Gibbs free energy of buddingtonite (NH (sub 4) AlSi (sub 3) O (sub 8) ) extrapolated from experiments and comparison to natural occurrences and polyhedral estimation

Two independent estimation methods for the standard state (289.15 K, 1 bar) free energy of formation for disordered buddingtonite predict -3527.8 and -3522.4 kJ/mol. The first method is based on a polyhedral summation approach calibrated on NH (sub 4) /K-mineral pairs that constrain the contribution of (NH (sub 4) ) (sub 2) O to -295.5 kJ/mol when combined with the data of Chermak & Rimstidt [A.M. 74-1023]. The second method extrapolates high-T K (super +) = NH (super +) (sub 4 ) exchange experiments between K-feldspar and buddingtonite to standard state with a density model. The recommended free energy for disordered buddingtonite is -3525.1 kJ/mol, and 213.7 kJ/mol for the additive K (super +) = NH (super +) (sub 4 ) exchange in silicates. Analysed waters from the Sulphur Bank mine (Lake County, California) thought to be near equilibrium with buddingtonite of composition Bd (sub 93) -KF (sub 6) suggest a free energy of -3519.5 kJ/mol for buddingtonite. The stability relationships of the NH (sub 3) -K (sub 2) O-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O system at standard state are shown in activity ratio diagrams including the NH (sub 4) -mica tobelite (Delta (sub f) G degrees = -5383.0 kJ/mol. Formation of buddingtonite is favoured by high aNH (super +) (sub 4 ) /aH (super +) ratios such as might be realized in some oil shales, black shales and anthracite coal beds.