The Crystal chemistry of the mineral solid solution by EXAFS spectroscopy.

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Rock-forming minerals, sulfides and various metal oxides generally form solid solutions with wide compositional range. Microstructures of atomic scale are basically important to elucidate the crystal chemistry, thermal history and physical property. This study has been done to clarify the crystal chemistry of the solid solution based on the local structure obtained from the analysis of EXAFS spectroscopy. The main results are summarized as follows; (1)The rock-forming mineral olivines with the various compositions in the Mg_2SiO_4 - Fe_2SiO_4 system have been investigated by EXAFS. The local structure of Fe atom in the olivine solid solution is independent on the composition and is nearly the same as in the structure of fayalite. (2) The sulfides minerals, cubanite and chalcopyrite have been investigated using the EXAFS at the Fe-K and Cu-K edges. The local structures of these minerals in the ordered phase are maintained even in the disordered phase of the high temperature form. The results are essentially important to elucidate the order-disorder transition mechanism in these compounds. (3) The stability and ionic conductivity versus composition of the fluorite- type solid solution in the Bi_2O_3-Gd_2O_3 system were explained by forming clusters of regular oxygen cube around Gd ions, which is compared with distorted oxygen polyhedra around Bi ions. It has been clarified in this study that the local structure of the specific kinds of atoms in the solid solution has close relation with its chemical composition, stability and physical property.