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Ikaite (CaCO_3·6H₂O) compressibility at high water pressure: a synchrotron X-ray diffraction study

Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK

^* E-mail: a.lennie{at}dl.ac.uk

Ikaite (CaCO_3·6H₂O), which forms in cold carbonate-rich marine environments, also crystallizes from calcite under aqueous conditions above ~0.5 GPa at room temperature. Using synchrotron X-ray powder diffraction, measurements have been made of pressure-induced changes in unit-cell dimensions of ikaite contained in a diamond anvil cell. Ikaite shows anisotropic compressibility along the crystallographic axes in the order a > c > b up to 4 GPa. Comparison with other phases shows the relative volume compressibility of ikaite to be greater than that of gypsum (CaSO₄.2H₂O), calcite and aragonite. The volume of ikaite is less than that occupied by equivalent volumes of CaCO₃ (calcite/aragonite) + 6H₂O at all pressures below the freezing point (H₂O(l) to ice VI). The bulk modulus of ikaite has been obtained from a fit to the Vinet equation of state, giving V₀ = 760.3 ± 0.1 ų, K₀ = 21.3 ± 1.4 GPa, and K₀' = 11.7 ± 1.7. Dissolution of CaCO₃ in H₂O (l) at high pressure resulting in crystallization of CaCO_3·6H₂O suggests equilibrium behaviour between a carbonate-rich fluid species and the condensed hydrated calcium carbonate. The potential for ikaite as a candidate phase for water transport in cold subduction zones is considered.

KEYWORDS: ikaite, compressibility, synchrotron, calcite

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