NEGATIVE FORMS OF DIOPSIDE CRYSTALS FROM THE KARYSH DISTRICT OF KHAKASSIA

G. A. Ivankin

Introduction

On the western slope of Mount Zavodskaya, approximately 1 km south of the Tereziya deposit, which is part of the Karysh group of skarn deposits in Khakassia, leaching cavities of an unknown mineral were found in the summer of 1946. These cavities are confined to quartz–feldspar formations that cut aplite-pegmatites.

In the summer of 1947 additional material was collected. Residual products of destruction were found in the cavities; these proved to be represented by manganese–iron titanium- and vanadium-bearing ochres impregnating the spongy aggregate of quartz, and by remnants of a destroyed mineral of the diopside–hedenbergite series.

Similar leaching cavities were also encountered in quartz–feldspar formations north of the Kladbishchenskoye skarn deposit and at the “Khakassia” deposit. In the latter case, a quartz–feldspar vein, up to 10 cm thick, cuts garnet skarns.

Exceptionally good impressions of the leached crystals attracted attention and prompted an investigation into their nature. The study of this material was carried out by the author under the supervision of Professor A. M. Kuzmin.

Below the results of studies of the material of interest to us are briefly presented.

Geology of the Mineral

The quartz–feldspar formations in which the leaching cavities of the unknown mineral were encountered compose a vein, 15–20 cm thick, traced along strike for 12–15 m. The vein has a symmetrical structure and consists of central and marginal zones. The central zone of the vein is composed predominantly of a coarse-grained aggregate of quartz, making up 75–85% of the zone, and rare crystals of orthoclase. Quartz not infrequently forms large, well-shaped prismatic crystals with developed faces of the hexagonal prism and rhombohedra. Sometimes the quartz crystals extend beyond the central zone of the vein, as if penetrating into the quartz–feldspar aggregate of the marginal zones.

Orthoclase, quartz, and, in smaller quantity, albite-oligoclase take part in the structure of the marginal zones of the vein. In addition, albite is present, which, in the form of perthitic intergrowths, penetrates the orthoclase. Weakly developed myrmekitic formations are rarely encountered.

The main mass of cavities, which are negative forms of crystals, is confined to the central part of the vein, where the cavities appear larger and more regular in outline. In the margi-

veins, the cavities are usually small, mostly of irregular shape, and occur relatively rarely.

The quartz–feldspar vein cuts the aplite–pegmatite dike at an angle of 25–30°; the latter cuts the granodiorites, extending in a latitudinal direction. Considerable talus of aplite–pegmatites, covering the entire slope of the mountain, did not allow the quartz–feldspar vein to be traced beyond the aplite–pegmatite dike.

Description of the leaching cavities

The cavities are distinguished by exceptionally good preservation of the boundary elements of the crystals of the leached mineral and often represent their ideal negative forms. Judging from the cavities, the crystals have a tetragonal-like cross section, while the impressions of the terminations indicate that they belong to the monoclinic system. The habit of the crystals is long-prismatic, with a ratio of diameter to length ranging from 1:5 to 1:10.

Fig. 1                  Fig. 2

Negative forms of diopside crystals in quartz–feldspar rock.

The largest cavities reach 1–1½ cm in diameter and 7–8 cm in length and, as noted above, are confined to the central part of the vein. They often radiate fan-like from the center of the vein toward the periphery (Fig. 2) and, as a rule, have the impression of only one crystal termination, directed away from the median plane of the vein. By volume, the cavities sometimes make up 10–15% of the specimens.

The general character of the occurrence of the cavities can be seen in Figs. 1 and 2.

Methods and results of the investigations

Methods of study

The good preservation, in the impressions, of the crystallographic faces of the leached mineral made it possible to apply, in order to determine the nature of the latter, the crystallochemical method. For goniometric study, models of the crystals were prepared from lead or wax.

The residual material of the products of destruction and the remnants of the undestroyed mineral were studied comprehensively using chemical and spectral methods.

Crystallographic study of the models

To obtain models of the crystals, two methods were used: 1) filling the cavities with molten lead, and 2) filling the cavities with wax. The first method is very convenient in that rather strong models are obtained, allowing them to be handled freely. The drawback of this method is that weakly developed faces on the models are not entirely distinct, because lead shrinks upon solidification. The most suitable material for these purposes may be type metal. Despite the indicated shortcomings of lead, models made from it are quite suitable for crystallographic studies, as was shown by measurements of a comparatively large number of casts.

To obtain clear signals, cover glasses were glued both onto the faces of already prepared models and onto the impressions of faces in the cavities before they were filled. In the latter case, models with embedded glass plates were obtained.

The second method, even more convenient, consists of the following: wax, slightly softened to a plastic state and used in crystallography, was pressed into the cavities. Cover glasses were first glued onto the impressions of the faces. The results obtained are good, but extreme care is required when removing the casts from the cavities so as not to damage them.

In preparing models of the crystals, split specimens were used predominantly, so that one half of the specimen contained one part of the negative form of the crystal, and the other half another. This made it possible to obtain models of the complete head and several models using one and the same cavity. The latter made it possible to determine the degree of perfection of the prepared models and to avoid accidental errors.

Models of the crystals prepared by one method or another were measured with a goniometer. The results of the measurements may be presented in the form of the following table, after first assigning symbols to the faces on the models:

\[ a : b = (100) : (010) = 90^\circ 00' \]

\[ m : m = (110) : (\overline{1}10) = 92^\circ 47' \]

\[ b : m = (010) : (110) = 43^\circ 36' 30'' \]

\[ c : a = (001) : (100) = 74^\circ 00' \]

\[ e : a = (okl) : (100) = 76^\circ 05' \]

\[ \mu : a = (hkl) : (100) = 61^\circ 15' \]

\[ \mu : b = (hkl) : (010) = 48^\circ 07' \]

\[ i : a = (\overline{h}kl) : (100) = 53^\circ 53' \]

\[ i : b = (\overline{h}kl) : (010) = 65^\circ 23' 40'' \]

On the basis of these measurements and casts, one may draw the stereographic and orthogonal projections of the crystal (Fig. 3), from which it is seen that the crystal belongs to the monoclinic-prismatic type of symmetry and is a combination of seven simple forms: three pinacoids \((a, b, c)\) and four prisms \((m, \mu, i, e)\).

To determine the constants of the crystal, and from them to decide to what chemical substance this type of crystal corresponds, it is necessary to choose correctly the crystallographic axes and the unit face. Following the general rules, we shall take as the crystallographic axis \(Y\) the axis of symmetry \(\{\perp (010)\}\); as \(Z\), the axis of the most highly developed zone of the faces \(a, b, m\); as the axis \(X\), in the present case, one may take a direction parallel to the face \(c\) \((001)\) and lying in the plane of symmetry.

Fig. 3. Stereographic and orthogonal projections of a diopside crystal.

The most responsible step is the choice of the unit face. It may be assumed that the face \(m\) will have the symbol \((110)\), since the prism faces \(m\) are always well developed; at times they are not inferior in size to the faces \(a\) and \(b\). Then let us consider the zone \(mcm''\) (Fig. 3). This zone unites the faces \(\{hkl\}\); the first two indices \(h\) and \(k\) will be unit indices, while \(l\) will be variable. Apart from the faces \(m(110)\) and \(c\{001\}\), not one of the actual faces falls in this zone. However, at the point of intersection of the three zones \(mcm''\), \(b\mu b'\), and \(aeia'\) there lies the pole of a possible face, which is situated on the crystal, with respect to \(\{100\}\), \(\{010\}\), and \([001]\), in the same way as the face \(i\), but is found in the positive octant. With a high degree of probability this face may be taken as the unit face. We shall denote it by \(V\) and give the calculations (Table 1).

Table 1

\[ a:b:c = \frac{0,4163}{0,3778} : \frac{0,4163}{0,4163} : \frac{0,4163}{0,6972} = 1,1019:1:0,5971 \]

\[ \beta = 74^\circ 00' - 106^\circ 00' \]

The ratio \(a:b\) can be refined by calculating it from the face \(m\), for which we have a large number of measurements:

\[ a:b = \frac{0,7241}{0,6633} : \frac{0,7241}{0,7241} = 1,0916:1. \]

Using handbooks, we arrive at the conclusion that the crystals studied are very close to crystals of diopside, for which, according to Raty:

\[ a:b:c = 1,09213:1:0,58931 \quad \text{and} \quad \beta = 74^\circ 10'09''. \]

The discrepancies may be considered insignificant (they do not exceed the angular value of \(10'\)) and quite permissible, since we measured crystals that were sufficiently large.

Thus, on the basis of the constants and the form of the crystals, we come to the conclusion that the cavities represent negative forms of diopside crystals.

Study of the residual material

To confirm the correctness of the conclusion drawn on the basis of the crystallographic study, the residual material of the products of destruction was investigated.

In the course of the study, among the brown ochres filling the cavities, fragments of a dark-green mineral were found; their distribution in the ochres indicates that it is precisely this mineral that undergoes destruction (Fig. 4).

For obtaining larger quantities of the indicated mineral, the brown ochres were treated with hydrochloric acid. With comparatively brief boiling, the brown ochres, predominantly ferruginous, passed into solution, while grains of a bottle-green mineral and quartz accumulated at the bottom of the test tube. In this way, from a comparatively small amount of ochre, 8–10 g, about 1.5 g of the required mineral was obtained. Separation of the mineral under study from quartz was carried out with the aid of bromoform.

Fig. 4. Remains of the primary mineral (black) in the ochre filling the cavities.

The mineral isolated in this manner showed the following properties. The color of the mineral is bottle-green. Under the microscope it is transparent, colorless or slightly greenish in thin splinters, and green in coarse ones. Pleochroism is not observed. The extinction angle \(Ng : c = 41^\circ\)—\(42^\circ\); elongation positive. The refractive index for \(Ng\), determined by the immersion method with an accuracy of \(\pm 0.004\), is 1.714. Cleavage is distinct; the cleavage planes intersect at an angle of about \(87^\circ\), are parallel to the third crystallographic axis, and correspond to the faces \(\{110\}\). The specific gravity, determined with a pycnometer for a 1.3 g sample, with an accuracy to \(\pm 0.005\), proved to be 3.265–3.270.

A chemical analysis of carefully selected material in an amount of about 1.5 g, performed by the analyst of the geochemical laboratory of the Department of Mineralogy, E. P. Antonova, showed the following composition of the mineral:

\[ \mathrm{SiO_2} \;—\; 48.85\%, \]

\[ \mathrm{TiO_2} \;—\; 0.023\%, \]

\[ \mathrm{Al_2O_3} \;—\; 0.59\%, \]

\[ \mathrm{FeO} \;—\; 10.00\%, \]

\[ \mathrm{MnO} \;—\; 0.88\%, \]

\[ \mathrm{MgO} \;—\; 12.62\%, \]

\[ \mathrm{CaO} \;—\; 23.93\% \]

\[ \Sigma = 96.893\% \]

Disregarding the contents of MnO, TiO₂, and Al₂O₃, the data of the chemical analysis may be recalculated into the following molecules, determining their participation in the composition of the mineral:

\[ \begin{aligned} \mathrm{CaSiO_3} &— 50.51\%,\\ \mathrm{MgSiO_3} &— 32.14\%,\\ \mathrm{FeSiO_3} &— 17.35\%\\ \hline \Sigma &= 100\%. \end{aligned} \]

Allowing for the isomorphism of CaSiO₃, MgSiO₃¹ and FeSiO₃, the chemical composition of the mineral may be represented as an isomorphous mixture of these molecules in the form \((\mathrm{Ca}, \mathrm{Mg}, \mathrm{Fe''})\mathrm{SiO_3}\), or as an isomorphous mixture of diopside and hedenbergite in the form \(m[\mathrm{CaMg(SiO_3)_2}] \cdot n[\mathrm{CaFe''(SiO_3)_2}]\), and for this particular case:

\[ 67.45[\mathrm{CaMg(SiO_3)_2}] \cdot 32.55[\mathrm{CaFe''(SiO_3)_2}]. \]

The data of the chemical analysis and the study of the physical and optical properties make it possible to refine the conclusion drawn on the basis of the crystallographic study and to assign the mineral to hedenbergitic diopside.

Fig. 5. Character of leaching of diopside; remnants of diopside (black) in spongy quartz.

Destruction of the mineral

The mineral on the surface is completely leached even in those places which appear to be difficult of access for surface agents of destruction (for example, water). At best, in the centers of large blocks of rock there have been preserved only brown ferruginous products of destruction and insignificant remnants of the primary mineral.

Destruction proceeds from the periphery of the crystals toward the center (Fig. 4). As a result of the destruction of the mineral, quartz² and a brown, predominantly ferruginous ocher are formed.

After leaching the brown ochers—ferric oxide hydrate—by dissolution, the picture of the destruction of the mineral can be clearly observed (Fig. 5). The quartz, apparently formed during the destruction of diopside, is deposited first of all along parting cracks and cleavage cracks.

From the parting cracks, filled with dense fine-grained quartz, as can be seen under the microscope, elongated quartz crystals extend, forming a spongy framework.

Spectral analysis of the products of destruction established contents of Mg, Fe, Ti, Si, Mn, V, traces of Cr and Ga.

Conclusion

Thus, the study of the impressions of crystals and of the residual material preserved in the cavities has led us to the conclusion that the leached mineral, in its morphological, physical, and chemical properties, belongs to hedenbergitic diopside and in its character will probably be close to the baikalite of the Slyudyanka deposit.

1. D. P. Grigor’ev. Isomorphism of compounds of magnesium and calcium. Problems of Mineralogy, Geochemistry and Petrography. Academy of Sciences of the USSR, 1946. ↩

2. The formation of quartz during the destruction of some metasilicates, for example aegirine, is indicated in the literature: A. K. Boldyrev et al., Course of Mineralogy, 1936. ↩