Physicochemical characteristics and thermodynamic properties of babingtonite

The comprehensive physicochemical study of babingtonite Ca_2.0(${\text{Fe}}_{0.6}^{2+}{\text{Mn}}_{0.2}^{2+}$Mg_0.2)_∑1.0${\text{Fe}}_{1.0}^{3+}$Si₅O₁₄(OH)_1.0 (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∙B₂O₃ 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 Ca₂Fe²⁺Fe³⁺Si₅O₁₄(OH) − manganbabingtonite Ca₂Mn²⁺Fe³⁺Si₅O¹⁴(OH): were estimated: Δ_f$H_{el}^{\mathrm{o}}$(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), Δ_f$S_{el}^{\mathrm{o}}$(298.15 K) = −1496.8 ± 1.8 and –1499.0 ± 2.6 J/(mol K), Δ_f$G_{el}^{\mathrm{o}}$(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_H₂O − t coordinates, as well as mineral associations of babingtonite in lgP_CO₂ – lgP_O₂ coordinates, characteristic of low-grade metamorphism and late skarn parageneses, were calculated.