Experimental determination of the solubility of natural wollastonite in pure water up to pressures of 5 GPa and at temperatures of 400–800 °C

The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO 3) has been determined at temperatures from 400 to 800 °C and pressures between 0.8 and 5 GPa 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 CaSiO 3 varies between 0.175 and 13.485 wt% and increases systematically with both temperature and pressure up to 3.0 GPa. Above 3.0 GPa 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 10.61 wt%. 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 then log ( m woll ) = 2.2288 - 3418.23 × T - 1 + 671386.84 × T - 2 + log ρ H 2 O × ( 5.4578 + 2359.11 × T - 1 ) . By 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: log ( m woll ) = 2.5930 - 3660.98 × T - 1 + 671402.32 × T - 2 + log ρ H 2 O × ( - 1.3609 + 6775.13 × T - 1 ) . ( all temperatures are in ° C ) This expression adequately describes wollastonite-I solubility between 0.2–3 GPa and 300–800 °C.