PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 127, issue 1

1964

ON THE METHODOLOGY OF ELECTROLYTIC STRUCTURAL ETCHING OF IRON-CONTAINING SULFIDES

V. I. BAZHENOV

(Presented by Prof. A. M. Kuzmin)

The importance of the method of structural etching of ore minerals in polished sections for revealing the structure of monomineral mineral aggregates, and also for establishing details of the internal structure of mineral grains, was noted as early as 1941 by A. G. Betekhtin and was emphasized by him later [1, 2]. For the structural etching of most sulfides, various reagents and their mixtures are used; their action leads to the removal of the thin amorphous film that forms on the polished surface of a polished section during its preparation and masks the internal structure of mineral aggregates and grains. But for the etching of some minerals no reagents have been found that give satisfactory results.

The method of electrolytic structural etching is usually applied to the etching of pyrite. We used this same method for etching other iron-containing sulfides as well: arsenopyrite, chalcopyrite, and pyrrhotite. As the current source, a pocket-flashlight battery with a voltage of 3.7 volts was used. The electrolyte was a 25–30-percent ammonia solution, which is applied in the form of a large drop to the polished surface of the polished section. A steel electrode (a steel needle), connected to the positive pole of the battery, is pressed against the sulfide outside the drop. A platinum electrode, connected to the negative pole of the battery, is introduced into the drop of electrolyte without touching the polished surface of the polished section. The completion of etching is determined visually by the appearance of a colored film. After etching is completed, the drop of ammonia is washed off with water and the section is dried in air. As a result of etching, a thin colored film appears, whose composition probably corresponds to a hydrate of iron oxide. The thickness, and consequently the color, of the film depends on the optical orientation of the mineral grains, on differences in chemical composition in different parts of the grain, etc. Therefore differently oriented grains, twins, and the zonal structure of grains are clearly distinguished by the color of the colored film. It should be borne in mind that the resulting film is very easily wiped off, and therefore the etched surface must under no circumstances be cleaned with cloth or have water removed from it with filter paper. After the etching results have been recorded, the film is easily removed with cloth.

Electrolytic etching of pyrite by the method described has been known for a long time. When it is etched for 60–80 seconds, the granular structure of the aggregates is established; in individual pyrite grains, growth zoning, colloform structure, etc., are revealed. As a result of etching, a breccia-like structure of pyrite aggregates is often manifested, caused by the presence of angular fragments of pyrite of an early generation cemented by later pyrite.

Arsenopyrite is etched by the method described for 1.5–2.0 minutes. In the polished sections studied, this reveals the granular character of the aggregate. In etched polished sections one can often see excellent growth zoning in the form of a series of bands concentric and parallel to one another and to the outer boundaries. Cataclasis is noted in some grains.

Pyrrhotite is etched more readily than all the other sulfides. With the etching procedure described, it is etched quite satisfactorily within 30–40 seconds. As a result of etching, the granular structure of aggregates is revealed in pyrrhotite. In the specimens studied, glide twins were also clearly distinguished.

It should be noted that in polymineral aggregates where, along with pyrrhotite, other iron-bearing sulfides are also present (pyrite, arsenopyrite, chalcopyrite), only pyrrhotite is subjected to etching. The remaining sulfides are attacked only when the pyrrhotite proves to be strongly overetched, while the etching time for pyrite, arsenopyrite, and chalcopyrite increases by a factor of 1.5–2.0.

Chalcopyrite is electrolytically etched for 1 minute. In etched polished sections, the granular structure of the aggregate is manifested in it. In many cases, polysynthetic twins are revealed in the grains.

When sulfides are etched, false etching effects sometimes arise [2], in the form of rounded spots caused by air bubbles adhering to the surface of the polished section. In some cases, during etching, a fine-grained structure is revealed within one and the same grain; against this background, both twinned and zonal structure of large grains is manifested. This phenomenon may mistakenly be taken for a process of recrystallization of the mineral. But it is undoubtedly a false etching effect. Its formation is apparently associated with the disintegration of the film into small polygonal areas as it dries.

A shortcoming of the etching procedure described is a certain limitation of its application. The presence in the ore of secondary minerals (limonite, chalcocite, covellite, etc.) interferes with the normal course of etching and makes it impossible to reveal the structure of mineral aggregates and grains.

The application of the described procedure of structural etching will probably also prove possible for other iron-bearing sulfides: marcasite, lellingite, and others.

REFERENCES

1. A. G. Betekhtin. On structural etching of polished sections. Tr. IGN AN SSSR, issue 41, ser. rudn. m-nii, No. 5, 1941.
2. A. G. Betekhtin. Textures and Structures of Ores. Publishing House of the Academy of Sciences of the USSR, 1958.