MINERAL FORMATION IN MICACEOUS MEDITERRANEAN RED SOILS OF SIERRA-NEVADA, GRANADA, SPAIN

We investigated the processes of mineral formation in three Alfisol pr ofiles of Sierra Nevada (southern Spain), with special emphasis on the little-studied process of mica inheritance, particularly as regards i ts quantitative aspects. X-ray diffraction, conventional and high reso lution transmission electron microscopy, selected-area electron diffra ction, and geochemical analysis of the soil solution were used, and th e granulometric fractions gravel, coarse sand, fine sand, silt and cla y were studied, as was unaltered parent rock. Most interesting was inh eritance of dioctahedral mica (illites) with small crystallochemical c hanges. The transition of mica from parent rock to clay was characteri zed by small crystallochemical changes affecting their structural form ulae, reduction of the 2M(1) polytype content, decrease in crystallite size, increase in crystal defects, and other changes in particle morp hology. 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 th e equilibrium phase within the chemical system. Reddening seems to hav e been caused by both inheritance and neoformation of iron oxides (hae matite + goethite).