On the Process of Pisolitization in Clays of the Bauxite-Bearing Strata (Western Kazakhstan)

A. N. Kondakov

(Presented by Prof. A. M. Kuzmin)

The origin of the pisolitic structure of bauxites is an essential point in understanding the bauxite-forming process. A. K. Sharov and A. K. Gladkovskii [3] attribute an allothigenic origin to pisolites, based on observations of the sorting of pisolites, oblique bedding, and the association of pisolites with fragments of other rocks. F. G. Pasova [2] regards pisolites as a product of diagenesis in a quiet silty environment, combined with agitation and redeposition of material. From the latter point of view, the formation of pisolites is governed by the physicochemical conditions of the environment, which in the present case lead either to kaolinization of the original rock or to its allitization. In this sense, the appearance of a pisolitic structure in the clays of the bauxite-bearing strata, which show no signs of secondary kaolinization of the original bauxitic material, may be used to explain the origin of pisolites in bauxites.

Observations were made in the worked-out quarry of the Kozyrevka mine. At the base of the clayey strata lying beneath the stony bauxites, all the signs are observed of a gradual redistribution of ferruginous matter, from a rather uniform distribution to its concentration into pisolites (Fig. 1). The parent rock is apparently composed of redeposited products of a weathering crust of kaolinite–beidellite composition (interpretation of thermal analysis and dehydration curves of the samples according to E. A. Shurygina [1]). It is represented by brownish-red and brownish-brown plastic, viscous clay with loop-like impregnating segregations of limonite and hydrohematite. By gradual transitions the rock is connected with plastic and viscous clay, but of a lighter color, with mottled brownish-yellow and brownish-red pigment. In turn, the latter passes into a light brownish-yellow mottled clay of beidellite–kaolinite composition, with broad development of diffuse, thin ferric-hydroxide Liesegang rings. Occasionally, isolated pisolites of loose brownish-brown hydrated iron oxide are observed in it. In iron content the rock differs little from the original clay (Table 1). Iron undergoes local redistribution and a mineral transition to a hydroxide composition. Simultaneously with iron, calcium, magnesium, manganese, and strontium are removed (Tables 1 and 2). In clays of transitional character, separate areas of pisolitic structure are observed. The described rocks

by gradual transitions with spotted bleached clay of bean-like structure, lighter and more porous, of kaolinitic composition. In the clay, very faintly colored brownish-yellow Liesegang rings are abundantly developed. The pisolites range in size from 2–3 to 15 mm. They consist of a loose earthy substance of hydrogoethite–hydrohematite composition and have both sharp boundaries, close to circular, and indented “bays.” A zonal structure is not discernible in the pisolites—

Fig. 1. Gradual character of the change of the original clayey rock into clay of bean-like structure (sketch of an outcrop):
1 — deluvial dump; 2 — brownish-black scoriaceous ferruginous rock; 3 — plastic viscous clay impregnated with brownish-red and brownish-brown iron compounds; 4 — plastic clay with a complex looped pattern of impregnation by brownish-yellow iron hydroxides; 5 — brownish-yellow light spotted clay with Liesegang rings of iron oxide hydrate and rare, unevenly distributed pisolites; 6 — patches of bleaching with a bean-like structure; 7 — light porous bleached clay of homogeneous bean-like structure with brownish-yellow light bands of Liesegang rings; 8 — sampling sites and their numbers.

except for those pisolites in which the core is made of bleached clayey material. Sometimes, in large pisolites, small pisolites of the second order are found. In chemical composition the rock

Table 1

Results of chemical analysis and measurement of pH and Eh of the corresponding samples

is characterized by further removal of iron, calcium, magnesium, zinc, and by a noticeable accumulation of copper. The iron content changes only slightly (Table 1), and at the same time the iron is concentrated in the pisolites in a new mineral form (hydrohematite).

Simultaneously with the change in the chemical and mineral composition of the rock, a regular change in the physicochemical conditions is observed (Table 1), established by measuring the oxidation-reduction potential and pH of moisture-saturated samples according to the generally accepted method.

The material presented confirms the determining role of physicochemical conditions in the formation of the mineral composition and final structure of the rock. Certain physicochemical conditions, in the bauxite-ore process as well, lead to a redistribution of matter into pisoliths, whose mineral composition corresponds to the conditions of the environment, just as is observed in clays of pisolitic structure.

Table 2

Results of semiquantitative spectral analysis, in percent

Note: The table gives the average content of elements from three analyses of each sample.

LITERATURE

1. N. I. Gorbunov, I. G. Tsyurupa, E. A. Shurygina. X-ray patterns, thermograms, and dehydration curves of minerals occurring in soils and clays. Publishing House of the Academy of Sciences of the USSR, 1952.

2. F. G. Pasova. On the conditions of bauxite formation. In: Bauxites, Their Mineralogy and Genesis. Publishing House of the Academy of Sciences of the USSR, 1959.

3. A. K. Sharova, A. K. Gladkovskii. Mineral composition, origin, and alteration of Lower Carboniferous bauxite of the eastern slope of the Urals and the Turgai trough. In: Bauxites, Their Mineralogy and Genesis. Publishing House of the Academy of Sciences of the USSR, 1959.