The stability and Raman spectra of ikaite, CaCO3 center dot 6H(2)O, at high pressure and temperature

Raman analyses of single crystals of ikaite, CaCO3-6H(2)O, synthesized in a diamond-anvil cell at ambient temperature yield spectra from 0.14 to 4.08 GPa; the most intense peaks are at 228 and 1081 cm(-1) corresponding to E-g (external) and A(1g) (internal) modes of vibrations in CO32 ions, respectively. These are in good agreement with Raman spectra previously published for ikaite in powder form at ambient temperature and pressure. Visual observations of a sample consisting initially of a mixture of calcite + water in a hydrothermal diamond-anvil cell yielded a P-T phase diagram up to 2 GPa and 120 degrees C; the boundary for the reaction ikaite <-> aragonite + water has a positive slope and is curved convexly toward the aragonite + water field similar to typical melt curves. This curvature can be explained in terms of the Clapeyron equation for a boundary between a solid phase and a more compressible liquid phase or largely liquid phase assemblage.