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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Anovitz, Lawrence M., Hemingway, Bruce S., Westrum, Jr. , Edgar F., Metz, Guy W., Essene, Eric J.
• Other Authors: Department of Chemistry, Univ. of Michigan, Ann, Arbor, MI 48109, U.S.A., Department of Geological Sciences, Univ. of Michigan, Ann Arbor, MI 48109, U.S.A., Department of Geosciences, Univ. of Arizona, Tucson, AZ 85721, U.S.A.; Department of Geological Sciences, Univ. of Michigan, Ann Arbor, MI 48109, U.S.A., U.S. Geological Survey, 959 National Center, Reston, VA 22092, U.S.A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 1987
• Subjects: Geology and Earth Sciences; Chemistry; Science; Greenland; Ilimaussaq
• Online Access: https://hdl.handle.net/2027.42/26500; http://www.sciencedirect.com/science/article/B6V66-48C8KHX-17K/2/d1dfd64912af39f1b4c71ad5e9b36552; https://doi.org/10.1016/0016-7037(87)90120-7

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. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/26500/3/0000036.pdf