The crystal structure of CaSiO3-walstromite, a special isomorph of wollastonite-II

The crystal structure of CaSiO3-walstromite has been solved and refined from a data set collected in situ from an inclusion in a diamond single crystal (1135 observed reflections, MoKalpha, Nonius CAD4, R = 3.46%). The crystal structure is almost identical to that of walstromite (Ca2BaSi3O9). The replacement of Ba2+ by Ca2+ causes no significant structural changes (ring silicate; according to F. Liebau: dreier single rings). The structural differences to wollastonite-H (Ca3Si3O9) are also relatively small: The Ca positions are almost the same, but the axial Si3O9-ring is isomorphously replaced by the same ring with the inverse axiality. This constitutes a new principle of isomorphous replacement. Based on density functional theory the equation of state has been predicted. This allows us to estimate the remaining pressure of the investigated CaSiO3 inclusion in the diamond to be approximate to3 GPa.