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% CaSiO3) has been determined at temperatures from 400 to 800 degrees 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 CaSiO3 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 x T-1 + 671386.84 x T-2 + log rho(H2O) x (5.4578 + 2359.11 x 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. Ana. 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 x T-1 + 671402.32 x T-2 + log rho(H2O) x (-1.3609 + 6775.13 x T-1). (all temperatures are in degrees C) This expression adequately describes wollastonite-I solubility between 0.2-3 GPa and 300-800 degrees C. (c) 2006 Elsevier Inc. All rights reserved.