THERMODYNAMICS OF FELDSPATHOID SOLUTIONS

We have developed models for the thermodynamic properties of nepheline s, kalsilites, and leucites in the simple system NaAlSiO4 - KAlSiO4 - Ca0.5AlSiO4 -SiO2 - H2O that are consistent with all known constraints on subsolidus equilibria and thermodynamic properties, and have integ rated them into the existing MELTS software package. The model for nep heline is formulated for the simplifying assumptions that (1) a molecu lar mixing-type approximation describes changes in the configurational entropy associated with the coupled exchange substitutions square Si double left right arrow NaAl and square Ca double left right arrow Na- 2 and that (2) Na+ and K+ display long-range non-convergent ordering b etween a large cation and the three small cation sites in the Na4Al4Si 4O16 formula unit. Notable features of the model include the predictio n that the mineral tetrakalsilite (''panunzite'', sensu stricto) resul ts from anti-ordering of Na and K between the large cation and the thr ee small cation sites in the nepheline structure at high temperatures, an average dT/dP slope of about 55 degrees/kbar for the reaction neph eline + kalsilite = tetrakalsilite (K-rich nepheline) over the tempera ture and pressure ranges 800-1050 degrees C and 500-5000 bars, roughly symmetric (i.e. quadratic) solution behavior of the K-Na substitution along joins between fully ordered components in nepheline, and large positive Gibbs energies for the nepheline reciprocal reactions [GRAPHI CS]