Phase relations of CaCO3 at high pressure and high temperature

Phase transitions in calcite, a naturally occurring crystalline form of CaCO3, have been investigated by three different experimental techniques: (1) in-situ X-ray diffraction (XRD) using synchrotron radiation to 6 GPa and 1750 degreesC in a cubic anvil press; (2) Raman scattering to 10 GPa at room temperature using a diamond-anvil cell, and (3) post-compression XRD on samples retrieved after heat treatment at temperatures to 2000 degreesC and pressures to 9 GPa in an octahedral anvil press. At room temperature, calcite I transformed into calcite II at 1.7 GPa and then to calcite III at similar to2 GPa. Calcite III persisted to at least 10 GPa. Elevation of temperature at 3, 4, and 6 GPa caused a sequence of transitions: calcite III --> aragonite --> disordered calcite ---> liquid, and aragonite was retained upon rapid cooling of the liquid. The melting curve of disordered calcite increased with pressure following a relation: T-m (degreesC) = 1338 + 82 P - 2.9 P-2 where P is in units of GPa.