STABILITY OF SODALITE RELATIVE TO NEPHELINE IN NACL-H2O BRINES AT 0.6 GPA AND 750 °C

Nepheline and sodalite bearing rocks occur in various localities around the Northeastern United States and Eastern Canada. Determining the relative stability of nepheline (NaAlSiO₄) and sodalite (Na₈Al₆Si₆O₂₄Cl₂) can place importance constraints on the minimum concentration of NaCl needed to form sodalite relative to nepheline, such as for the York River nepheline gneisses, Bancroft, Ontario (Anderson & Cermignani 1991 Can Min) and the Beemerville carbonatite-alkalic rock complex in New Jersey (Maxey 1976 GSA Bull). Previous work on the stability relations of nepheline and sodalite have been determined at 0.06 - 0.2 GPa (Wellman 1970 J. Petrol.) in NaCl brines and at 0.7 - 0.9 GPa (Sharp et al. 1989 GCA) in the absence of water. This study investigated the reaction 6NaAlSiO₄ + 2NaCl = Na₈Al₆Si₆O₂₄Cl₂ at 750 °C and 0.5 - 0.6 GPa in the presence of brine concentrations ranging from 0.02 - 1.0 X_NaCl (= mole fraction of NaCl /(NaCl + H₂O)) to provide additional information on the hydrothermal formation of sodalite.

The starting materials were synthetic phases including nepheline, sodalite, and halite made in the system Na₂O-Al₂O₃-SiO₂-NaCl-H₂O. Sodalite was made at 800 °C, 0.2 GPa (water-free) in a cold-seal vessel and nepheline was made at 750 °C, 1.3 GPa (with 2 wt% water) in a piston-cylinder press. The halite was reagent-grade NaCl. Experiments were done in sealed Pt capsules for durations of 70-72 hours in a piston-cylinder press with all NaCl pressure media.

The results indicated that sodalite forms over a wide range of brine concentrations from 1.0 - 0.25 X_NaCl (18 m), while nepheline forms at brine concentrations below 0.05 X_NaCl (2.9 m). This corresponds to sodalite formation in a brine with activity of NaCl (a_NaCl) of 0.2 or greater, while nepheline forms in brines with a_NaCl below 0.02, using the activity expression of Aranovich & Newton (1996 CMP). In this study, sodalite occurs well below halite saturation of 0.62 X_NaCl (Driesner & Heinrich 2007 GCA). The nepheline-sodalite system investigated demonstrates sodalite’s ability to remain stable at low brine concentrations. This indicates that sodalite is a mineral common in high-grade, low-silica, alkali metasomatic conditions at mid-crustal metamorphic locations and may be a useful paleo-salinity indicator at low salinities.