Smolyaninovite from the Gorny Altai

A. I. Bazhenov

(Presented by Professor A. M. Kuzmin)

In 1956 L. K. Yakhontova described a new mineral—smolyaninovite, corresponding in composition to cobalt arsenate. This mineral is a fairly common hypergene formation in the case of skutterudite–smaltine, safflorite–smaltine, and rammelsbergite–smaltine mineralization.

L. K. Yakhontova gives a detailed description of the conditions under which smolyaninovite occurs, its physical properties, and its chemical composition.

By the time the indicated articles appeared, the author had largely completed the processing of materials from one of the deposits of the Gorny Altai, in the oxidation zone of which an analogous mineral had been established; however, the conditions of its formation in the present case differ from those described by L. K. Yakhontova.

The present article sets forth the results of the study of smolyaninovite, supplementing the information reported by L. K. Yakhontova. Some of the smolyaninovite specimens were kindly given to the author by the geologist P. I. Slynko.

Smolyaninovite has a very limited distribution within the deposit described, which is connected, on the one hand, with the weak development of the oxidation zone, conditioned by the specific geomorphology of the district, situated under conditions of a sharply dissected high-mountain landscape. As a consequence, suboxidized ores are exposed at the surface, often bearing only traces of oxidation. On the other hand, conditions favorable for the appearance of smolyaninovite also occur rather rarely. It should be noted that the mineral develops chiefly in areas directly exposed at the surface and does not appear in deeply situated parts of the oxidation zone, where the latter is more or less fully developed.

The appearance of smolyaninovite is by no means always connected with the oxidation of cobalt arsenide ores “containing closely similar quantities of smaltine and safflorite,” as A. K. Yakhontova (1956) believes. Under the conditions of the deposit described, the mineral develops as a result of the oxidation of cobalt ores, to which cobalt-bearing sulfoarsenides are admixed in insignificant quantity. A very important condition for the appearance of smolyaninovite is the presence of carbonate material, whose role is played by vein, coarsely crystalline, manganiferous calcite. Especially favorable for the development

smolyaninovite areas enriched in cobalt sulfoarsenides and containing a small amount of calcite.

Smolyaninovite develops by replacement of cobalt sulfoarsenides and, as a rule, does not appear in vein calcite, which sharply distinguishes it from other secondary cobalt minerals. Only as an exception does smolyaninovite fill cleavage cracks in adjacent calcite grains. The process of replacement of cobaltite can be well traced under microscopic study (Fig. 1). At first smolyaninovite appears as the finest veinlets developing along the boundaries of cobaltite grains. Then the number and thickness of these veinlets rapidly increase, as a result of which a typical mesh-like replacement structure appears. With further development of the process, only relics of the original mineral are preserved in the mass of smolyaninovite.

Fig. 1. Replacement of cobaltite (black) by a tangled-fibrous aggregate of smolyaninovite (Sm). Nicols II. ×160.

Macroscopically, smolyaninovite forms crusts of reddish-brown color, the thickness of which sometimes reaches 8–10 mm. These crusts always bear cracks arising as a result of dehydration, which break the aggregates into polygonal areas (Fig. 2). A network of irregularly arranged cracks is also observed in microscopic study of the mineral. The increased cracking greatly hinders the preparation of thin sections, since the aggregate easily falls apart into the smallest fragments under slight force. Dehydration cracks affect the aggregate of smolyaninovite and quickly die out in weakly oxidized cobaltite. With further development of the process, the reddish-brown color of the mineral changes to brownish-yellow, resembling the color of erythrite. Correspondingly, the hardness of the mineral decreases from 4.0 to 2.0–2.5 (on the Mohs scale). The streak of smolyaninovite is light, brownish-yellow. The specific gravity was determined by hydrostatic weighing and pycnometrically, and in both cases proved to be 2.453.

In thin sections the mineral has a brownish-yellow color and lacks pleochroism. As a rule, the tangled-fibrous structure of the aggregate is clearly visible. The size of individual, largest fibers does not exceed 0.008 × 0.001 mm. The usual sizes of the fibers are considerably smaller, as a result of which the fibrous aggregate of smolyaninovite clearly shows undulatory extinction. The elongation of the fibers is positive. Extinction relative to elongation is parallel. Owing to the insignificant sizes of the individuals and the impossibility of separating them, in immersion it was possible to measure only the mean refractive index and to determine the limits within which it varies. According to many measurements, the value of the index varies from 1.632 to 1.639, which somewhat exceeds the results of analogous measurements reported by L. K. Yakhontova—1.620–1.630. The birefringence of smolyaninovite is low. Measurements of it with a calcite rotary compensator gave very constant values of this magnitude, fluctuating within the limits 0.007–0.008. It was not possible to determine the axiality and optical sign because of the impossibility of separating more or less large individuals.

Microscopic study showed the monomineralic composition of the ag-

aggregate, and no other mineral varieties, about which L. K. Yakhontova writes, were found in any thin section.

The X-ray study of the mineral was carried out in the laboratory of the West Siberian Geological Administration under the direction of B. G. Erenburg. The photographs were taken in cameras of 57.3 mm diameter, with iron radiation at a voltage of about 30 kV and a current of about 10 mA. The interplanar spacings were corrected from separate photographs of a mixture with ammonium bromide.

Fig. 2. Aggregate of smolyaninovite. Dehydration cracks are clearly visible. On the surface of the aggregate, rosettes of gypsum (white) are visible. Natural size.

The results of the X-ray study are given in Table 1, which also includes the results of an X-ray investigation of smolyaninovite borrowed from L. K. Yakhontova.

It follows from Table 1 that the smolyaninovite described here differs, apparently, by a more complete recrystallization of the substance in comparison with the variety described by L. K. Yakhontova.

L. K. Yakhontova, analyzing the results of the X-ray study of smolyaninovite, comes to the conclusion that the behavior of the mineral as a finely dispersed substance is due to “the varying degree of hydration of individual fibrils and the insufficient ordering of their packing, with a general imperfection of the structure.” The following feature should also be noted: L. K. Yakhontova indicates that smolyaninovite, after grinding in a mortar, gives no lines on the powder photograph. Samples of the mineral described here that were subjected to X-ray study were ground in a mortar, and nevertheless the number of lines

Table 1

in the powder diagram is considerably greater than in the specimens of L. K. Yakhontova. All this gives grounds to suppose that the variety of smolyaninovite described here possesses a considerably more ordered packing and a more perfect structure.

X-ray metric analysis confirmed the homogeneity of the mineral: in the powder diagram not even traces of lines corresponding to the structures of other minerals were established, with the exception of very diffuse and weak lines of cobaltin, which indicates the high dispersion of its particles, apparently representing the finest particles not yet replaced by it.

Heating of smolyaninovite samples in a closed tube, and then the data of chemical analysis, showed a significant content of water. In order to obtain heating curves, homogeneous material was carefully selected.

The thermal study of the mineral was carried out in the laboratory of the Department of Silicate Technology of the Tomsk Polytechnic Institute and was conducted by the method of “double heating” (A. V. Kazakov and K. S. Andrianov, 1936). The heating curve (Fig. 3) has an endothermic effect at a temperature of 260°, and in this large peak there is observed

Fig. 3. Differential heating curve of smolyaninovite.

an additional endothermic effect at 80°. Exothermic effects appear at temperatures of 600° and 795°, with the first maximum being especially sharply expressed.

The endothermic effect is due to dehydration, and the inflection at a temperature of 80°, apparently, is caused by the completion of the release of hygroscopic water, which is also consistent with the data of L. K. Yakhontova, who noted that, according to dehydration results, this water is released at temperatures of 40–100°.

The reason for the appearance of the exothermic effects is not yet entirely clear. It can only be assumed that they are caused by recrystallization (coarsening of the crystals) of smolyaninovite.

After completion of the thermal analysis, the powder acquires a light brownish-yellow color. It should be noted that the exothermic maxima on the heating curve are very characteristic and are easily recorded in thermal analyses of mixtures containing even an insignificant amount of smolyaninovite.

In order to study the qualitative composition of the mineral, optical spectral analysis was performed, and for the principal components, semiquantitative X-ray spectral analysis. The results of the optical spectral analysis, carried out by V. K. Chistyakov in the laboratory of the Department of Mineralogy of Tomsk State University, are given in Table 2.

Table 2

X-ray spectral analysis was carried out in the laboratory of the West Siberian Geological Administration by B. G. Erenburg. According to the data obtained, the content of the most important elements in smolyaninovite is determined by the following values: Co > 10%; Ni—3%; Fe < 10%; As > 10%.

For quantitative chemical analysis, the selected material was checked under the microscope.

Chemical analysis of smolyaninovite was carried out in the laboratory of the Tomsk Complex Expedition of the West Siberian Geological Administration. Table 3 contains the results of the analysis and the recalculation to the chemical formula of the smolyaninovite described, as well as analogous data borrowed from L. K. Yakhontova.

In calculating the results of the chemical analysis to the formula of the mineral, the possibility of isomorphous substitutions between cobalt, nickel, magnesium, and manganese, as well as between trivalent iron and aluminum, was allowed. Similar assumptions were also made by L. K. Yakhontova in calculating the formula of smolyaninovite. The possibility of isomorphous substitutions between Co^+2, Ni, Mg and Ca is noted by A. G. Betekhtin (1950) and V. M. Goldschmidt (1955).

According to the calculation presented in Table 3, the formula of the mineral may be written as 2.5 As₂O₅ · R₂O₃ · 4.5 RO · 13.2 H₂O or 2.5 As₂O₅ · (Al, Fe)₂O₃ · 4.5 (Co, Ni, Mg, Mn, Ca) O · 13.2 H₂O. This formula is fairly close to the results of recalculation of one of the chemical analyses of smolyaninovite obtained by L. K. Yakhontova and presented in Table 3: 3.2 As₂O₅ · (Al, Fe)₂O₃ · 4.3 (Co, Ni, Mg, Ca) O · 12.9 H₂O. It should be noted that, according to the various analyses of L. K. Yakhontova, the value of the coefficient for the group of divalent elements varies from 4.0 to 4.9.

Table 3

Attention is drawn to the difference in the contents of cobalt, nickel, and iron in the smolyaninovite described here and in the variety studied by L. K. Yakhontova. This difference is due to peculiarities in the composition of the primary minerals.

In the deposit described, smolyaninovite develops, as noted above, on cobaltite, in which, according to chemical analysis, the contents of these elements are determined by the following figures: Co—34.18%, Ni—0.01%, Fe—3.33%. In the deposit whose materials were studied by L. K. Yakhontova, the primary mineral—smaltite—is characterized by a high nickel content, “close to the cobalt content, and by the maximum amount of iron—2.5%.”

It is interesting to note the appreciable concentration in the smolyaninovite of this deposit of iron and especially nickel, the content of which increases 100-fold in comparison with its amount in the primary cobaltite.

Smolyaninovite is usually observed in close paragenetic association with gypsum and erythrite.

Gypsum forms rosettes and crusts on the surface of smolyaninovite aggregates; more rarely it appears in cavities of the latter in the form of druses of small colorless platy crystals. Spectral analysis shows its rather pure composition: besides strong calcium lines, medium lines of cobalt and strontium, weak lines of magnesium and arsenic, and only traces of manganese, iron, titanium, copper, aluminum, and molybdenum were noted in it.

Erythrite always appears later than smolyaninovite. Raspberry-colored platy crystals of erythrite, quite often in the form of druses, grow on the surface of smolyaninovite. The size of individuals elongated along [001] does not exceed 0.8 × 0.4 × 0.1 mm. Very often the crystals are curved along the plane (010). The mineral possesses perfect cleavage on (010); much more rarely a parting perpendicular to the elongation is manifested.

Erythrite crystals possess distinct pleochroism. Pleochroism scheme: Ng—rose-red, Nm—pale, pinkish-violet, Np—pale, slightly pinkish. The plane of the optic axes is perpendi-

pendicular to (010). The axis Np coincides with the axis [010], \(cNg' = 30^\circ\); \(Ng = 1.699\); \(Nm = 1.662\); \(Np = 1.622\); \(Ng - Np = 0.077\).

Spectral analysis showed the following composition of erythrite: strong lines of cobalt and arsenic, medium lines of magnesium and iron, weak lines of calcium, thereby confirming the isomorphism of these elements noted above.

Of great interest is the appearance of an unknown mineral, spatially closely associated with erythrite and smolyaninovite and showing very limited development in the deposit.

Fibrous, considerably more rarely platy, individuals of this variety are sometimes observed in cavities of smolyaninovite and are very closely associated with erythrite. The color of this variety is pinkish-reddish, but weaker than the color of erythrite.

Pleochroism is weakly expressed, considerably weaker than in erythrite. Pleochroism scheme: Ng—light pinkish-reddish, Nm—almost colorless, pinkish-violet, Np—colorless. Thus, the pleochroism scheme is analogous to that of erythrite and differs only in its reduced intensity.

The fibers and platy individuals of the mineral are elongated along Ng. In the same direction a distinct cleavage is manifested. Extinction relative to the cleavage and elongation is straight. The refractive indices \(Ng = 1.674\), \(Np = 1.660\), \(Ng - Np = 0.014\), are considerably lower than those of erythrite.

Microchemical tests give positive results for cobalt and arsenic. Because the mineral shows very limited development, it was not possible to select material in sufficient quantity for more detailed study, in particular for chemical analysis.

With further development of the oxidation zone, smolyaninovite becomes coated on the surface with yellowish ochres, which still require study. Erythrite also appears in considerable quantities, its veinlets cutting the aggregate of smolyaninovite.

Since in the present deposit smolyaninovite is associated with sulfoarsenide cobalt mineralization, the conclusion of L. K. Yakhontova on the significance of this mineral as an indicator of arsenide-cobalt mineralization of smaltine composition must be broadened, and smolyaninovite, along with erythrite and minerals of the cobalt hydroxide group, may also be used as an indicator of cobalt sulfoarsenides. At the same time, it should be borne in mind that diagnosis of the mineral is rather difficult; its appearance is connected with certain conditions in terms of the paragenesis of the primary minerals, as a result of which the significance of smolyaninovite as an indicator is not so very great.

Nevertheless, in studying oxidation zones of cobalt deposits and in examining outcrops containing erythrite, which is the most stable cobalt arsenate in the oxidation zone, one should very carefully investigate aggregates resembling jarosite or limonite, since the possibility of the appearance of smolyaninovite under these conditions is not excluded.

REFERENCES

1. Betekhtin A. G. — Mineralogy. Gostgeolizdat, 1950.
2. Kazakov A. V. and Andrianov A. A. — Methodology of thermal analysis. In the collection “To Academician V. I. Vernadsky,” Publ. Acad. Sci. USSR, vol. 2, 1936.
3. Yakhontova, L. K. — A new mineral—smolyaninovite. Tr. MGRI, vol. 29, 1956.
4. Yakhontova, L. K. — A new mineral—smolyaninovite, DAN SSSR, vol. 109, No. 4, 1956.
5. Goldschmidt V. M. — Geochemistry, 1955.

Corrections and Noted Misprints

In S. A. Stroitelev’s article “Investigation of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.