Thermodynamic Model for Predicting Interactions of Geothermal Brines with Hydrothermal Aluminum Silicate Minerals

Abstract

We report our continued progress on the development of a thermochemical model of aluminum silicate mineral solubilities in aqueous solutions containing H+, Na+, K+, Al3+, Cl?, Si(OH)4, SiO(OH)3?, OH?, Al(OH)2+, Al(OH)2+, Al(OH)30, Al(OH)4? as a function of pH to high salt concentrations (I ? 5 m). Prior conference proceedings outlined our progress in developing an accurate model for the potassium free system to 100oC, and some preliminary results for that system to 300oC. In this article we report a fully developed model for the potassium free system to 300oC and preliminary results to 100oC for the full system. The model, which incorporates the Pitzer specific interaction equations (PITZER, 1987) accurately predicts fluid compositions for the low Al (< 10-5 m) and Si(OH)4 (< 10-4 m) concentrations commonly encountered in the intermediate pH ranges typical of most natural fluids. With available solubility or free energy of reaction data, the solubility of complex Na and K hydrothermal aluminosilicate minerals can now be predicted as a function of solution composition and pH in this system to high temperature. Phase equilibrium diagrams illustrating the capabilities of this model are presented.