Phase relations of CaCO3 at high pressure and high temperature

Phase transitions in calcite, a naturally occurring crystalline form of CaC O3, have been investigated by three different experimental techniques: (1) in-situ X-ray diffraction (XRD) using synchrotron radiation to 6 GPa and 17 50 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 sam ples retrieved after heat treatment at temperatures to 2000 degreesC and pr essures 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 simila r to2 GPa. Calcite III persisted to at least 10 GPa. Elevation of temperatu re at 3, 4, and 6 GPa caused a sequence of transitions: calcite III --> ara gonite --> disordered calcite ---> liquid, and aragonite was retained upon rapid cooling of the liquid. The melting curve of disordered calcite increa sed with pressure following a relation: T-m (degreesC) = 1338 + 82 P - 2.9 P-2 where P is in units of GPa.