Mineralogical development of the silt fractions of a Podzoluvisol on loess in the Paris Basin (France)

Much of the Paris Basin is covered with loess, dominated by particles 2-50 mu m in diameter, which has undergone strong pedogenesis. We have investigated the mineralogical changes that have accompanied the development of a Podzoluvisol in such material. Four silt-size and two clay-size fractions from five horizons of a profile were separated and analysed by X-ray diffraction, chemical analysis, thermogravimetric analysis and transmission electron microscopy. The minerals present in the silt-size fractions are quartz, K- and Na-feldspars, mica, chlorite, vermiculite and kaolinite, Quartz and feldspars are more abundant in the 2-10-mu m fraction, micas in the 0.2-5-mu m fraction and chlorite in the <0.2-mu m fraction. These differences suggest preferential physical breakdown of phyllosilicates, particularly chlorites. Silt-size particles seem to be composed of crystals of the same mineralogy, but only 1/40-1/50 of the size. Their disintegration contributes to enrich the fine clay fraction of the loessial material in chlorite. The proportion of chlorite increases as particle size decreases in all horizons. It also increases with depth for particle sizes larger than 5 mu m. These observations indicate that physical disintegration is an essential process in the pedogenesis.