Heat capacity measurements for cryolite (Na3AlF6) and reactions in the system NaFeAlSiOF
The heat capacity of cryolite (Na 3 AlF 6 ) 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: S 0 298 = 238.5 J / mol K "> S 0 298 = 238.5J/mol K S 0 T − S 0 298 = 145.114 ln T+ 193.009∗10 −3 T− 10.366∗ 10 5 T 2 − 872.89 J / mol K (273−836.5 K ) "> S 0 T −S 0 298 = 145.114lnT+ 193.009∗10 −3 T−10.366∗ 10 5 T 2 − 872.89J/mol K(273−836.5K) Δ S Trans = 9.9 J / mol K "> ΔS Trans = 9.9J/mol K S 0 T − S 0 298 =198.414 ln T+73.203∗ 10 −3 T−63.814∗ 10 5 T 2 −1113.11 J / mol K (836.5−1153 K ) "> S 0 T −S 0 298 =198.414 ln T+73.203∗ 10 −3 T−63.814∗10 5 T 2 −1113.11J/mol K(836.5−1153K) with the transition temperature between α- and β-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 f O 2 , f F 2 , a SiO 2 and T conditions of formation for assemblages in alkalic rocks. A sample application suggests that log f O 2 is approximately −19.2, log f F 2 is −31.9 to −33.2, and a SiO 2 is −1.06 at assumed P T "> PT conditions of 1000 K, 1 bar for the villiaumite-bearing Ilimaussaq intrusion in southwestern Greenland.