Heat capacity measurements for cryolite (Na3AlF6) and reactions in the system Na---Fe---Al---Si---O---F

Abstract

The heat capacity of cryolite (Na3AlF6) has been measured from 7 to 1000 K by low-temperature adiabatic and high-temperature differential scanning calorimetry. Low-temperature data were obtained on material from the same hand specimen in the calorimetric laboratories of the University of Michigan and U.S. Geological Survey. The results obtained are in good agreement, and yield average values for the entropy of cryolite of: S0298 = 238.5 J/mol KS0T-S0298 = 145.114 ln T+ 193.009*10-3T- 10.366* 105/T2- 872.89 J/mol K (273-836.5 K)[Delta]STrans = 9.9J/mol KS0T-S0298 =198.414 ln T+73.203* 10-3T-63.814*105/T2-1113.11 J/mol K (836.5-1153 K) with the transition temperature between [alpha]- and [beta]-cryolite taken at 836.5 K.These data have been combined with data in the literature to calculate phase equilibria for the system Na---Fe---Al---Si---O---F. The resultant phase diagrams allow constraints to be placed on the fO2, fF2, aSiO2 and T conditions of formation for assemblages in alkalic rocks. A sample application suggests that log fO2 is approximately -19.2, log fF2 is -31.9 to -33.2, and aSiO2 is -1.06 at assumed P/T conditions of 1000 K, 1 bar for the villiaumite-bearing Ilimaussaq intrusion in southwestern Greenland.