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

X-ray diffraction studies conducted at high pressure with a beryllium pressure vessel show that compression parallel to the c-axis of CaCO(3)-I (calcite), as the phase boundary of CaCO-(3)II (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 CaCO(3)-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 A, b0 = 8.42 +/- .01 A, and c0 = 7.14 +/- .01 A (Z = 10).