On Two Varieties of Pyrite

A. D. Nozhkin

(Submitted by the Department of Mineral Resources)

In studying the mineralogy of one of the hydrothermal ore occurrences of the Yenisei Ridge, two morphological varieties of pyrite were encountered, differing in crystal form, in certain properties in reflected light, and also in their content of impurity elements. Both varieties are observed as uniform disseminations in hydrothermally altered sandstones and gravelites of Lower Cambrian age.

The first of them is represented by small (0.2–0.5 mm) crystals having the form of an octahedron, the apices of which are often blunted by faces of a pentagonal dodecahedron (Fig. 1). The crystal faces are smooth, without the slightest signs of striation. In comparison with ordinary pyrites, the variety described possesses a reduced polishing hardness, as a result of which aggregates in polished sections always have a smooth surface, without scratches or pits. In reflected light the mineral has a yellowish-white color with a faint greenish tint, clearly established at the contact with grains of pyrite of the other variety. In crossed nicols it polarizes noticeably in yellowish and gray tones. It should be emphasized that the greenish tint and noticeable polarization are specific properties of the pyrites described. The internal structure of the crystals is zo-

Fig. 1. Crystals of the first variety of pyrite:
1 — octahedron;
2 — octahedron (111), blunted by faces of the pentagonal dodecahedron (210).
Crystals of the second variety of pyrite:
3 — pentagonal dodecahedron;
4 — combination of pentagonal dodecahedron with cube (100).

... zonal. Distinct zoning is usually detected immediately, without preliminary etching. The outer zones, as a rule, repeat the outlines of the inner ones. Such zoning is the usual growth zoning, reflecting, according to existing views [6, 9, 10, 12], a successive change in the external environmental conditions.

To refine the composition of the mineral, microchemical tests were carried out for a number of elements; as a result, the presence of arsenic, cobalt, and nickel was established. The microchemical data were confirmed by chemical analyses performed in the laboratory of the Tomsk Complex Expedition of TPI (Table 1).

Table 1

The elevated arsenic content, amounting to as much as 3.21%, is noteworthy. The maximum arsenic content in pyrite noted in the literature is 2.7% [5, 6].

The second variety of pyrite occurs as larger (0.5–1 mm) crystals having the form of a pentagonal dodecahedron or a combination of a pentagonal dodecahedron with a cube (Fig. 1). In the latter case, the faces of the pentagonal dodecahedron determine the habit of the crystals. On the cube faces there is coarse step-like, sometimes fine and intense, combination striation.

According to V. I. Vernadsky [3], G. K. Abdullaev [1], and a number of other investigators, the appearance of combination striation is most characteristic of pyrites of hydrothermal origin and is connected with the physicochemical conditions of the mineral-forming solutions; in particular, it depends to a considerable degree on the quantity and character of impurities in the solution. S. A. Stroitelev [8] experimentally proved the connection of combination striation with rapid changes in crystallization conditions during the period of mineral formation.

Under the microscope it is evident that, compared with the first variety, this variety of pyrite has a higher relief and polishing hardness—the surface of the grains is slightly rough. The color is whitish yellow with a pink tint. In oil, under strong illumination, weak anisotropy is noticeable. Electrolytic etching does not reveal zoning. Where both varieties occur together, overgrowth of pyrite of the second variety on crystals of the first can be observed.

The chemical analyses performed (Table 1) reveal arsenic, cobalt, and nickel in concentrations different from those established in the first variety of pyrite. Here the arsenic content is lower, while that of cobalt and nickel is somewhat higher.

It is characteristic that these elements are found in appreciable amounts by spectral analysis in hydrothermally altered green-colored sandstones and, with the exception of nickel, are entirely absent in unaltered red-colored formations. The latter indicates the introduction of arsenic and cobalt by hydrothermal solutions.

Thus, the material studied shows that the two described varieties of pyrite, which differ morphologically, in their optical properties, and in the amount of impurity elements, represent two generations of different ages, of which the first is the earlier. At the same time, judging from the identical suite of impurity elements, the time gap between them was insignificant—the composition of the solutions changed little.

The presence of impurity elements in pyrites is also reflected in certain features of their properties. In particular, they differ in polishing hardness, shades of color, and anisotropy. In turn, the difference between these varieties in the quantitative content of impurity elements, together with other physicochemical conditions, determines the specificity of the crystallographic habit and of certain physico-optical properties of each of them.

It should be emphasized that the dependence of the crystallographic forms of a mineral on the content of impurities has been noted by a number of researchers [2, 4, 7]. However, information on the influence of impurity elements on the optical and other properties of minerals and, in particular, of pyrite is insufficient. There are, for example, indications by F. N. Shakhov [11] of anisotropy in colloform pyrites containing arsenic.

A change in the color and hardness of pyrite due to the presence of nickel in it is noted in the mineralogical handbook [6].

LITERATURE

1. G. K. Abdullaev. Striation on the faces of pyrite crystals. Zap. Vses. Min. ob-va, part 88, issue 4, 1959.

2. G. G. Bakli. Growth of Crystals. GINL, Moscow, 1954.

3. V. I. Vernadskii. On combinational striation of crystal faces. Izv. AN SSSR, vol. I, No. 1, 1907.

4. V. D. Kuznetsov. Crystals and Crystallization. Moscow, 1953.

5. Minerals of the USSR. Vol. 2, Publishing House of the Academy of Sciences of the USSR, 1940.

6. Minerals. Handbook, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1960.

7. I. N. Sitkovskii, G. K. Abdullaev. Morphological features of pyrite crystals from the Bitti-Bulak deposit. Zap. Vses. Min. ob-va, part 86, issue 4, 1957.

8. S. A. Stroitelev. On the causes of formation of combinational striation of crystals. Zap. Vses. Min. ob-va, part 90, issue 6, 1961.

9. E. V. Frantskaya. On the internal structure of pyrites from the gold-ore deposit in the Eastern Transbaikal region. Zap. Vses. Min. ob-va, part 85, issue 2, 1956.

10. V. K. Cherepnin. On the zonality of growth of pyrite grains in the Ursk deposits of Salair. Zap. Vses. Min. ob-va, part 86, issue 4, 1957.

11. F. N. Shakhov. The Principal Ore-Forming Minerals. Publishing House of ZSGU, 1941.

12. P. Ya. Yarosh. Some details of the internal structure of pyrite grains from vein deposits of the Urals. Zap. Vses. Min. ob-va, part 82, issue 4, 1953.