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<sup>3</sup>/mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm<sup>3</sup>/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 <i>C</i>2, <i>Cm</i>, or <i>C</i>2/<i>m</i> from the systematic absence of reflections, and with the refined cell parameters of <i>a</i> = 8.707(7), <i>b</i> = 9.315(6), <i>c</i> = 6.154(4) Å, 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/cm<sup>3</sup> for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm<sup>3</sup> 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<sup>3</sup> at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.


  • Journal of mineralogical and petrological sciences

    Journal of mineralogical and petrological sciences 100(1), 31-36, 2005-02-01

    一般社団法人 日本鉱物科学会

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