Model of Earth and Planetary Chemistry: Prediction of Solid/Liquid/Gas Equilibria in Aluminosilicate, Evaporite and Carbonate Systems to High Concentration and Temperature 0- 250C

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Aluminosilicate minerals are found throughout the earth's surface and crust, commonly as feldspars in metamorphic and igneous rock, as clays in well-weathered soils and as authigenic constituents of evaporites. This one year research project develops modeling technology, based on the Pitzer representation of the aqueous solution free energy, to accurately predict the chemical behavior of natural fluids in contact with aluminosilicate minerals to high solution concentration and temperature. The Pitzer models developed by this group (TEQUIL, geotherm.ucsd.edu), can predict properties such as the activity of solute species, the solubility of minerals and the vapor pressure above solutions for formation fluids below 300C in evaporite, carbonate and silica systems with accuracies near that of data. The TEQUIL models include many of the solution species necessary to determine the saturation status of natural fluids in contact with aluminosilicate minerals but do not include Al solution species. The aqueous chemistry of aluminum is complex, and, because of the low solubility of Al containing minerals, precise data are difficult to obtain. In this project it is planned to generate a model of aluminum aqueous chemistry to ~250C in sodium chloride solutions using recently determined effective equilibrium constants for mononuclear Al speciation reactions via solubility studies and potentiometric titrations (Wesolowski, Palmer, Benezeth, Bourcier, Castet). This solution model will be the first high accuracy equation of state that summarizes these data and is tailored to provide mixing properties in the high concentration region. It is also planned to, then, initiate development of a thermodynamic data base for aluminosilicate minerals (including alkali feldspars) that is consistent with the solution models obtained. This work will develop the methods necessary to include many solids in this extremely important class of rock forming minerals in a high concentration, temperature-dependent model.