Mineral formation in micaceous Mediterranean Red soils of Sierra Nevada, Granada, Spain

Abstract

We investigated the processes of mineral formation in three Alfisol profiles of Sierra Nevada (southern Spain), with special emphasis on the little‐studied process of mica inheritance, particularly as regards its quantitative aspects. X‐ray diffraction, conventional and high resolution transmission electron microscopy, selected‐area electron diffraction, and geochemical analysis of the soil solution were used, and the granulometric fractions gravel, coarse sand, fine sand, silt and clay were studied, as was unaltered parent rock. Most interesting was inheritance of dioctahedral mica (illites) with small crystallochemical changes. The transition of mica from parent rock to clay was characterized by small crystallochemical changes affecting their structural formulae, reduction of the 2M1 polytype content, decrease in crystallite size, increase in crystal defects, and other changes in particle morphology. We propose the term ‘pedocrystallochemical evolution’ for the transition. The changes reverse the processes that take place in micas when rocks are formed by sedimentation, diagenesis and metamorphism. The kaolinite in the soils is a result of neoformation, and acts as the equilibrium phase within the chemical system. Reddening seems to have been caused by both inheritance and neoformation of iron oxides (haematite + goethite).