Thermodynamic properties of analcime solid solutions

Abstract

Analcime (Na_xAl_xSi_3-xO₆ · [(3-x)/2]H₂O, where x varies from ∼0.78 to ∼1.06) is one of the most common rock-forming zeolites. It forms in a wide range of geologic environments that span a range of temperature and pressure from ambient to magmatic conditions. Cluster variation method analysis of ²⁹Si magic angle spinning nuclear magnetic resonance spectra indicates 1) the presence of at least two distinct states of short range Si/Al disorder [low (less disordered) and high (more disordered) analcime], and 2) that configurational entropy associated short-range Si-Al disorder within each of these states increases regularly with increasing Si content. Hydrofluoric acid (HF) solution calorimetry at 50°C was used to determine the enthalpy of formation of five pure analcime samples of varying composition (range of x approximately 0.95 to 1.05) and Si-Al disorder. Enthalpies of formation from the elements at 25°C (ΔH_f) for these samples fall on a linear trend, except for one sample of high analcime for which ΔH_f was about 6.1 ± 3.0 kJ/mol less stable than a low analcime of the same composition. Comparison with the results of previous calorimetric studies indicates negligible excess enthalpies of mixing in both low and high analcime solid solutions (that is, the solid solutions are athermal). The configurational entropies derived from cluster variation analysis were in turn used to derive activity-composition relationships for low analcime solid solutions whose compositions are bounded by an aluminous endmember (Na_1.05Al_1.05Si_1.95O₆ · 0.975H₂O) and a siliceous endmember (Na_0.75Al_0.75Si_2.25O₆ · 1.125H₂O). These relationships were used to retrieve thermodynamic properties for the endmembers from experimental observations of equilibria between analcime, albite, and aqueous solutions. Retrieved values of ΔH_f are in excellent agreement with the calorimetric results of this study. Comparative analysis of equilibrium observations in the literature indicate that one sample of analcime from the Mont St. Hilaire alkaline intrusive complex used for analcime solubility measurements is high analcime. The Gibbs energy of disordering at 298.15 K, 1 bar consistent with the retrieval calculations is ∼ 6 kJ/mol. The thermodynamic properties of disordering for analcime indicate that hydrated low analcime is stable with respect to hydrated high analcime everywhere in Earth’s crust. Phase relations between low analcime, quartz, albite, and aqueous solutions calculated from the retrieved thermodynamic data indicate that at quartz equilibrium, low analcime should become more Si-rich with increasing temperature and pressure and that the composition of analcime is a sensitive function of the chemical potential of SiO₂. Stable equilibrium between analcime, albite, quartz and H₂O occurs at much lower temperatures than suggested by earlier phase equilibrium experiments. The breakdown of analcime plus quartz to form albite in geologic systems probably reflects metastable equilibrium in which the composition of analcime did not equilibrate with quartz.