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The stability and Raman spectra of ikaite, CaCO₃·6H₂O, at high pressure and temperature

Anat Shahar^1,2,*, William A. Bassett^1,, Ho-kwang Mao², I-Ming Chou³ and Wendy Mao²

¹ Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, U.S.A
² Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. 20015, U.S.A.
³ 954 National Center, United States Geological Survey, Reston, Virginia 20192, U.S.A.

Correspondence: E-mail: bassett{at}geology.geo.cornell.edu

Raman analyses of single crystals of ikaite, CaCO₃·6H₂O, 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 CO^2– ₃ 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 °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.