X-ray Diffraction Data on Two High-Pressure Phases of Calcium Carbonate

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Abstract
X-ray diffraction studies conducted at high pressure with a beryllium pressure vessel show that compression parallel to the c-axis of CaCO3-I (calcite), as the phase boundary of CaCO-3II (modified calcite type) is approached, results in a relative reduction of the c-axis of phase II of several times that of the a-axes. Such data are not consistent with previously proposed anion disorder in CaCO3-II. At 18.7 ± .5 kb and room temperature a new phase of CaCO3 has been observed that is assumed to be phase III found by Bridgman. At this pressure the x-ray diffraction pattern can be indexed as orthorhombic KNO3-IV type, with a0 = 8.90 ± .01 Å, b0 = 8.42 ± .01 Å, and c0 = 7.14 ± .01 Å (Z = 10).