The stability of sodalite in the system NaAlSiO4-NaCl

Abstract

The reaction SODALITE = [beta]-nepheline + NaCl (s) was reversed in solid-medium apparatus (923 K, 7.6-8.0 kbar; 973 K, 7.4-7.6 kbar; 1073 K, 7.8-8.0 kbar; 1173 K, 8.4-8.6 kbar) and the reaction SODALITE = carnegieite + NaCl (l) was reversed at 1 bar (1649-1652 K). The calculated slope for the first reaction is -11 bar/K compared to the experimentally constrained slope above 973 K of +5 bar/K. This major discrepancy can be eliminated if the entropy of sodalite (Na8Al6Si6O24Cl2) is 61.7 J/mol [middle dot] K greater than the measured value of 848.1 +/- 4.0 J/mol [middle dot] K at 298.15 K. This entropy contribution is equal to that expected from 58% disorder of Al and Si on the tetrahedral sites, but could also be caused by a reversible phase change. The experimental reversals between 923 K and 973 K can be fit with a dP/dT of -11 bar/K, suggesting that the excess entropy for sodalite is present only above 923 K. A phase diagram for the NaAlSiO4-NaCl system that is consistent with the measured thermochemical data and the experiments between 973 and 1650 K can be generated if the 61.7 J/mol [middle dot] K entropy contribution is included in the S2980 of sodalite. This entropy contribution must be removed below 973 K for the experiments to fit with calculations. Previously unreported thermodynamic data estimated in this study are [Delta]G2980 for sodalite (-12697 kJ/mol) and carnegieite (NaAlSiO4) (-1958 kJ/mol), S2980 of carnegieite (129.6 J/mol [middle dot] K) and compressibility of NaClliquid (V298P (cm3) = 31.6 [middle dot] [1 - 24.7 [middle dot] 10-3 [middle dot] P + 800 [middle dot] 10-6 [middle dot] P2]) (T in K; P in kbar).Sodalite is a high-temperature, low-pressure phase, stable well above the solidus in sodic silica-undersaturated magmas enriched in NaCl, and its presence constrains NaCl activities in magmas. Estimates of minimum NaCl (l) activities in the Mont St-Hilaire sodalite syenites are 0.05 at 1073 K and 0.13 at 1273 K. Density calculations are consistent with the field observations that sodalite phenocrysts will float in a nepheline syenite liquid. This explains the enrichment of sodalite in the upper levels of the sodalite syenites at Mont St-Hilaire and elsewhere.