Northeastern Section - 53rd Annual Meeting - 2018

Paper No. 8-10
Presentation Time: 8:00 AM-12:00 PM


JENKINS, David M. and SCHNEIDER, Jonathan, Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902-6000

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 XNaCl (= 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 XNaCl (18 m), while nepheline forms at brine concentrations below 0.05 XNaCl (2.9 m). This corresponds to sodalite formation in a brine with activity of NaCl (aNaCl) of 0.2 or greater, while nepheline forms in brines with aNaCl below 0.02, using the activity expression of Aranovich & Newton (1996 CMP). In this study, sodalite occurs well below halite saturation of 0.62 XNaCl (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.