Mineralogical development of the silt fractions of a Podzoluvisol on loessin 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 investiga ted the mineralogical changes that have accompanied the development of a Po dzoluvisol in such material. Four silt-size and two clay-size fractions fro m five horizons of a profile were separated and analysed by X-ray diffracti on, 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 fe ldspars 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 sugges t preferential physical breakdown of phyllosilicates, particularly chlorite s. Silt-size particles seem to be composed of crystals of the same mineralo gy, but only 1/40-1/50 of the size. Their disintegration contributes to enr ich the fine clay fraction of the loessial material in chlorite. The propor tion of chlorite increases as particle size decreases in all horizons. It a lso increases with depth for particle sizes larger than 5 mu m. These obser vations indicate that physical disintegration is an essential process in th e pedogenesis.