Computer Science
Scientific paper
Aug 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.1943s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 8, pp.1943-1954
Computer Science
3
Scientific paper
The reaction SODALITE = -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 (Na 8 Al 6 Si 6 O 24 Cl 2 ) is 61.7 J/mol · K greater than the measured value of 848.1 ± 4.0 J/mol · 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 NaAlSiO 4 -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 · K entropy contribution is included in the S 298 0 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 G 298 0 for sodalite (-12697 kJ/mol) and carnegieite (NaAlSiO 4 ) (-1958 kJ/mol), S 298 0 of carnegieite (129.6 J/mol · K) and compressibility of NaCl liquid ( V 298 P (cm 3 ) = 31.6 · [1 - 24.7 · 10 -3 · P + 800 · 10 -6 · P 2 ]) ( 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.
Bohlen Steven R.
Essene Eric J.
Helffrich George R.
Sharp Zachary D.
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