On the mineralogical zoning of the crust of weathering of the Estonian crystalline basement

In the present work the mineralogical zoning of the crust of weathering of the Estonian crystalline basement is characterised. The factual data used consist of the results of an x-ray investigation of 753 samples from 29 boreholes (Figs. 1 and 2). The clay minerals in the fraction with a grain size less than0 .0 0 1 mm can be classified into four groups on the basis of the quantitative phase analysis: 1 ) kaolinite (dominating); 2 ) montmorillonite-hydromica, chlorite and hydromica (quite widespread); 3)trioctahedral montmorillonite (it is a common mineral in thelower part of the crust of weathering of baik rocks; 4) montmorillonite-chlorite (an exotic mineral in the lower part of the crust of weathering of basic rocks). When the degree of weathering is considered, the regularity in the occurrence of clay minerals in the crust of weathering becomes evident both in a given section and as well statistically. Mineralogical zoning is well observed in the Otepaa 2, Tapa, Kaagvere l.Sigulda F 124, Vanakiila F 130 and in some other  borehole sections of the crust of weathering. To clearing the general trends in the vertical allocation of clay minerals in sections a graph was drawn (fig. 3), which characterizes the frequency of occurrence of clay minerals in terms of a rock type, the degree of weathering and the quantitative content. The sections of the crust of weathering are divided into 3 groups according to their compositional rocks: 1 ) acid rocks (granites, granite-gneisses, biotite-faldspar-gneisses), 2 ) rocks with an intermediate compositio n (biotite-amphibole-gneisses, gneisses with a high content of aluminium oxide) — these rocks approximately correspond to andesite-diorite — and 3 ) basic rocks (amphibolites, amphibole-pyroxene-gneisses, gabbroes and serpentinites). The idealized profiles of the crust of weathering of these rock groupes as well the ones referred to particular boreholes are given in Fig. 4. It can be seen that montmorillonites, montmorillonite- chlorites, montmorillonite-hydromica and kaolinite can be characterized with certain places in the sections. Due to the great variety of polygenetic forms of hydromica and of chlorite, the occurrence of these minerals is not so distinctly characterizable. Four clay minerals were identified in the crust of weathering of the acid rocks: montmorillonite-hydromica (with a sporadic distribution), chlorite, hydromica and kaolinite. A zonality consisting of three parts is distinguished (from bottom to top): 1 ) hydromica-chlorite, 2) hydromica-kaolinite, 3) kaolinite. Along with chlorite, hydromica and kaolinite, the characteristic
minerals in the section of the crust of weathering of the rocks with arn average composition are montmorillonite-chorite, montmorillonite-hydromica and montmorillonite (exotically). The following
mineralogical zones can be distinguished (from bottom to top): 1 ) montmorillonitic-chloritic or hydromicaceous-montmorillonitic-chloritic , 2 ) kaolinitic-montmorillonitic-hydromicaceous, 3) kaolinitic.
In the crust of weathering of the basic rocks trioctahedral montmorillonite is always present. The following mineralogical zones can be distinguished in the crust of weathering (from bottom to top): montmorillonitic, hydromicaceous-montmorilloniticchloritic, kaolinitic-montmorillonitic-hydromicaceous and kaolinitic. Some circumstances leading to mineralogical zoning such as the composition of rocks, tectonic splitting etc. are discussed in more detail.