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.