Clay minerals from aluminous gneiss weathering crust of the Estonian crystalline basement

At NE part of Estonian crystalline basement in the complex of Alutaguse zone the most developed types of rocks are high-alumina gneisses. The thickness of their crust of weathering fluctuates in wide range from 4.5 m to 125 m and more in dependence of fissility of rocks. The X«»ray powder diffraction data show the most distributed clay minerals in fraction of about 0.001 mm in the crust of weathering are kaolinite and mixed-layered montmorillonite-hydromica. Less distributed are hydromica, chlorite, montmorillonite and montmorillonite-chlorite. In rare cases a chamosite occured.
The following zonal occurance of clay minerals in the crust of weathering is observed from down to up: 1) a zone of montmorillonite-hydromica with a kaolinite, 2) a zone of kaolinite. Such a zonation is breaked by hydrothermal developing of chlorite and hydromica and by developing of hydromica and chamosite upon influence of a transgressing basin on the crust of weathering.