Heat capacity measurements of CaAlSiO4F from 5 to 850 K and its standard entropy

Abstract

Heat capacity (CP) data of Al-F-bearing titanite are presented that yield the standard entropy S°298.15 of F-Al-titanite CaAlFSiO4 (FAT). CP of synthetic FAT was measured with relaxation calorimetry and differential scanning calorimetry between 5 and 764 K. The results constrain S°298.15 to be 115.4 ± 2.0 J/(mol.K) and subsequently the standard Gibbs free energy of formation from the elements, ΔfG°, of CaAlSiO4F to be between –2583 ± 3.0 and –2588 ± 3.0 kJ/mol, and the standard enthalpy of formation from the elements, ΔfH°, to lie between –2728 ± 3.0 and –2733 ± 3.0 kJ/mol depending on the thermodynamic data retrieval approach. These data, in turn, can be used to quantitatively model high-grade and UHP fluid-rock interaction. The calculation of future petrogenetic grids involving F-bearing minerals and titanite solid solutions in the system CaTiSiO4O–CaAlSiO4F will only be possible by expanding existing internally consistent thermodynamic databases to the F-system.