Mineral Composition of Pelitic Sediments in the Rhenohercynian Zone

Abstract

Pelitic sediments of the Rhenohercynian Zone were examined with regard to mineral composition by infrared spectroscopy. This method offers the possibility for exact and rapid quantitative analyses of sedimentary rocks containing illite, chlorite, kaolinite, quartz, feldspars and calcite. Ey their clay mineral/(quartz + feldspar)− ratios and their carbonate contents pelitic rocks were classified into three main types: common shales (215 samples), silty shales (131 samples) and calcareous shales (35 samples).

For each class the average mineral composition was determined. Thus, the average mineral composition of common shales over the whole stratigraphic sequences in the Phenohercynian basin comes to 60 % clay minerals (46 % illite, 13 % chlorite, 1 % kaolinite), 35 % quartz, 4 % feldspars, 1 % carbonate.

It has been found, especially for the common shales, that the average total clay contents in nearly all stratigraphic units remain more or less constant. Because the clay mineral fraction is predominantly composed of illite and chlorite which show a reciprocal development with time, an increase in chlorite is always compensated by a decrease in illite and vice versa. This is caused largely by different primary MgO contents in the stratigraphic units. In contrast to the MgO poor Ordovician and Gedinnian sediments, the higher MgO contents in the Upper Devonian sediments produced in the course of diagenesis and anchimetamorphism relatively more chlorite and less illite with a stronger phengitic character.