CRYSTAL-CHEMISTRY OF KAOLINITE AND FE-MN OXIDES - RELATION WITH FORMATION CONDITIONS OF LOW-TEMPERATURE SYSTEMS

Recent data on the crystal chemistry of Earth's Surface minerals obtained using spectroscopic methods are reviewed, Two methods, X-ray Absorption Spectroscopy (XAS) and Electron Paramagnetic Resonance (EPR), have received particular attention, The results described include: intracrystalline distribution of cations in kaolinite and some other phyllosilicates and Fe- and Mn-oxides; interpolyhedral relationships in oxide minerals and hydrous gels; complexation mechanisms for transition elements at surfaces of oxides and silicates (outer-sphere, mono- and polynuclear inner-sphere complexes); and finally, the significance of radiation-induced defect centers: Mineralogical and geochemical implications include: non-ideal mixing of elements in solid-solutions; the existence of specific modes of intersite distribution including the segregation of atoms; recognition, at the molecular level, of reactive sui face sites that have structural control over sorption efficiency and reaction kinetics; and the transport of short-lived isotopes and their relationship with processes of weathering, hydrothermal alteration, and formation of sedimentary basins, Information from spectroscopic methods has resulted in a better understanding of some of the geochemical processes that control the surface and bulk composition of minerals, One application is the study minerals, of mineral formation at the Earth's surface, Defect centers and paramagnetic impurities are effective finger prints of kaolin origin, The presence of Mn2+ outersphere and vanadyl innersphere complexes provides clues about the evolution of physico-chemical conditions during the formation of laterites and sediments, respectively, A second application is to environmental geochemistry and, in particular, to the migration and containment of radioactive and toxic wastes, An understanding of heavy metal trapping by mineral surfaces provides a rationale for modeling soil contamination and for choosing appropriate materials for rehabilitation. Furthermore, comparison of a total accumulated dose with present-day geochemical data provides an assessment of radionuclide migration in the geosphere.