A thermodynamic model for the system [formula omitted] near the upper critical end point based on quartz solubility experiments at 500–1100 °C and 5–20 kbar

Abstract

A thermodynamic model of SiO2–H2O mixing in sub- and supercritical fluids has been developed based on new and existing experimental data on the solubility of quartz in H2O. To supplement previously published data, we conducted new solubility experiments at 15 and 20kbar and 900–1100°C using hydrothermal piston–cylinder methods. At concentrations below ∼10mol% SiO2, solubility was measured by single-crystal weight loss. At higher concentrations, solubility was determined by bracketing the presence and absence of quartz in quenched charges using multiple isothermal and isobaric runs with varying SiO2–H2O ratios. These data were combined with previously published results to construct a thermodynamic model of SiO2–H2O mixing. Following studies of silicate melts, the model takes oxygen in the fluid to be in three forms: free, molecular H2O, Si-bridging oxygens (Obr2-), and the terminal hydroxyls (OHtm-) of silanol groups. The equilibrium exchange of oxygen between these forms can be written 12H2O+12Obr2-=OHtm-. The standard Gibbs free energy change of this reaction (ΔG∘) was incorporated into a subregular solution model for mixing of SiO2 liquid and H2O fluid. The P–T dependences of ΔG∘ and interchange energies were derived by an error minimization algorithm, producing thirteen independent fit parameters. The model is applicable from 5 to 20kbar and 500°C to the dry melting curve of quartz. It reproduces experimentally derived quartz solubility data to 3.8% on average (1σ=5.3%). The model also predicts hydrous melting of quartz, critical melt–vapor mixing, activity–concentration relations, partial molar volume and entropy of aqueous silica, water speciation, and the thermal expansivity, isothermal compressibility, and isobaric heat capacity of a fluid in equilibrium with quartz. The model predicts a critical end point in the SiO2–H2O system at 1067°C and 9.33kbar, in very good agreement with the accepted location at ∼1080°C and 9.5–10kbar. The model is also in good agreement with previous estimates of the extent of silica polymerization. The results of this study clearly demonstrate that there is an explicit link between polymerization chemistry and critical mixing of silicate–H2O solutions.