Mineral solution equilibria—II. An experimental study of mineral solubilities and the thermodynamic properties of aqueous CaCl2 in the system CaO-SiO2-H2O-HCl

Speciation of aqueous calcium chloride and the solubility of wollastonite represented by the reaction wollastonite + 2HCl° → CaCl₂° + quartz + H₂O were experimentally investigated at 1 and 2 kbar in the range 425–600°C using rapid-quench hydrothermal techniques and a modified Ag + AgCl buffer technique (Frantz and Popp, 1979). Variation in the measured concentration in HCl° as a function of total dissolved calcium was used to identify associated aqueous CaCl₂° as the predominant calcium species in the fluid at temperatures above 500°C at 2 kbar. The data were used to calculate the equilibrium constant for the above reaction as a function of temperature and pressure, from which the difference in Gibbs free energy of formation between CaCl₂° and HCl° at 1 and 2 kbar, 450°–600°C was calculated. Solubility constants for minerals in the system MgO-CaO-SiO₂-H₂O-HCl-CO₂ were calculated using the data from this study and from Frantz and Popp (1979). Calculated mineral solubilities were used to calculate the solution compositions and solid alteration products resulting from interactions of a Ca-Mg silicate mineral (diopside) with hydrothermal solutions containing a range of different total chloride concentrations. High total chloride (2.0 m) in the solution results in Si-Mg enrichment in the solids and Ca enrichment in the fluid, whereas low total chloride (0.008 m) results in Mg enrichment in the solids and Ca-Si enrichment in the fluid.