Thermodynamic Modeling of the Solubility of Quartz in Water up to High Temperatures and Pressures

Available data on speciation of silica in aqueous solutions show that the main forms of silica are the monomer Si­(OH)4 and the dimer Si2O­(OH)6, with shares of higher polymers growing with an increase of temperature and the total concentration of silica in solution. Thermodynamic properties of the monomer and dimer in the ideal gas state are determined from the quantum-chemical calculations. The fugacity coefficients of these species in water up to water densities of 1200 kg m–3 and temperatures of up to 1350 K are estimated from an empirical correlation based on the stoichiometry of hydroxide species. The fine-tuning of the model is made by comparing calculations with the experimental solubility and speciation data. It was found that to match the speciation results reported as the share of the monomer, it is necessary to consider polymerization of silica beyond the dimer, which can be achieved without increasing the number of fitting parameters. The resulting model reproduces well the experimental results on the solubility of quartz in water, on the distribution of silica between the vapor and liquid phases of water at subcritical temperatures, and on the speciation of silica in water in equilibrium with quartz.