The standard thermodynamic properties of fluor-topaz, Al2SiO4F2, have been calculated from low- and high-temperature heat-capacity measurements and from high-temperature, oxide-melt calorimetry. Fluor-topaz (from Topaz Mountain, Thomas Range, Utah) containing 0.04 wt. percent water was used in all the experiments. Adiabatic calorimetry performed from 10.6 to 379.2 K gives S298S0 of 105.4 ± 0.2 J/mol · K. Combined heat capacities determined by adiabatic calorimetry (200–380 K) and differential scanning calorimetry (340–800 K) were fit to the following polynomial (equation valid 200–1,000 K):

The enthalpy of the reaction CaF2 + Al2O3 + SiO2 = CaO + Al2SiO4F2 was measured at 970 K by oxide-melt calorimetry and gave ΔH970=91.88±3.56 kJ. From our heat-capacity measurements and ancillary data, we calculate ΔH298=96.12±3.95 kJ for the reaction and ΔHf,298=3084.45±4.70 kJ/mol, and ΔGf,298=2910.66±4.74 kJ/mol for fluor-topaz.

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