MICROPROBE AND X-RAY-DIFFRACTION STUDIES OF CLAY COATINGS IN LEACHED SOILS DERIVED FROM LOESS

Point analyses of Si, Al, K, Mg, Ca, Fe, Mn, as well as of Zn and Pb o f clay coatings in soil thin sections from illuvial horizons of four l eached soils were carried out using an electron microprobe (EMA). The K, Mg, Si, and Al content, the Si/Al-molar ratio of coatings and XRD-a nalyses indicate that the clay coatings consist mainly of illitic clay minerals. This suggests either a mineral-specific displacement of ill ite or the formation of illitic clay coatings from migrated smectite o r expanded illite (''illitization'') through subsequent K uptake. The clay coatings show, with the exception of Fe, Mn, Zn and Pb, only litt le variation in their chemical composition within the same horizon. Be tween the horizons of the investigated soils, however, distinct differ ences in the total element content, especially in the K content may oc cur. The strong vertical variation of the K content in clay coatings i n some horizons is probably due to acidic conditions and lateral infil tration which lead to an increased weathering of illite and the deplet ion of K. The EMA results show that Pb is predominantly bound in Fe-Mn -Oxide coatings on the surface of clay minerals or in Fe-Mn-Oxide prec ipitates within clay coatings. Microanalyses of K depleted clay coatin gs indicate on the other hand an increased fixation of Zn in expanded illites.