GIBBS FREE-ENERGY OF BUDDINGTONITE (NH4ALSI3O8) EXTRAPOLATED FROM EXPERIMENTS AND COMPARISON TO NATURAL OCCURRENCES AND POLYHEDRAL ESTIMATION

Two independent estimation methods for the standard state (298.15 K, 1 bar) free energy of formation for disordered buddingtonite (NH4AlSi3O 8) predict -3527.8 and -3522.4 kJ/mol, respectively. The first method is based on a polyhedral summation approach calibrated on NH4/K-minera l pairs that constrain the contribution of (NH4)(2)O to -295.5 kJ/mol when combined with data of Chermak & Rimstidt. The second method extra polates high temperature K(+)double left right arrow NH4+ exchange exp eriments of Lorch between K-feldspar and buddingtonite to standard sta te with a density model. The recommended free energy for disordered bu ddingtonite is -3525.1 kJ/mol, and 213.7 kJ/mol for the additive K(+)d ouble left right arrow NH4+ exchange in silicates. Analyzed waters fro m the Sulphur Bank mine (Lake County, California) thought to be near e quilibrium with buddingtonite of composition Buddingtonite(93)-K-felds par(6) suggest a free energy of -3519.5 kJ/mol for buddingtonite. This is within combined uncertainties of the estimated free energies, or m ay suggest supersaturation with respect to buddingtonite. The stabilit y relationships of the NH3-K2O-Al2O3-SiO2-H2O system at standard state are portrayed in activity ratio diagrams including the NH4-mica tobel ite (Delta(f)G(0) = -5383.0 kJ/mol). Formation of buddingtonite is fav ored by high a(NH4)(+)/a(H)(+) ratios such as might be realized in som e oil shales, black shales and anthracite coal beds.