Chemical, mineralogical and (micro)morphological study of coarse fragmentsin Mediterranean Red Soils

We studied the gravels derived from mica schists and micaceous quartzites o f three Mediterranean Red Soils (Haploxeralf and Cryoboralf) from the Sierr a Nevada (Andalusia, Spain) with emphasis on the following aspects: morphol ogy (visual evidence of weathering, thin section description and scanning e lectron microscopy); composition (X-ray diffraction, extractable Fe, Al and Si, and electron microprobe analysis); and chemistry (pH, cation exchange capacity, specific surface area and organic carbon content). The gravels ha d undergone considerable modification during weathering in soil. A loss of the primary rock structure was detected, together with a decrease in compac tness, an increase in roundness, color value and chroma. Neoformed kaolinit e was present and relatively high contents of extractable forms of iron, al uminium and silica (citrate-dithionite and oxalate) were detected. From an analytical point of view, the surface properties (CEC and SSA) increased an d the equilibrium pH decreased in the clasts which had undergone most weath ering. The SEM study shows that the fabric changes from laminar schistose i n the parent rock to other types in the gravels due to the weathering proce ss. Electron microprobe analysis revealed the presence of phyllosilicates t ogether with iron oxides in the illuvial infilling materials of fissures in the most altered clasts. These infillings affect the properties of the coa rse fragments. The relationship detected between the properties of fine ear th and those of the gravel fractions suggest that the coarse fragments have a significant influence on the properties of the soil. (C) 1999 Elsevier S cience B.V. All rights reserved.