A program to compute chemical equilibrium in geological systems has been developed. The program can handle a large number of components over wide T and P ranges. If reliable laboratory results have been obtained the program can be used to produce accurate thermodynamic data. It does not consider liquid or solid solutions but can be adapted to do so at some future date. The program provides a means of assessing the quality of published thermodynamic data by comparing equilibria calculated using this data with experimentally positioned reactions. The parameters tested are molar volume, entropy, enthalpy and free energy.

An error in the value of the molar volume for beta-tridymite was found when the system SiO2 was investigated. The MgO-SiO2 and CaO-SiO2 systems emphasize the need for further modifications so that eutectics and liquid solutions can be considered. The other binary systems (MgO-CO2, MgO-H2O, and CaO-H2O) demonstrate the need for data, which covers a wider P-T range, so that reactions at higher P can be studied.

The method of extrapolation used in this study proved to be reliable. The program was successfully applied to the study of metamorphic equilibria of geological interest; i.e. the tremolite reaction. The effects of increasing or decreasing the relative proportions of volatile species can be studied. Ternary systems can be studied over the complete composition range below the solidus at any T or composition intervals. Although cosmochemical systems were not presented in this report, the program can consider such problems at low pressures (10^-6 bars).

The program can be used to determine dependence of a particular mineral assemblage on P, T and fluid composition. Once reliable thermodynamic data is available, the program can be used to replace or at

least supplements some of the laboratory methods used in the study of phase equilibria. With a few modifications, the program can be used to study a variety of geochemical processes. It can be used to construe phase diagrams for systems that have not been experimentally studied

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