Experimental and thermodynamic study of heterogeneous and homogeneous equilibria in the system NaAlSiO4-SiO2

Internally consistent thermodynamic datasets available at present call for a further improvement of the data for nepheline (Holland and Powell 1988; Berman 1991). Because nepheline is a common rock-forming mineral, an attempt has been made to improve on the present state of knowledge of its thermodynamic properties. To achieve that goal, two heterogeneous reactions involving nepheline, albite, jadeite and a-quartz in the system NaAlSiO₄-SiO₂ have been reversed bylong duration runs in the range 460 ≤ T(°C) ≤ 960 and 10 ≤ P(kbar) ≤ 22. Given sufficiently long run times, thealbite run products approach internal equilibrium with respect to their Al,Si order-disorder states. Using appropriate thermochemical, thermophysical, and volumetric data, Landau expansion for albite, and the relevant reaction reversals, a refined thermodynamic dataset (Δ_fH_i⁰ and S_i⁰) has been derived for nepheline, jadeite, a-quartz, albite, and monalbite. Our refined data agree very well with theircalorimetric counterparts, but have smaller uncertainties. The refined dataset for Δ_fH_i⁰ and S_i⁰, including their uncertainties and correlation, help generate the NaAlSiO₄-SiO₂ phase diagram including 2a confidence interval for eachP-T curve (Fig. 5). Editorial responsibility: W. Schreyer