EXPERIMENTAL AND THERMODYNAMIC STUDY OF HETEROGENEOUS AND HOMOGENEOUSEQUILIBRIA IN THE SYSTEM NAALSIO4-SIO2

Internally consistent thermodynamic datasets available at present call for a further improvement of the data for nepheline (Holland and Powe ll 1988; Berman 1991). Because nepheline is a common rock-forming mine ral, an attempt has been made to improve on the present state of knowl edge of its thermodynamic properties. To achieve that goal, two hetero geneous reactions involving nepheline, albite, jadeite and alpha-quart z in the system NaAlSiO4-SiO2 have been reversed by long duration runs in the range 460 less than or equal to T(degrees C) less than or equa l to 960 and 10 less than or equal to P(kbar) less than or equal to 22 . Given sufficiently long run times, the albite run products approach internal equilibrium with respect to their Al,Si order-disorder states . Using appropriate thermochemical, thermophysical, and volumetric dat a, Landau expansion for albite, and the relevant reaction reversals, a refined thermodynamic dataset (Delta(f)H(i)(0) and S-i(0)) has been d erived for nepheline, jadeite, alpha-quartz, albite, and monalbite. Ou r refined data agree very well with their calorimetric courterparts, b ut have smaller uncertainties. The refined dataset for Delta(f)H(i)(0) and S-i(0), including their uncertainties and correlation, help gener ate the NaAlSiO4-SiO2 phase diagram including 2 sigma confidence inter val for each P-T curve (Fig. 5).