Mineral equilibria in the system nepheline-alkali feldspar-plagioclase and their petrological significance
The purpose of this work is a theoretical consideration of equilibrium conditions in the system nepheline - alkali feldspar - plagioclase - alkali chloride aqueous solution based on the thermodynamic treatment of experimental data (Orville, 1963; Iiyama et al., 1963; Debron et al., 1961) for particular systems. The excess thermodynamic functions (Z E , S E , H E , V E ) of solid solutions were calculated for it. These data enable us to derive the diagrams of isotherms for the distribution of sodium components between nepheline and alkali feldspar ; between plagioclase and alkali feldspar ; and among plagioclase , nepheline , and alkali feldspar for the temperature range 400-1000° C. In order to check the calculated diagrams we analysed the minerals from the binary and ternary assemblages of the systems nepheline $ alkali feldspar $ plagioclase from various igneous massifs of nepheline syenites of the world. Temperatures obtained from different geothermometers were found to coincide satisfactorily.