absorption of part of the light by a finely dispersed mechanical admixture of carbonaceous, siliceous, and clayey substances (V. S. Kormilitsyn, A. A. Ivanov [9]) or by the presence of small quantities of isomorphous impurities of iron and manganese (A. N. Winchell and G. Winchell [5], A. Serra, 1947). According to V. I. Sobolevsky, an admixture of manganese caused the appearance of violet, dark-violet, and lilac varieties of fluorite.

Another group of investigators explains the color of fluorite by the presence in them of atomic calcium and fluorine (N. S. Lavrovich, A. N. Winchell and G. Winchell, A. E. Fersman, V. I. Sobolevsky, and others). In his popular book The Colors of Minerals (1936), A. E. Fersman, along with many reasons responsible for the coloration of minerals, dwells especially on the color of minerals depending on the state of excitation of atoms and ions [23] without a change in chemical composition and in the absence of any chromophores, but under the condition that, for example, in blue halite, sodium atoms that have acquired the electron of chlorine appear. In essence, this is what de Boer (1937) called a color center, an \(F\)-center in crystals, arising as a result of localization of an electron near a vacant anion site and the consequent neutralization of a certain number of sodium ions. As applied to mineralogy, the theory of coloration by the appearance of \(F\)-centers was used by S. I. Stishev [18] to explain the appearance of coloration in halites, fluorites, spodumenes, quartz, pacmanites, amazonites, and calcites. A similar appearance of \(F\)-centers in the indicated minerals, including fluorite crystals, is conceived in the presence of radioactive radiation, the source of which may be located in the fluorite itself or outside it, “somewhere in the vicinity” [18]. It is believed that under natural conditions, for example, the coloration of halite under radioactive radiation from the isotope \(\mathrm{K}^{40}\) may appear after 20,000 years [15].

However, this theory of fluorite coloration is at variance with reality. Thus, for example, fluorites from the Suugli deposit have a zonal structure. In Fig. 5 one of the

Fig. 5. Zonal structure of polychrome fluorite. The zone begins with colorless fluorite 1, is replaced by green (2), and ends with thick-green (3)

many specimens with a similar zonal structure, which can be divided into separate zones. Each zone, for example V, VI, and VII, begins with a small zonule (1), composed of a fine-grained aggregate of white and transparent fluorite. It rests on a surface made up of green crystals (3) of unequal size belonging to the underlying zone IV. On this white zonule a colored, in the present case green (2), zone V is formed. As one moves upward, the sizes of the crystals in the zone increase, and the surface of zone V ends with relatively large cubic crystals (3) of fluorite. As for the color, it is green, in other cases violet; as one moves from below upward, from the base of the zone to its roof, the density of the coloration increases. At the transition to zone VI, the uneven surface of zone V, caused by protruding CaF₂ crystals (3), is overlain by fine-grained white (transparent) fluorite beginning the new zone VII, on which colored green fluorspar is then superimposed, and zone VII ends with a surface bearing partially faceted crystals of fluorite of cubic habit (3) protruding on it. This surface, as in the previous cases, is sharply overlain by a zonule of white fluorite, beginning the formation of the next, VIII zone, etc.

Looking closely at such regularly constructed zonal aggregates of fluorite, we draw the following conclusions: 1) sharp boundaries between zones testify to the presence of a certain interruption in time between the end of formation of one zone and the beginning of another, later one; 2) the formation of fluorite zones begins with the deposition of a zonule (1) of fine-grained white (colorless) fluorite, the crystallization of which apparently proceeds from a supersaturated solution; then, with the beginning of the appearance of the colored zone, the growth rate slows, and toward the end of the formation of the zone it dies out completely; 3) the color of fluorite crystals within each zone changes regularly from its base upward. Thus, in the example under consideration the zone begins with the deposition of white (transparent) fluorite, which is then replaced by colored fluorite, with the intensity of coloration increasing as the crystals grow in the zone. Sometimes, in the colored part of a zone, for example a green one, one can see the appearance of second-order zones: narrow violet zonules of 1–2 mm in the green, or in violet parts of zones—also narrow green zonules. From the observed facts it follows that the coloration of fluorite appeared simultaneously with the growth of the crystals, and the regular change in coloration within the zone indicates that the pigment determining the color of fluorite also changed quantitatively in a regular way with crystal growth. That is, if crystal defects and electron localization are invoked to explain the coloration, then it is natural to suppose that the number of $F$-centers also increases quantitatively in accordance with the growth of the intensity of coloration.

The next group of scholars places the coloration of fluorites in dependence on the entry into the mineral of rare earths in the form of an isomorphous impurity replacing calcium in fluorspar. A. Pshimbryam [15] explains the coloration of fluorite by the presence in its composition primarily of samarium, europium, and ytterbium. In confirmation of this, O. K. Kozlova [6] cites data on the content of rare earths in fluorspar.

from various deposits in order to evaluate the raw material for obtaining synthetic fluorite. It follows from this article that its author studied more than 65 samples from different deposits. Of these, green-colored fluorites account for 31%, of which only 18% of the spars contain samarium and europium, while the remaining 13% contain only europium accompanied by gadolinium. The group of violet fluorites, blue and pink, averages 38%, of which 9% of the fluorites contain samarium, 25% contain only europium with gadolinium. Colorless and white fluorspars constitute 31% of the total number studied by O. K. Kozlova; of these, 6% contain samarium and europium, and 25% contain europium, gadolinium, terbium, and didymium; moreover, in white and colorless fluorites, in comparison with those colored by these impurities, they are present in high concentrations: samarium—\(0.8 \cdot 10^{-3}\%\), europium—\(1.1 \cdot 10^{-3}\), gadolinium—\(2.0—2.6 \cdot 10^{-3}\), terbium \(2.0 \cdot 10^{-3}\), and didymium \(1.4 \cdot 10^{-3}\). Analysis of the materials of O. K. Kozlova [6] gives no grounds for considering the appearance of the green coloration to be caused by the presence of samarium alone. The intensity of the coloration of fluorites is not in agreement with the content of samarium in them.

The experimental studies of I. V. Stepanov and P. P. Feofilov, undertaken in order to prove that samarium is responsible for the green coloration of fluorites when it is added to a CaF\(_2\) charge in an amount of \(10^{-5}\)—\(10^{-4}\%\) in the presence of about \(10^{-3}\) g of graphite powder. The synthetic fluorite obtained as a result of the experiment was colored green [31]. Later, in joint studies on the growth of synthetic CaF\(_2\) under reducing conditions with the addition to the charge of about 0.1% of graphite substance by weight of the grown crystal and rare earths (samarium, europium, ytterbium) in an amount of about \(10^{-2}\%\) in the form of fluorides, I. V. Stepanov and M. P. Feofilov obtained an artificial colorless and transparent fluorite. It is in precisely this form that its specimens are shown in the photograph of the authors [17]. The synthetic fluorite obtained by them contained impurities of two kinds with isotropic and anisotropic properties [17], which proved to be crystals of transparent graphite (A. K.). Thus, it may be considered that I. V. Stepanov and M. P. Feofilov [17] and O. K. Kozlova [6] showed that TR, being present in fluorspar, are not responsible for its coloration.

There is yet another group of scientists who explained the allochromatic coloration, for example, of brown and yellow calcites, smoky quartz and morion, and other minerals, by the presence in them of organic matter [4, 1; 23, 9]. A distinctive feature only of the indicated minerals was that, when heated within the range of \(100—600^\circ\mathrm{C}\), they were easily decolorized due to its combustion [5] or its evaporation [9]. A. M. Kuz’min’s experiments on the decolorization of transparent yellow, brownish-yellow, brown, violet, and pink cleavage pieces of Turukhansk spar and spars from other deposits showed that when they were heated within \(125—180^\circ\mathrm{C}\) and above, hydrocarbons began to move; in part, evaporating, they condensed in the near-surface part of the crystal in the form of a thin oily brown film along cleavage cracks and in parts of the mineral separated along cleavage. With rapid heating, yellow calcites became brown as a result of the tarification of heavy bitumens. Morions were decolorized upon heating within \(400—800^\circ\mathrm{C}\). With slow heating, fractionation of hydrocarbons occurred, their removal by evaporation from the body of the crystals, and partly the condensation of heavy bitumens on the surface of the crystal in the form of brown films, which then disappeared. With rapid heating to \(800^\circ\mathrm{C}\), the crystals burst explosively and scattered into small fragments, which remained colorless for 4—5 years.

In the Khaidarkan fluorites of the black-violet, light-violet, and colorless varieties, spectral analyses established the presence of manganese (0.003–0.2%), titanium (0.001–0.003%), and copper (0.003–0.3%); the presence of cerium, lanthanum, and yttrium (0.003–0.0005%) was also established [12]. In addition, special studies were carried out on the carbon and hydrogen content. In black-violet fluorite it was established: C—0.33 and H—0.37%; in light-violet—C—0.16 and H—0.73; and in white—C—0.09 and H—0.01% [12]. During the bleaching of black-violet fluorites from the Severo-Yenisei taiga, the appearance of brown films along cleavage cracks was also observed. The fluorescence of fluorites, especially brown ones, is often explained by the presence in them of organic matter [28]. According to Ford, the color of dark-violet fluorspars is due to the presence of hydrocarbons [28]. Haberland [20] and Müller [31] relate the intensity of the color and fluorescence of fluorites to the content in them of finely dispersed bituminous products.

In connection with what has been said, the question involuntarily arises as to what promotes the removal, during heating, of gas-liquid inclusions from minerals. But the removal of gas-liquid inclusions can also occur without heating. Frequent causes promoting the removal of gas-liquid inclusions are cracks, as was the case in the bleaching of morions. But there are also general causes that promote the removal of mobile media. Thus, crystals, according to A. V. Shubnikov [16], L. N. Ovchinnikov (1951, 1952, 1959), and A. M. Kuz’min [10, 11], are to one degree or another porous because, in the process of crystallization, they are built up by crystallites (submicrons, according to A. V. Shubnikov) of different sizes, and they do not always fit ideally densely and spatially parallel with respect to one another. Taken together, all this accounts for the appearance in crystals of pores, channels, and sutures of various sizes [9, 10], in which, during crystallization, the mother solution is trapped. Under heating and pressure, these same pores and channels, connected with one another in one way or another, promote the removal of gas-liquid inclusions, and they are also the routes by which pores and voids are refilled by epigenetic gas-liquid formations. It is therefore quite natural to expect that mobile media may be removed from crystals completely or partially. In this latter case, the hydrocarbons remaining in the crystal, under the action of radiant energy in the presence of oxygen, may again become resinified and thereby cause a visible coloration of the crystals.

In conclusion to our article, we shall briefly dwell on the coloration of Yakutian diamonds, the nature of which still remains unclear and, in its character, resembles the coloration of fluorites. The most widespread green color in diamonds, of various shades, is explained by small contents of an iron impurity [26]. Violet colorations of diamond of various shades and densities are considered not to depend on an iron impurity, but, by analogy with the nature of the coloration of cherry-smoky diamonds of the Urals [27], the indicated colors may be explained by the presence in them of finely dispersed graphite. In spottily colored diamonds, morphologically complex crystalline formations, green, blue-green, smoky, and brown colorations are related to the presence of graphite, inclusions of which are most frequent in Yakutian diamonds [26]. In this case, graphite in the form of round leaves, plates, rosettes, flakes, and the finest finely dispersed particles is embedded in the mass of the diamond and is distributed in the parallel {111} growth planes. The authors of Diamond Deposits of Yakutia [26], in agreement with A. A. Kukharenko [27], regard graphite in diamonds as an epigenetic

inclusion, with which one can hardly agree: diamond and graphite are syngenetic formations.

Thus, the causes of the appearance of colorations in fluorite and diamond are manifold; however, the colors usual for them are closely connected with the presence in them of inclusions of graphite and hydrocarbons in various degrees of dispersion.

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