SOME RESULTS OF ELECTRON-MICROSCOPIC STUDY OF THE COAL-BEARING STRATA OF THE KANSK-ACHINSK BASIN

V. L. Kokunov

(Presented by the scientific seminar of the Department of Combustible Minerals)

Clayey rocks make up from 40 to 60% of the section of the coal-bearing strata of the Kansk-Achinsk brown-coal basin and, in addressing questions of the genesis of coal-bearing deposits, naturally attracted our close attention.

The mineral composition of clays and argillites was studied by us with the aid of an electron microscope in combination with staining of clays by organic dyes and electron diffraction. The considerable number of samples and the short time available for carrying out the work to some extent predetermined the choice of research method; therefore it was necessary to choose the simplest method, quite suitable for mass determinations: the method of viewing suspensions under an electron microscope with subsequent staining of the suspension with MG.

Preparation of suspensions from the original samples for electron-microscopic study was carried out according to the method of M. F. Vikulova (1957) and consisted of the following. A piece of rock weighing about 5 g was covered in a porcelain mortar with distilled water and left for 24 hours, after which the transparent layer of water above the sample was decanted to remove electrolytes; fresh water was added, two or three drops of ammonia were added to improve dispersion, and the clay was brought into a suspended state by rubbing with a rubber finger. The resulting suspension was poured into a test tube; the test tube was shaken several times and left for 24 hours to settle so that fractions \(>0.001\) mm would settle to the bottom of the test tube. After settling, the upper layer of suspension, 5–7 cm high, with particles \(<0.001\) mm, was poured into a clean test tube and diluted with distilled water to the required concentration (weak opalescence). A drop of this suspension was placed on a grid support film and, after drying in a thermostat, was examined under an electron microscope at a magnification of 6–12 thousand times.

The remaining suspension was stained with 1 cm³ of 0.01% MG and divided among three test tubes. Into one, after 2–4 hours, another 0.5 cm³ of MG was added; into another, 4–5 drops of KCl; the third was left unchanged. After 24 hours the staining of the suspension was observed against a white background in daylight. The color of the suspension was determined by comparison with the ten-color standard scale of the All-Union Geological Institute (VSEGEI).

The final diagnosis of the mineral composition of the studied samples was carried out according to electron-microscopic descriptions

taking into account the staining results, which made it possible to diagnose the minerals with greater confidence.

In all, we studied about 500 samples of clayey rocks; as a result of processing them, both monomineral clays and polymineral clays with characteristic mineral associations of clay minerals were identified. For the most characteristic minerals, electron-diffraction photographs were taken and the parameters of the elementary cells were calculated, while electron-microscopic photographs were made for each sample.

In the section of the coal-bearing Jurassic, a rather rich complex of clay minerals was encountered; the characteristics of the individual mineral species are given below in order of the frequency of their occurrence.

Kaolinite has the widest distribution in the section of the coal-bearing sequence and, according to the degree of destruction of the crystallographic outlines, can be divided into 4 groups.

The first group is represented by well-crystallized pseudohexagonal plates of isometric or elongated form, with distinct outlines and sizes from 0.15 to 4 microns. The plates have a uniform thickness, but one that differs among different particles. All faces are intact (Fig. 1).

The second group is also represented by well-crystallized pseudohexagonal plates, but with only 1–3 preserved crystal faces. The remaining faces have been destroyed during transport and redeposition. The particles likewise have a uniform thickness; usually they are black or semitransparent (Fig. 2).

The third group is represented by fragments of kaolinite crystals that have completely lost their crystallographic outlines (not a single face is preserved). The sizes and shapes of the particles vary; most often, however, this group of kaolinite-crystal fragments has sizes from 0.1 to 1 micron. The boundary lines of the particles are distinct; the angles are usually rounded. This group of kaolinite grains has undergone more distant transport or repeated redeposition (Fig. 3).

Finally, the fourth group of kaolinite grains has the appearance of particles of various sizes and shapes that have partly lost the sharpness of their boundaries. The loss of sharpness of the boundaries, in the opinion of M. F. Vikulova (1957), I. D. Zkhus (1959), and others, reflects the initial stage of qualitative alteration of kaolinite and its transition into other minerals—beidellite and montmorillonite.

Hydromica is, no less than kaolinite, a widespread mineral in the clayey rocks of the coal-bearing Jurassic.

Electron-microscopic observations showed that hydromica encountered in the section of the coal-bearing sequence can be subdivided into 4 varieties or groups.

The first group is represented by leaves of various sizes and shapes, possessing nonuniform thickness; moreover, the nonuniformity is manifested sharply, in a stepped manner. Usually the greatest thickness is at the center of the leaf, and it is less toward the edges (Fig. 4-I).

The second group of hydromicas is characterized by scales, usually of small size (up to 1 micron) and of varied form, with poorly expressed nonuniform thickness, as shown in Fig. 4-II. Sometimes large scales of hydromica occur, up to 5 microns in size, having uniform thickness. In such cases they are usually semitransparent and give a spot electron-diffraction pattern characteristic of single crystals.

Fig. 1.

Fig. 2.

Fig. 3.

The third group of hydromicas is represented by flakes of various sizes and shapes which, unlike those of the preceding groups, have lost the sharpness of their boundaries.

The fourth group of hydromicas is encountered rarely and is represented by strongly elongated narrow plates of unequal thickness. In the opinion of D. D. Kotelnikov (1962), this variety of hydromica is of authigenic origin.

With methylene blue, hydromica is stained in blue or violet-blue tones; after the addition of KCl it either does not change color, or changes to blue-light-blue. Dried drops of the sediment of the stained suspension are characterized by light-blue-green tones under standard staining (0.7–1.2 cm³ MG) and by blue tones under intensive staining (1.5 cm³ MG).

Fig. 4—I–II.

Montmorillonite has a less wide distribution than kaolinite and hydromica, and in the section of the Rybinsk depression it is encountered rarely; in the Nazarovo depression, however, this mineral forms part of the clayey rocks of the Makarovskaya suite and the coal-barren subsuite of the Itatskaya suite. It is characterized by flakes and plates of irregular form, with blurred, entirely indistinct outlines. Usually these flakes and plates form lumpy aggregates of cloudy character.

With methylene blue this mineral gives a blue-light-blue to light-blue-green coloration, which after the addition of KCl changes to green. Drops of the sediment of the stained suspension, after drying, acquire a blue-light-blue to green coloration.

Montmorillonite is also confirmed electronographically, by calculation of the unit-cell parameters from electron diffraction patterns.

Accessory minerals. In addition to the three minerals listed above, which are widely developed in the section of the coal-bearing strata, the study of clayey rocks noted the presence of a whole series of other minerals, usually entering into the composition of clays as accessories. These include allophane, dickite and halloysite, monotermite and an Mg-silicate mineral (beidellite), as well as palygorskite and sepiolite. Of the listed accessory minerals, only dickite has been confirmed electronographically; the remaining minerals were identified in the course of electron-microscopic observations and from staining data.

In the described deposits, in the fraction with particle sizes < 0.001 mm, together with clay minerals, non-clay terrigenous and authigenic minerals also occur as accessories and separate inclusions—quartz, pyrite, calcite, chlorite, iron oxides, siderite, and siliceous formations. These minerals were distinguished in the electron-microscopic study of the rocks by the shape of grains characteristic of these minerals; pyrite is also confirmed electronographically.

In concluding the characterization of the clay minerals studied, it should be emphasized that the overwhelming majority of them are terrigenous, as is convincingly shown by the fact that we are most often dealing with fragments of minerals. This is a very important conclusion,

makes it possible to explain the regularities in the distribution of mineral types of clays in the section of the coal-bearing strata of the basin, as will be discussed below.

In the Jurassic deposits of the Rybinsk and Nazarovo depressions, monomineral clayey rocks occur rarely. In most cases the clayey rocks here represent one or another association of clay minerals, i.e., they are represented by polymineral clays.

Let us give a brief characterization of the mineral composition and distribution, in the section, of the types of clays.

More or less monomineral clays occur in the section of the Jurassic of the Rybinsk and Nazarovo depressions at least twice—in the base of the Lower Jurassic deposits and in the base of the Middle Jurassic deposits. At the same time, monomineral clays at the base of the Middle Jurassic deposits are developed not only in the northern part of the Rybinsk depression, as was shown by V. A. Borisov (1961), who linked their genesis with redeposition of the weathering crust of the Yenisei Ridge, but also in the central and southern parts of the Rybinsk and Nazarovo depressions. As a rule, they are represented by kaolinite–monothermite clays, more rarely by montmorillonite clays; moreover, electron-microscopic observations showed that the clay-mineral particles composing them are almost always strongly destroyed, bear traces of transport and redeposition, and do not have the well-crystallized kaolinite and dickite characteristic of weathering crusts in situ. In appearance these are light-gray, almost white, fairly dense clays, deserving attention as raw material for the refractory industry. Montmorillonite clays, unlike kaolinite–monothermite clays, have a distinctly expressed greenish tint and can be successfully used for preparing high-quality drilling muds. The confinement of monomineral clays to age boundaries makes it possible to use the geological map to delineate the areas of their distribution.

As already noted above, the widest distribution among the clayey formations of the coal-bearing Jurassic is enjoyed by polymineral clays, representing one or another association of clay minerals. In their composition we distinguish hydromica–kaolinite clays with kaolinite predominating over hydromica, and kaolinite–hydromica clays with hydromica predominating over kaolinite.

The listed types of clays are distributed in the section of the coal-bearing Jurassic of the Rybinsk, Nazarovo, and Kansk-Taseevo depressions in the following way.

At the base of the Lower Jurassic deposits, as a rule, monomineral, usually kaolinite, clays are distributed, or clays with a hydromica–kaolinite association of clay minerals. Higher in the section, an ever greater amount of hydromica appears in the clays, and still higher there appear mixed-layer minerals or minerals of the montmorillonite group.

The same sequence in the distribution of mineral types of clays in the Middle Jurassic deposits is repeated three times in the section of the Middle Jurassic of the Rybinsk and Nazarovo depressions. This sequence is especially clearly revealed at the Aban deposit.

This is, undoubtedly, a reflection of the cyclicity of sediment accumulation, which in turn reflects the history of tectonic movements in the source areas.

The most developed current conception of the process of formation of sedimentary series is the one clearly formulated in the works-

Fig. 5. Direct lithotype of the suite.

Layering of quartz sandstones and kaolinite clays (argillites) with coal seams.

Layering of feldspar–quartz sandstones and hydromica–kaolinite clays (argillites) with coal seams.

Layering of quartz–feldspar sandstones and kaolinite–hydromica clays (argillites).

Layering of arkosic sandstones with beds of conglomerates, gravelites, and hydromica clays (argillites).

Fig. 6. Reverse lithotype of the suite.

Layering of arkosic sandstones and hydromica clays (argillites).

Layering of quartz–feldspar sandstones, coals, and kaolinite–hydromica clays (argillites).

Layering of feldspar–quartz sandstones, coals, and hydromica–kaolinite clays (argillites).

Layering of quartz sandstones and kaolinite clays (argillites) with coal seams.

works of V. P. Kazarinov (1962), the doctrine of sedimentary series distinguished by lithologic-facies analysis (Fig. 5).

The process of formation of such a lithotype of a suite is a consequence of the destruction of a newly formed, uplifted source area. But in our case we are dealing with an area in which a thick weathering crust developed, caused by a break in the accumulation of sediments, beginning at the end of \(C_3\) and through \(T\) inclusive. This predetermines the reverse order in the deposition of clay minerals, as shown in Fig. 6.

In an epoch of intensive erosion of a thick weathering crust, kaolinite is first removed and deposited at the base of the redeposited sediments. Then, as the source area approaches the lower boundary of the weathering mantle, hydromicas and even partly decomposed feldspar and micas begin to be deposited in the area of sedimentation.

It is precisely such a sequence in the clay deposits of the coal-bearing strata, forming reverse lithotypes, that we observe in the Kansk-Achinsk basin.

This makes it possible to use the results of electron-microscopic studies for purposes of stratigraphic correlation both of sections of closely spaced boreholes and of Jurassic sections isolated from one another.

Thus, electron-microscopic study of clay formations can be used for the following purposes:

1. Establishing the mineralogical composition of clay rocks.
2. Reconstructing the conditions of formation (paleogeographic conclusions).
3. Distinguishing lithotypes of suites and correlating sections on their basis.

LITERATURE

V. A. Borisov. Quartz-kaolin formations of the northern part of the Rybinsk depression. Materials on the Geology of Krasnoyarsk Krai, issue 2, 1961.

M. F. Vikulova. Methodological guide to petrographic-mineralogical study of clays. Gostoptekhizdat, 1957.

I. D. Zhuk. Clay minerals of the coal-bearing deposits of the southwestern part of the Moscow Basin. Soviet Geology, No. 46, 1959.

V. P. Kazarinov. Sedimentary series and tectonic phases of the Paleozoic of Siberia. Transactions of SNIIGGiMS, issue 16, 1962.

D. D. Kotelnikov. On the relationship of morphological features of clay minerals to the conditions of their formation in sedimentary rocks. Doklady of the Academy of Sciences of the USSR, vol. 146, No. 4, 1962.