Introductory Studies of High-Pressure Polymorphism to 24,000 Bars by X-Ray Diffraction with Some Comments on Calcite II

A technique is described by which X-ray diffraction studies may be made of substances at pressures in the neighborhood of 24,000 bars. Potassium iodide transforms from the NaCl structure to the CsCl structure above 18,000 bars. Cadmium displayed no transitions at 16,000 bars. Aragonite and vaterite showed no transition to 24,000 bars. A thermodynamic argument shows calcite II and calcite III to be unstable with respect to aragonite. X-ray patterns of calcite II and thermodynamic evidence are consistent with the interpretation that calcite II is an anion-disordered form of normal calcite.