Gibbs free energy of buddingtonite (NH4AlSi3O8) extrapolated from experiments and comparison to natural occurrences and polyhedral estimation
Mäder, Urs K.; Ramseyer, Karl; Daniels, Eric J.; Althaus, Egon
European Journal of Mineralogy Volume 8 Number 4 (1996), p. 755 - 766
published: Jul 29, 1996
manuscript accepted: Mar 22, 1996
manuscript received: Oct 5, 1994
ArtNo. ESP147050804009, Price: 29.00 €
Abstract Two independent estimation methods for the standard state (298.15 K, 1 bar) free energy of formation for disordered buddingtonite (NH4AlSi3O8) predict -3527.8 and -3522.4 kJ/mol, respectively. The first method is based on a polyhedral summation approach calibrated on NH4/K-mineral pairs that constrain the contribution of (NH4)2O to -295.5 kJ/mol when combined with data of Chermak & Rimstidt. The second method extrapolates high temperature K+⇔NH4+ exchange experiments of Lorch 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+ ⇔ NH4+ exchange in silicates. Analyzed waters from the Sulphur Bank mine (Lake County, California) thought to be near equilibrium with buddingtonite of composition Buddingtonite(93)-K-feldspar(6) suggest a free energy of -3519.5 kJ/mol for buddingtonite. This is within combined uncertainties of the estimated free energies, or may suggest supersaturation with respect to buddingtonite. The stability relationships of the NH3-K2O-Al2O3-SiO2-H2O system at standard state are portrayed in activity ratio diagrams including the NH4-mica tobelite (ΔfG° = -5383.0 kJ/mol). Formation of buddingtonite is favored by high aNH4+/aH+ ratios such as might be realized in some oil shales, black shales and anthracite coal beds.