Ikaite (CaCo_3.6H_2O) compressibility at high water pressure: a synchrotron X-ray diffraction study

摘要

Ikaite (CaCO_3.6H_2O), which forms in cold carbonate-rich marine environments, also crystallizes from calcite under aqueous conditions above approx 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 (CaSO_4.2H_2O), calcite and aragonite. The volume of ikaite is less than that occupied by equivalent volumes of CaCO_3 (calcite/ aragonite) + 6H_2O at all pressures below the freezing point (H_2O(l) to ice VI). The bulk modulus of ikaite has been obtained from a fit to the Vinet equation of state, giving V_0 = 760.3 + - 0.1 A~3, K_0 = 21.3 + - 1.4 GPa, and K_(0') = 11.7 + -1.7. Dissolution of CaCO_3 in H_2O (l) at high pressure resulting in crystallization of CaCO_3.6H_2O 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.