Mineral evolution of acid forest soils in the Strengbach catchment (Vosges mountains, N-E France)

Sand, silt and clay fractions were analysed to characterise different soil-forming processes of two acid forest soils from the Strengbach catchment. On the northern slope, a podzolic soil was formed on only slightly hydrothermally altered Brezouard granite and on the southern slope was an acid brown soil derived from granite strongly influenced by hydrothermal activity. During hydrothermal alteration, the parent material of the acid brown soil became enriched in fine-grained white mica. Simultaneously, the rock became more coherent. In dissolution voids of albite, favourable conditions for the subsequent neoformation of smectite were created. Physical weathering, during the Quaternary period, produced high amounts of clay by release of illite and smectite from coarser particles. Although soil conditions are favourable to podzolisation, the hydrothermal decrease in Al-rich minerals results in lower amounts of poorly crystalline compounds and less intensively hydroxy-Al interlayered 2:1 phyllosilicates. Illite evolved partly into interstratified illite-vermiculite. An 'intrazonal' acid brown soil was developed at this site. On the less hydrothermally altered material, clay percentages are lower and podzolisation developed as expected. In the surface horizon, illite-smectite was derived from illite. Dissolution of minerals and production of poorly crystalline compounds were even enhanced by lateral accumulation of albite-rich material. (C) 1998 Elsevier Science B.V.