The crystal data and stability of calcite III at high pressures based on single-crystal X-ray experiments

Single-crystal X-ray experiments have been performed for calcite, calcite II, and calcite III at 300 K and pressures up to 4.1 GPa. The volume decrease from calcite to calcite II is estimated to be 0.54 cm(3)/mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm(3)/mol in the calcite to calcite II transition at 1.4 GPa reported previously based on piston-cylinder volume-compression experiments. The crystal data of calcite III have been determined using single-crystal X-ray analyses at 300 K and pressures between 2.3 and 4.1 GPa. The calcite III crystal is found to be monoclinic with the space group either C2, Cm, or C2/m from the systematic absence of reflections, and with the refined cell parameters of a = 8.707(7), b = 9.315(6), c = 6.154(4) angstrom, and beta = 115.18(2)degrees at 2.3 GPa. These crystal data of calcite III at 2.3 GPa yield a density of 2.94 g/cm(3) for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm(3) at pressures from 1.9 to 2.9 GPa, using powder X-ray analyses based on orthorhombic structural models for calcite III, but is generally consistent with the reported data by the piston-cylinder experiments. Thus the present study reveals the density of calcite III is substantially less than that of aragonite (3.02 g/cm(3) at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.