The crystal structure of calcite III

The crystal structure of calcite III has been deduced from existing high pressure powder X-ray diffraction patterns, based on the assumption that it is a displacive modification of the calcite I structure. The structure is monoclinic with space group C2 and a Z of 6. There are two Ca and two C positions, and five O positions, and atom coordinates have been refined by distance-least-squares methods to give reasonable octahedral coordination for Ca and parallel, planar CO3 groups. Unit cell parameters refined from a published powder diffraction pattern at 4.1 Gpa are: a = 8.746(8)Angstrom; b = 4.685(5)Angstrom; c = 8.275(8)Angstrom; and beta = 94.4 degrees. The structure has a calculated density of 2.949 Mg/m(3) at 4.1 GPa which is less than that of aragonite at this pressure and consistent with early piston cylinder studies. This implies that calcite III is indeed a metastable intermediary between calcite I and aragonite.