Enthalpies of formation of CaAl₄O₇ and CaAl₁₂O₁₉ (hibonite) by high temperature, alkali borate solution calorimetry

The enthalpies of formation of CaAl₄O₇ and CaAl₁₂O₁₉ (hibonite), by alkali borate solution calorimetry at 1063 K are: ΔH⁰_f,CaAl4O7, (1063 K; from oxides) = -[25.6 ± 4.7] kj g.f.w. and ΔH⁰_f,CaAl12O19 (1063 K; from oxides) = -[33.0 ± 9.7] kj g.f.w. Using experimental enthalpy data for CaAl_{4O7 and estimated values for CaAl12O19}, the standard enthalpies of formation of these compounds from the elements at 298 K are: ΔH⁰_f,CaAl4O7 (298 K; from elements) = -4,007 ± 5.2 kj g.f.w. and ΔH⁰_f,CaAl12O19 (298 K; from elements) = -10,722 ± 12 kj g.f.w. Comparison with high-temperature galvanic cell data for the Gibbs energy of formation of CaAl₁₂O₁₉ allows a calculation of the standard entropy of hibonite, S⁰_hibonite (1100 K) = 1280.2 J K g.f.w. This value is only about 2% larger than the oxide sum. Hence it is inferred that the standard entropy of hibonite at 298 K is probably only slightly larger than the oxide sum value of 343.7 J K g.f.w. The present data were used to extrapolate Kumar and Kay's (1985) data for the Gibbs energies of formation of hibonite and CaAl₄O₇ to the temperature range 1500-1700 K. These data were then used in equilibrium thermodynamic calculations of the condensation of a gas of solar composition. Contrary to calculations of Kornacki and Fegley (1984), who used thermodynamic data of Allibert et al. (1981) for calcium aluminates, our results show no stability field for CaAl₄O₇ in a gas of solar composition at 10^-3 to 10^-5 atm total pressure. At 10^-3 atm pressure, hibonite forms by reaction of corundum with the gas at 1725 K, begins to react with the gas to form gehlenite at 1607 K and disappears completely in a reaction to form spinel at 1494 K. The absence of CaAl₄O₇ from hibonite-, spinel-rich inclusions in carbonaceous chondrites cannot be used as an argument against a condensation origin for these objects.