PHYSICOCHEMICAL CHARACTERISTICS AND THERMODYNAMIC PROPERTIES OF BABINGTONITE

The comprehensive physicochemical study of babingtonite Ca(Fe Mn Mg)Fe SiO(OH) (Herborn, Hessen, Germany) was carried out using powder X-ray diffraction, electron probe microanalysis, IR, Raman and Mössbauer spectroscopy. The enthalpy of formation of the studied babingtonite from the elements was determined for the first time using high-temperature solution calorimetry on a Calvet microcalorimeter in a melt of the composition 2PbO•BO at T = 973 K (−6911.6±10.2 kJ/mol). The value of its standard entropy was estimated and the values of the standard entropy and Gibbs energy of formation were calculated (338.8±2.0 J/(mol K), −1501.3±2.0 J/(mol K) and −6464.0±10.2 kJ/mol, respectively). The thermodynamic constants of the end members of the isomorphic series babingtonite CaFeFeSiO(OH) — manganbabingtonite CaMnFeSiO(OH): were estimated: ΔH°(298.15 K) = −6868.0±10.4 and −6876.9±9.9 kJ/mol, S°(298.15 K)=341.2±1.8 and 343.9±2.6 J/(mol K), ΔS°(298.15 K)=−1496.8±1.8 and −1499.0±2.6 J/(mol K), ΔG°(298.15 K)=−6422.0±10.4 and −6430.0±9.9 kJ/mol, respectively. The stability fields of babingtonite for oxidation-reduction conditions determined by two different buffers — quartz-fayalite-magnetite and magnetite-hematite — in P − t coordinates, as well as mineral associations of babingtonite in lg P — lg P coordinates, characteristic of low-grade metamorphism and late skarn parageneses, were calculated.