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 ca tchment. On the northern slope, a podzolic soil was formed on only sli ghtly hydrothermally altered Brezouard granite and on the southern slo pe was an acid brown soil derived from granite strongly influenced by hydrothermal activity. During hydrothermal alteration, the parent mate rial 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 sm ectite 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 podzoli sation, 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 inters tratified illite-vermiculite. An 'intrazonal' acid brown soil was deve loped 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 e nhanced by lateral accumulation of albite-rich material. (C) 1998 Else vier Science B.V.