VACANCY DISTRIBUTION IN A SYNTHETIC NONSTOICHIOMETRIC PYROXENE. M. Okui, F. Marumo, H. Sawada, Department of Earth Sciences, Nihon University, Japan

Nonstoichiometric pyroxenes, which were supposed to characteristic of high-pressure products, were synthesized under ambient pressure, and the structure was refined to elucidate the structural details. The crystals were grown from a silicate melt with a chemical composition belonging to the system diopside-anorthite-esseneite. Microprobe analysis showed that these crystals contain practically no Mg and about one-eighth of M(1)+M(2) sites were vacant. This amount of vacancies is outstandingly large among the values of nonstoichiometric pyroxenes so far reported. The chemical formula derived from the results of microprobe analysis and structure refinement is (Ca_0.75Fe_0.10u_0.15)(A1_0.78Fe_0.1u_0.06)(Al_0.50Si_1.50)O₆. The vacancies are distributed over both M(1) and M(2) sites, but predominant in M(2). Compared with diopside, the mean of T-O distance is long due to the replacement of Si with Al, M(1)-O is short due to the replacement of Mg with Al and Fe, M(2)-O is long probably due to the large amount vacancies. As a result, the crystal has a cell volume comparable with that of diopside. In this point the present crystal is distinct from the nonstoichiometric pyroxenes known to date.