Weathering and soil forming processes under semi-arid conditions in two Mexican volcanic ash soils

Weathering and neoformation of allophane, imogolite and halloysite in volcanic ash soils have been studied extensively in humid climates but little data are available on these soils in arid and semi-arid conditions. Weathering and soil formation under semi-arid conditions on acid pyroclastics have been investigated in two profiles, both at the micro-site scale and on bulk samples. XR microdiffraction and SEM-EDX microanalysis were performed on soil thin sections. Chemical, particle-size, XRD and mineral identification analysis were performed on conventional soil samples. Results show the interplay of various sub-processes : intense division of the coarse minerals into small fragments 50-200 micrometers in size ; diagenesis of noncrystalline products in the zone of contact with the parent minerals ; transformation of noncrystalline minerals into halloysite in the compact soil microstructures and preservation of the noncrystalline minerals in the topsoil ; desiccation and condensation of the crystalline and noncrystalline minerals into microaggregates in the topsoil. The results of these interacting weathering processes are silty-loam soils with no cohesion and high susceptibility to wind erosion. Differences appearing between different analytical methods at different sampling scales need special precautions in explaining the results. (Résumé d'auteur)