Thermodynamic modeling of solubility and speciation of silica in H2O-SiO2 fluid up to 1300C and 20 kbar based on the chain reaction formalism

Abstract

Recent systematic studies of mineral solubilities in water to high pressures up to 50 kbar call for a suitable thermodynamic formalism to allow realistic fitting of the experimental data and the establishment of an internally consistent data base. The very extensive low-pressure (<5 kbar) experimental data set on the solubility of SiO2 in H2O has in the last few years been extended to 20 kbar and 1300 degrees C, providing an excellent experimental basis for testing new approaches. In addition, solubility experiments with different SiO2-buffering phase assemblages and in situ determinations of Raman spectra for H2O-SiO2 fluids have provided both qualitative and quantitative constraints on the stoichiometry and quantities of dissolved silica species. We propose a thermodynamic formalism for modeling both absolute silica solubility and speciation of dissolved silica using a combination of the chain reaction approach and a new Gibbs free energy equation of water based on a homogeneous reaction formalism. For a given SiO2-buffer (e.g., quartz) and the coexisting H2O-SiO2 fluid both solubility and speciation of silica can be described by the following two reactions: monomer-forming standard reaction: SiO2(s) + 2(H2O)L = (SiO2)center dot(H2O)(2) (A) polymer-forming chain reaction: (SiO2)(n-1)center dot(H2O)(n) + (SiO2)center dot(H2O)(2) = (SiO2)(n)center dot(H2O)(n+1) + (H2O)(L), (B) where 2