Zur Feldspatverwitterung und ihrer Bedeutung für die Tonmineralbildung

Abstract

Weathering of feldspars and their importance for the formation of clay mineralsIn quaternary sediments weathering and transformation of layer silicates, and in particular of biotites play a dominant role in the formation of clay minerals in soils of the moderately (semi) humid climate. In contrast, conflicting results were published concerning the participation of feldspar weathering for pedogenetic clay mineral formation. Another question is, whether formation of clay minerals in the fractions 0,6–0,2 μ m and<0,2μ m by chemical weathering of micas and feldspars is the dominating process, or the formation of „pseudo clay minerals”︁ in the fraction 2–0,6 μ m by cryoclastic desintegration of micas and feldspars, as proposed by several authors. It is not yet clear, which clay minerals are formed from the weathering products of feldspars. — A necessary condition for a conclusive study of these problems is the vertical sedimentary homogeneity of the soil forming material.In eight loess soils investigated of very different genesis (a syrosem, chernozems, brown forest soils and a „braunlehm”︁) there were no indications for clay formation in the coarse clay fraction (2–0,6μm) by physical weathering of feldspars. This was also true for the loess‐syrosem of young Würm age, where the former climatic conditions should have promoted this process.In the chernozems, the brown forest soils and the braunlehm the proportion of illite and Smectite newly formed were about 25–40% and 45–50% respectively. They are derived mainly from layer silicates but also from feldspars. In the chernozems 15–20%, the recent brown forest soils 20–25%, in a brown forest soil from the last interglacial period more than 30% and in the braunlehm of middle pleistocene age more than 40% of the original feldspars were destroyed by chemical weathering and had participated in clay formation, especially in the formation of smectites.