Introduction

A review of thermodynamic properties of plagioclase, nepheline and alkali feldspar solid solutions is the subject of this paper with the aim of revising two-feldspar and nepheline–feldspar geothermometers. Sources of experimental data on equilibria of the solid solutions in the system NaAlSiO4–KAlSiO4–CaAl2Si2O8–SiO2 and data on the calorimetry of NaAlSi3O8/–KAlSi3O8 and NaAlSi3O8–CaAl2Si2O8 joins are presented in Table 7.1. The table shows that the volume of data on the equilibria exceeds that of the calorimetric data by an order of magnitude. Thermodynamic properties of the nepheline–kalsilite solid solution have not been studied calorimetrically at all. Data on equilibria of ternary solid solutions in the systems CaAl2Si2O8–NaAlSi3O8 – KAl Si3O8 and KAlSiO4–NaAlSiO4–SiO2 are very limited. Temperature estimates from different versions of two-feldspar thermometer (Ryabchikov, 1965, Perchuk & Ryabchikov, 1968, Stormer, 1975, Perchuk & Aleksandrov, 1976, Whitney & Stormer, 1977, Brown & Parsons, 1981, 1985, Haselton et al., 1983) based on the binary and ternary solution models show considerable divergence.

Attempts to develop a thermodynamic model of the ternary solid solution in the system CaAl2Si2O8–NaAlSi3O8-KAlSi3O8 on the basis of experimental data by Seck (1971,1972) and Johannes (1979) have been made by several authors (Saxena & Ribbe, 1972, Powell & Powell, 1977a, Ghiorso, 1984, Price, 1985, Green & Usdansky, 1984, 1986) with the purpose of creating a two-feldspar geothermobarometer, but due to low solubility of Ca in alkali feldspar and that of K in plagioclase the mixing properties derived have large uncertainties.