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

We studied the gravels derived from mica schists and micaceous quartzites of three Mediterranean Red Soils (Haploxeralf and Cryoboralf) from the Sierra Nevada (Andalusia, Spain) with emphasis on the following aspects: morphology (visual evidence of weathering, thin section description and scanning electron 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 had undergone considerable modification during weathering in soil. A loss of the primary rock structure was detected, together with a decrease in compactness, an increase in roundness, color value and chroma. Neoformed kaolinite was present and relatively high contents of extractable forms of iron, aluminium and silica (citrate-dithionite and oxalate) were detected. From an analytical point of view, the surface properties (CEC and SSA) increased and the equilibrium pH decreased in the clasts which had undergone most weathering. The SEM study shows that the fabric changes from laminar schistose in the parent rock to other types in the gravels due to the weathering process. Electron microprobe analysis revealed the presence of phyllosilicates together with iron oxides in the illuvial infilling materials of fissures in the most altered clasts. These infillings affect the properties of the coarse fragments. The relationship detected between the properties of fine earth and those of the gravel fractions suggest that the coarse fragments have a significant influence on the properties of the soil. (C) 1999 Elsevier Science B.V. All rights reserved.