Physicochemical Study of Titanium-Bearing Garnets

— Four samples of natural Ti-bearing garnets from Odihincha, Maimecha-Kotui alkaline province, Krasnoyarsk Krai, Russia, were studied using electron microprobe analysis, X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman and Mössbauer spectroscopy, and thermal analysis. The enthalpies of formation from elements were determined with a high-temperature heat-flux Tian–Calvet microcalorimeter by means of the melt solution calorimetry. The first ever values of (298.15 K) were determined as follows: −5861.1 ± 11.3 kJ/mol for (Ca 3.00 Na 0.02 Fe 2+ 0.01 ) ( Al 0.21 Mg 0.04 )[(Si 2.85 )O 11.92 (OH) 0.08 ]; −5915.2 ± 9.0 kJ/mol for Ca 3.01 ( Mg 0.05 )[(Si 2.69 Al 0.16 )O 11.96 (OH) 0.04 ]; −5902.5 ± 9.1 kJ/mol for (Ca 2.97 )( Mg 0.05 )[(Si 2.64 Al 0.11 )O 11.96 (OH) 0.04 ]; and −5945.7 ± 10.2 kJ/mol for (Ca 2.90 Na 0.04 Mg 0.01 )( Mg 0.13 Zr 0.08 ) [(Si 2.33 Al 0.07 )O 11.84 (OH) 0.16 ]. The standard Gibbs free energies of formation of these garnets were calculated using the values obtained for the formation enthalpies and estimated for the entropies. Also, the enthalpies of formation of the end-members of the isomorphic schorlomite–morimotoite series were derived. The thermodynamic constants were used in quantitative modeling of the stability fields of these minerals.