First Principles Calculation of the Stability of Iron Bearing Carbonates at High Pressure Conditions

Carbonate minerals such as ferromagnesite (Mg,Fe)CO ${}_3$ are suggested to be a possible major deep-carbon host in the lower mantle, because ferromagnesite is possibly stabilized by Fe spin crossover under pressure. However, the behavior of Fe-bearing carbonates under lower mantle pressure conditions has not been suitably examined thus far. Thus, in this study, we investigate the high-pressure stability of ferromagnesite and possible high-pressure structures with the chemical composition of (Mg ${}_{0.833}$ Fe ${}_{0.167}$ )CO ${}_3$ via first principles calculation using internally consistent local density approximation with Hubbard parameter (LDA+U) method, which can more accurately account for the electronic state of Fe than the LDA and generalized gradient approximation (GGA) approaches. The enthalpy values obtained via our calculations suggest that (Mg ${}_{0.833}$ Fe ${}_{0.167}$ )CO ${}_3$ undergoes phase transition from the R $\overline{3}$ c structure (high spin) to the P $\overline{1}$ (high spin) at 50 GPa, and to C2/m (high-spin) structure above 80 GPa, under static 0 K conditions. Therefore, no spin transitions in these carbonate minerals is expected under the lower mantle pressure conditions. View Full-Text