Formation of Mg-Orthocarbonate through the Reaction MgCO3 + MgO = Mg2CO4 at Earth's Lower Mantle P-T Conditions

Orthocarbonates of alkaline earth metals are the newly discovered class of compounds stabilized at high pressures. Mg-orthocarbonates are the potential carbon host phases, transferring oxidized carbon in the Earth's lower mantle up to the core-mantle boundary. Here, we demonstrate the possibility for the formation of Mg2CO4 in the lower mantle at pressures above 50 GPa by ab initio calculations. Mg2CO4 is formed by the reaction MgCO3 + MgO = Mg2CO4, proceeding only at high temperatures. At 50 GPa, the reaction starts at 2200 K. The temperature decreases with pressure and drops down to 1085 K at the pressure of the Earth's core-mantle boundary, approximately 140 GPa. Two stable structures, Mg2CO4-Pnma and Mg2CO4-P2(1)/c, were revealed using a crystal structure prediction technique. Mg2CO4-Pnma is isostructural to mineral forsterite (Mg2SiO4), while Mg2CO4-P2(1)/c is isostructural to mineral larnite (beta-Ca2SiO4). Transition pressure from Mg2CO4-Pnma to Mg2CO4-P2(1)/c is around 80 GPa. Both phases are dynamically stable on decompression down to the ambient pressure and can be preserved in the samples of natural high-pressure rocks or the products of experiments. Mg2CO4-Pnma has a melting temperature more than 16% higher than the melting temperature of magnesite (MgCO3). At 23.7, 35.5, and 52.2 GPa, Mg2CO4-Pnma melts at 2661, 2819, and 3109 K, respectively. Acoustic wave velocities V-p and V-s of Mg2CO4-Pnma are very similar to that of magnesite, while universal anisotropy of Mg2CO4-Pnma is stronger than that of magnesite, as well as the coefficient A(U) is larger for orthocarbonate. The obtained Raman spectra of Mg2CO4-Pnma would help its identification in high-pressure experiments.

Automated summary

Orthocarbonates of alkaline earth metals are a newly discovered class of compounds stabilized at high pressures. Mg2CO4, a potential carbon host phase, can be formed in the lower mantle at pressures above 50 GPa and high temperatures. It has two stable crystal structures, with higher melting temperatures and stronger anisotropy compared to magnesite, and its Raman spectra can aid in its identification in high-pressure experiments.