エネルギー最小化法による特殊条件下の鉱物結晶構造の予測

Abstract

The energy minimization method (WMIN, Busing (1981)) has been successfully applied to predicting the crystal structures of minerals of geological interest at high pressure. The repulsive parameters, ionic radius A and ionic compressibility B of some ions have been revised. The changes of the crystal structures with the increase of pressure are summarized as follows : (1) Degrees of shortening of longer bonds are larger than those of shorter bonds; (2) Unshared edges become shorter at the higher rate than the shared edges; In some cases the shortest shared edge remains almost constant; (3) The shape of polyhedron becomes regular, because unshared edges of cation polyhedra tend to approach the length of shared edges. It is often found that the mean Si-O distance increases when the phase transition; however, the mean Si-O distance of a particular phase in the stability field does not differ very much from that of the phase stable at 1 atm. The inter-diffusion coefficients of four transition metals in the forsterite structure previously obtained have been compared with their self-diffusion coefficients in olivine structures estimated on the basis of the potential energy maps by using the WMIN program. We have also applied the WMIN method to predicting the crystal structures of minerals at high temperature and to examining the cation distribution in olivine. Our studies have laid the foundation of the discussion based on the potential energy function about the crystal structures of minerals and about the rate processes applicable to geological phenomena.