Abstract
Babingtonite, Ca2.0( \({\text{Fe}}_{{0.6}}^{{2 + }}{\text{Mn}}_{{0.2}}^{{2 + }}\) Mg0.2)∑10. \({\text{Fe}}_{{1.0}}^{{3 + }}\) Si5O14(OH)1.0 from Herborn (Hessen, Germany) was comprehensively studied using powder X-ray diffraction, electron probe microanalysis, and IR absorption, Raman, and Mössbauer spectroscopies. The enthalpy of babingtonite formation from the elements was determined for the first time by high-temperature solution calorimetry on a Calvet microcalorimeter in 2PbO · B2O3 melt at T = 973 K as –6911.6 ± 10.2 kJ/mol. Its standard entropy was estimated as 338.8 ± 2.0 J/(mol K), and the standard entropy and Gibbs energy of formation were calculated: –1501.3 ± 2.0 J/(mol K) and –6464.0 ± 10.2 kJ/mol, respectively. The thermodynamic constants of the end-members of the babingtonite Ca2Fe2+Fe3+Si5O14(OH)–manganbabingtonite Ca2Mn2+Fe3+Si5O14(OH) isomorphous series were estimated: \({{\Delta }_{f}}H_{{{\text{el}}}}^{^\circ }\) (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), \({{\Delta }_{f}}S_{{{\text{el}}}}^{^\circ }\) (298.15 K) = –1496.8 ± 1.8 and –1499.0 ± 2.6 J/(mol K), and \({{\Delta }_{f}}G_{{{\text{el}}}}^{^\circ }\) (298.15 K) = ‒6422.0 ± 10.4 and –6430.0 ± 9.9 kJ/mol, respectively. The stability fields of babingtonite in the \({{P}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) –T coordinates were calculated for the redox conditions controlled by the quartz–fayalite–magnetite and magnetite–hematite buffers, and mineral associations of babingtonite characteristic of low-grade metamorphism and late skarn assemblages were determined in the \({\text{log}}{\kern 1pt} {{P}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}{\kern 1pt} - {\kern 1pt} {\text{log}}{\kern 1pt} {{P}_{{{{{\text{O}}}_{{\text{2}}}}}}}\) plane.
