The Stability of Sodalite in a Synthetic Syenite plus Aqueous Chloride Fluid System

Natural feldspathoidal syenites may be approximated by assemblages containing some or all of the phases sodalite, nepheline, one or two alkali feldspars, and aqueous chloride fluid in the system NaAISi3O8-KAISi3O8-NaAISiO4-KAISiO4-NaCI-KCI-H2O. The stability of sodalite in these assemblages was studied in the range 500–700 °C and 600–2000 bars fluid pressure. Sodalite appears to be a stable phase on the vapor-saturated liquidus in this system over a wide range of pressure. At or near the vapor-saturated liquidus minimum in this system, three distinct types of sodalite-bearing syenite can crystallize. Nepheline-sodalite-one alkali feldspar rocks, nepheline-sodalite-two alkali feldspars rocks and sodalite-analcime-bearing rocks crystallize below 1600 bars, between 1600 and 2750 bars and above 2750 bars fluid pressure, respectively. The effects of closed-system cooling on the assemblage sodalite-nepheline-two alkali feldspars-aqueous fluid are different and distinguishable from the effects of metasomatism. Closed-system cooling results in replacement of K-feldspar by albite, feldspathoids remaining nearly unchanged, while metasomatism generally results in sismultaneous enrichment or impoverishment in sodalite plus K-feldspar.