Abstract

The standard 298°K, 1 bar Gibbs free energy of formation and enthalpy of formation of the following minerals were calculated from reversed hydrothermal equilibrium data: Diaspore, tremolite, zoisite, prehnite, laumontite, wairakite, clinochlore, muscovite, paragonite, pyrophyllite. Calculation was by a procedure previously described by the author, and uncertainties, due to uncertainties in the hydrothermal data and in the thermochemical data input, are included. Where entropy data did not exist, they were calculated from pairs of hydrothermal p-T brackets. The zeolites have anomalously high entropy values, reflecting the highly mobile state of molecular H2O in the structure. The calculated Gibbs free energy of muscovite is about 10 kcal more negative than calorimetric measurements; however, much of this discrepancy could be caused by a gross error in the basic thermochemical parameters for the aluminum silicates (corundum, kyanite, andalusite, sillimanite, kaolinite) suggesting error in the thermochemistry of corundum. The inconsistency of about 3-4 kcal/gram atom of Al is implicit in the available thermo chemical values of aluminum-bearing minerals, but there is at present no a priori means to decide which sets of data are wrong. Until this point is resolved, the validity of all thermochemical calculations for mineralogical phase reactions involving aluminum is suspect.

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