Mineral Equilibria in the System Nepheline-Alkali Feldspar-Plagioclase and their Petrological Significance

The purpose of this work is a theoretical consideration of equilibriumconditions in the system nepheline-alkali feldspar-plagioclase-alkalichloride aqueous solution based on the thermodynamic treatmentof experimental data (Orville, 1963; Iiyama et al., 1963; Debronet al., 1961) for particular systems. The excess thermodynamicfunctions (Z^E, S^E, H^E, V^E) of solid solutions were calculatedfor it. These data enable us to derive the diagrams of isothermsfor the distribution of sodium components between nephelineand alkali feldspar; between plagioclase and alkali feldspar;and among plagioclase, nepheline, and alkali feldspar for thetemperature range 400–1000 C. In order to check the calculated diagrams we analysed the mineralsfrom the binary and ternary assemblages of the systems nepheline$alkalifeldspar$plagioclase from various igneous massifs of nephelinesyenites of the world. Temperatures obtained from differentgeothermometers were found to coincide satisfactorily.