THE STABILITY OF IKAITE, CACO₃·6H₂O, AT HIGH PRESSURE AND TEMPERATURE

We report the first Raman spectra of a single crystal of ikaite, CaCO3·6H2O, synthesized in a diamond anvil cell at ambient temperature. Spectra taken from 0.14 to 4.08 GPa, at room temperature have the most intense peaks at 228 and 1081 cm-1, corresponding to the lattice vibrations and CO symmetric stretch, respectively. These are consistent with Raman spectra previously published for ikaite in powder form at ambient temperature and pressure. We used visual observations of a sample consisting initially of a mixture of calcite + water in a gasketed diamond anvil cell to map an extended water saturated calcium carbonate phase diagram up to 2 GPa and 120°C. Temperatures were determined with thermocouples and pressures were calculated using the melting points of ice and the equation of state of H2O. The phase boundary for the reaction ikaite<->aragonite + water which defines the upper temperature limit of the stability field of ikaite is curved convex 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. Observations of coexisting solid phases immersed in a solution have proven to be a valuable technique for mapping equilibrium phase relations as a function of temperature and pressure.