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First principles molecular dynamics simulations have been carried out at various temperatures and pressures starting with either Ca2+ or CaO in a reactive volume of 63 H2O molecules. In the case of aqueous Ca2+, the ion is surrounded by six H2O molecules in the first hydration shell at 300 K/0.3 GPa, with rare exchange between first and second hydrations shells. At 900 K/0.9 GPa, the coordination number in the first hydration shell fluctuates between six and eight, the average being 7.0. CaO immediately reacts with the surrounding H2O molecules to form Ca2+ + 2OH-. The hydroxyl ions form transient Ca(OH)+ and Ca(OH)2 complexes and have a mean residence time in the first coordination shell of Ca2+ of 6 ± 4 ps at 500 K and 3 ± 3 ps at 900 K, respectively. At 500 K/0.5 GPa, the time-averaged relative concentrations of the transient Ca2+, Ca(OH)+, and Ca(OH)2 species are 14%, 55%, and 29%, while at 900 K/0.9 GPa, they are 2%, 34%, and 63%. [ABSTRACT FROM AUTHOR]

Abstract: The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO3) has been determined at temperatures from 400 to 800°C and pressures between 0.8 and 5GPa in piston-cylinder apparatus with the weight-loss method. Chemical analysis of quench products and optical monitoring in a hydrothermal diamond anvil cell demonstrates that no additional phases form during dissolution. Wollastonite-I, therefore, dissolves congruently in the pressure–temperature range investigated. The solubility of CaSiO3 varies between 0.175 and 13.485wt% and increases systematically with both temperature and pressure up to 3.0GPa. Above 3.0GPa wollastonite-I reacts rapidly to the high-pressure modification wollastonite-II. No obvious trends are evident in the solubility of wollastonite-II, with values between 1.93 and 10.61wt%. The systematics of wollastonite-I solubility can be described well by a composite polynomial expression that leads to isothermal linear correlation with the density of water. The molality of dissolved wollastonite-I in pure water is thenBy combining the present experimental data with literature data on the solubility of quartz (Manning, C.E., Boettcher, S.L., 1994. Rapid-quench hydrothermal experiments at mantle pressures and temperatures. Am. Mineral. 79, 1153–1158.) and wollastonite-I+quartz (Xie, Z., Walther, J.V., 1993b. Wollastonite+quartz solubility in supercritical NaCl aqueous solutions. Am. J. Sci. 293, 235–255.) in pure water, an analogous expression can be derived for the solubility of wollastonite in quartz-saturated aqueous solution as follows:This expression adequately describes wollastonite-I solubility between 0.2–3GPa and 300–800°C. [Copyright &y& Elsevier]