Mineralogy of clay fractions of some soils on loess in northern France

Abstract

Important field characteristics of soils on loess deposits in northern France result primarily from processes of clay migration. Two successive steps can be distinguished. The first has uniformly distributed brown “primary” clay coatings in a brown matrix (Hapludalfs), the second has features of hydromorphy and degradation, especially bleached tongues, penetrating into the horizon of clay accumulation, with gray “secondary” clay coatings in the lower parts. The mineralogy of fine and coarse clay in the two kinds of soils shows an identical mixture of different materials: quartz, kaolinite, mica, chlorite, an heterogeneous complex of minerals having micaceous, smectitic, vermiculitic and chloritic layers, and smectite. The first four minerals predominate in the coarse clay, the complex and the smectite in the fine clay. The only fundamental difference between primary and secondary stages is a difference in distribution of free iron, the latter being characterized by a strong local redistribution due to redox processes. The coarse clay in the upper part of the glossic soils has an obvious increase in chlorite, relative to the other minerals. The chemical composition indicates that this chlorite is trioctahedral, excluding a process of secondary chloritization.