Critical evaluation of the thermodynamic data for titanite, in conjunction with a heat capacity equation that takes into account the P21/a ↔ A2/a transition and a more complete P-V-T data set for titanite, indicates that: (1) enthalpy of formation values greater than −2600 kJ/mol are not supported by calorimetry; (2) calculated internally consistent feasible solutions converge to enthalpy of formation from the elements values between −2608 and −2600 kJ/mol and 113 to 121 J/mol·K for the entropy; (3) the practice of adjusting the enthalpy of formation to fit phase equilibrium experiments may be erroneous, in contrast, it is the currently accepted entropy of 129.20 ± 0.84 J/mol·K that may need revision to a smaller value. Consequently, we optimize standard-state properties for end-member titanite (P21/a structure) consistent with the thermodynamic data in the program QUILF. In addition, we use a modified version of the program to calibrate equilibria among titanite (CaTiOSiO4), Fe-Mg-Ti ilmenite and spinel, Ca-Mg-Fe pyroxenes and olivine, and quartz. Calculations at 1 and 3 kbar, and 650, 850, and 1100 °C, in the system CaO-MgO-FeO-Fe2O3-TiO2-SiO2, suggest that the

\[reactions:\ augite\ +\ ilmenite\ =\ titanite\ +\ spinel\ phase\ +\ quartz\]


\[augite\ +\ ilmenite\ +\ quartz\ =\ titanite\ +\ orthopyroxene\]

impose well defined fO2, aSiO2, and XFeOpx restrictions to the assemblages (1) titanite + spinel phase + quartz, (2) titanite + orthopyroxene, (3) augite + ilmenite, and consequently titanite stability. The absence of quartz (aSiO2< 1 with respect to Qz) favors the sub-assemblage Fe-Mg-Ti spinel + titanite over augite + ilmenite, and the latter over titanite + orthopyroxene. Nonetheless, in the absence of quartz, no titanite-bearing assemblage is stable relative to olivine + orthopyroxene + augite + ilmenite + Fe-Mg-Ti spinel. From the phase relations, we can also infer that in quartz-saturated rocks: (1) titanite can coexist with orthopyroxene only at low XFeOpx values, regardless of fO2 conditions, whereas the assemblage titanite + orthopyroxene + spinel phase requires high fO2, relative Mg-enrichment (from high to moderate XFeOpx), and temperatures ≤650 °C. (2) The association of titanite with a spinel phase is generally indicative of relatively oxidizing conditions. (3) The assemblage titanite + olivine requires fO2 condition close to FMQ and relative Fe-enrichment (from moderate to high XFeOpx), and temperatures ≤650 °C.

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