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

Abstract

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 cm3/ mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm3/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) A, and β = 115.18(2) at 2.3 GPa. These crystal data of calcite III at 2.3 GPa yield a density of 2.94 g/cm3 for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm3 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/cm3 at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.