Effects of Composition and Iron Spin State on the Structural Transition of (Mg,Fe)CO3 in the Earth's Lower Mantle

Abstract:

Iron-bearing magnesium carbonates (Mg,Fe)CO₃ are believed the major carbon carriers in the Earth’s deep lower mantle; they may play a crucial role in the Earth’s deep carbon cycle. Knowledge of the physical and chemical properties of these carbonates is thus essential for our understanding of the mantle’s role in global carbon cycle. Experiments have shown that (Mg,Fe)CO₃ ferromagnesite (calcite structure) can be stable up to 80-100 GPa. At 45-50 GPa, ferromangsite undergoes a high-spin to low-spin transition, accompanied by a volume reduction and elastic anomalies. Starting ~100 GPa, ferromagnesite goes through a complicated structural transition. The detail of this transition and the atomic structures of high-pressure (Mg,Fe)CO₃ phases are still highly debated. Experimental observations and theoretical results are inconsistent so far. In experiments, several distinct high-pressure (Mg,Fe)CO₃ structures have been reported, including a P21/c phase [1] and a Pmm2 phase [2]. In theory, a C2/m phase [3] and a P-1 phase [4] have been suggested, while the Pmm2 phase is not found. One possible reason for such a discrepancy is that all available theoretical calculations so far are based on pure MgCO₃, while experimental works are performed using (Mg,Fe)CO₃ with high iron concentration ( > 50%). Clearly, the concentration of iron and the possible iron spin crossover can significantly affect the stability of these high-pressure (Mg,Fe)CO₃ phases. Here, we use density functional theory + self-consistent Hubbard U (DFT+U_sc) calculations to study this structural transition. The effects of composition and iron spin state on these (Mg,Fe)CO₃ phases are also discussed. Our results can be expected to provide insightful information for better understanding the Earth’s deep carbon cycle.

[1] E. Boulard et al., Proc. Natl. Acad. Sci. USA 108, 5184 (2011).

[2] J. Liu et al., Sci. Rep. 5, 7640 (2015).

[3] A. R. Oganov et al., Earth Planet. Sci. Lett. 273, 38 (2008).

[4] C. J. Pickard and R. J. Needs, Phys. Rev. B 91, 104101 (2015).