In this paper we report progress on the development of a thermodynamic model that correctly predicts solvent/solute activities and monomeric aluminum hydrolysis speciation as well as solid-liquid equilibria in the H+, Na+, Al3+, Cl?, Si(OH)4, SiO(OH)3?, OH?, Al(OH)2+, Al(OH)2+, Al(OH)30, Al(OH)4? system as a function of pH to high salt concentrations (I ? 5 m NaCl), for temperatures up to 300oC and for saturation pressures. This model, which incorporates the Pitzer specific interaction equations (Pitzer, 1987, 1991), accurately predicts the 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. For high and low pH regions where the formation of polymeric Al hydrolysis species is low, the model will apply to higher total aluminum concentrations. The successful prediction of the solubility of aluminiosilicate solid phases falling within this system is described.

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