About the mineralogy of the crust of weathering of the Estonian crystalline basement
Aasta | 1971 |
---|---|
Pealkiri originaal | O минералогии коры выветривания кристаллического фундамента Эстонии |
Kirjastuse koht | Tartu |
Ajakiri | Tartu Riikliku Ülikooli Toimetised |
Köide | 286 |
Leheküljed | 52-163 |
Tüüp | artikkel ajakirjas |
Joonised | 40 |
OpenAccess | |
Eesti autor | |
Keel | vene |
Id | 3884 |
Abstrakt
For the first time a generalized account is given of the mineralogy of the crust of weathering of the crystalline basement on the basis of investigation with X-ray methods. In addition the results
of microscopic observations are used. The investigation was carried out in the Cabinet of Mineralogy of the Tartu State University in the course of contractual work. The results of the X-ray analysis of 350 hand specimens from 30 core samples were used (Fig. 1). The data of a few early Debye-Scherrer patterns showed that the main minerals in the crust of weathering of the crystalline basement are kaolinite and hydromica. However, this work brings out a large number of new clay minerals. The occurrence of mixed-layered clay minerals with a various degree of order such as montmorillomte hydromica, montmorillonite- chlorite and montmorillonite, chlorite, chamosite, sepiolite, vermiculite and talc was established. This work consists of two parts. In the first part a review is given of the more thoroughly investigated sections of the crust of weathering of the crystalline basement and the second part deals with the mineralogy and genesis of clay minerals. A scale of 0, I, II and III is used for characterizing the hypergene changes. 1) Degree 0 denotes a practically fresh rock in which hypergene changes are observed as separated little nests or on the planes of crevices. 2) The rocks of Degree I have preserved their original texture and structure; the minerals are partially decomposed. 3) The rocks of Degree II consist mainly of clayey productsof chemical weathering with small amondts of residual and resistant minerals. 4) A clayey rock which has fully lost the structure and texture of the source rock belongs to Degree III. Every concrete rock specimen can be classified in terms of the degrees referred to above. In every section the characteristic type of the crust of chemical weathering is denoted by using the terms. The sections of the crust of weathering are divided into the following 9 groups according to their rock content: I — biotite g ra nites, II — biotite-feld'spar-gneisses, III — gneisses with a high content of aluminium oxide, IV — amphibolites and gneisses-shales with a basic content, V — gabbroes, VI — serpentinites, VII —magnetitie quartzites, VIII — sections with variable rock types and IX — sections where later hydrothermal changes are clearly observed. In every group of rocks at least more thoroughly investigated section is given and they are illustrated with graphic ma terial (petrographical characterization, quantitative rations of clay minerals, localities of sample collection). In the case of more interesting sections typical difractograms of clay minerals are given. The chapter on the mineralogy of clay minerals comprises the analysis of probability of X-ray identifications and a review about the occurrence of clay minerals (Fig. 23). The density of occurrence of clay minerals can be arranged in the following order: kaolinite, hydromica, chlorite, montmorillonite-hydromica, chamosite and sepiolite; vermiculite and talc are met with in some cases. In the crust of weathering of the crystalline basement clay minerals occur rarely in a pure form; usually they constitute mixtures of two or three components. In most cases a regularity of locating can be observed which is expressed as a zonal occurrence of clay minerals (up to 5 zones ). The occurrence of clay minerals is characterized graphically in accordance with their quantitative content, the degree of chemical weathering of a rock and natural associations (Figs. 24, 25, 26
and Table III). A few clay minerals are roentgenographically described both by means of diffractometrical curves and on the basis of the data of Debye-Scherrer patterns. In the same place an attempt is made to bring out the specific features of the structure of clay minerals in connection with macroscopic and microscopic observations. On the basis of petrographic investigations a short review of the genesis of clay minerals is given. In many cases a microscopic characterizationof clay minerals and marks for their diagnosis are given. At the end of this work a review is given of the later changes
of clay minerals. The following four categories of change are discussed: 1) The possibility of epochs of chemical weathering with different ages. 2) The influence of a transgre s s ing basin on the crust of weathering (the formation of hydromica and chamosite). 3) Possible changes induced by underground waters of clay minerals buried in the crust of weathering. 4) Hydrothermal changes that have had an influence on the crust of weathering. The categories 2 and 4 are more fully discussed.
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