A first-principles investigation on the enthalpy landscape for the hibonite solid solution: Implications for a nebular barometer

International audience ; Hibonite, nominally CaAl 12 O 19 , is among the first minerals thermodynamically predicted to have formed in the early history of our solar system. It can incorporate significant amounts of Ti (≤15 wt%, ∼2 cations per formula unit) into its crystal structure as both Ti 4+ and Ti 3+ . The main pathways for Ti incorporation in the solar nebula include a direct substitution of Ti 3+ replacing Al 3+ and a coupled substitution in which Ti 4+ and Mg 2+ replace two Al 3+ . Additionally, the formation of oxygen vacancies can also reduce a Ti 4+ cation to Ti 3+ by trapping a free electron. The relative amounts of these cations potentially reflect the fugacity of oxygen (fO 2 ), a fundamental thermodynamic parameter, that prevailed when hibonite first formed or last equilibrated. However, the Ti content and its oxidation state in hibonite does not depend solely on fO 2 . The composition of the system is, thus, a key factor in changing the Ti 4+ /∑Ti ratio of the structure concurrently with the fO 2 . Therefore, it is necessary to understand the energetics, complex crystal chemistry, and substitution reactions of hibonite in order to relate the Ti oxidation state to the fO 2 of the nebular system in which condensed. To that end, we report DFT calculations (0 K) to determine the ground-state energies and the enthalpy of formation (ΔH) of hibonite solid solutions that span the range reported in meteorites. Our results show that coupled substitution is energetically favored (ΔH=-96.70 kJ.mol -1 , from oxides). In comparison, the formation of oxygen vacancies is energetically unfavorable, but similar to Ti 3+ direct substitution for Al 3+ (ΔH=∼60 kJ.mol -1 , from oxides), which is commonly observed in hibonite. It is therefore necessary to consider oxygen vacancies as a potential mechanism for controlling the incorporation of Ti³⁺ into hibonite, in addition to direct replacement reactions. We provide here the first reliable estimation of the formation enthalpies for the hibonite solid solution that ...