Formation of Mg-Orthocarbonate through the Reaction MgCO3+ MgO = Mg2CO4at Earth’s Lower Mantle P–TConditions

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 Mg2CO4in the lower mantle at pressures above 50 GPa by ab initiocalculations. Mg2CO4is 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-Pnmaand Mg2CO4-P21/c, were revealed using a crystal structure prediction technique. Mg2CO4-Pnmais isostructural to mineral forsterite (Mg2SiO4), while Mg2CO4-P21/cis isostructural to mineral larnite (β-Ca2SiO4). Transition pressure from Mg2CO4-Pnmato Mg2CO4-P21/cis 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-Pnmahas a melting temperature more than 16% higher than the melting temperature of magnesite (MgCO3). At 23.7, 35.5, and 52.2 GPa, Mg2CO4-Pnmamelts at 2661, 2819, and 3109 K, respectively. Acoustic wave velocities Vpand Vsof Mg2CO4-Pnmaare very similar to that of magnesite, while universal anisotropy of Mg2CO4-Pnmais stronger than that of magnesite, as well as the coefficient AUis larger for orthocarbonate. The obtained Raman spectra of Mg2CO4-Pnmawould help its identification in high-pressure experiments.