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

Abstract

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-P21/c, were revealed using a crystal structure prediction technique. Mg2CO4-Pnma is isostructural to mineral forsterite (Mg2SiO4), while Mg2CO4-P21/c is isostructural to mineral larnite (β-Ca2SiO4). Transition pressure from Mg2CO4-Pnma to Mg2CO4-P21/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.

Orthocarbonates of Ca, Sr, and Ba—The Appearance of sp3-Hybridized Carbon at a Low Pressure of 5 GPa and Dynamic Stability at Ambient Pressure

“…Subsequent experimental synthesis has confirmed the existence of this structure at a pressure above 20 GPa or possibly lower . Recently, we have also shown the existence of Mg-orthocarbonate, which is stabilized at pressures above 50 GPa . Our results on the stability of the orthocarbonates of the other alkaline-earth metals, in particular Sr 2 CO 4 - Pnma , have been presented in the form of a conference presentation and the corresponding book of abstracts .…”

“…However, some of the structures of orthocarbonates are dynamically stable and can be recovered on decompression after high-pressure synthesis. Totally, there are five such structures: Mg 2 CO 4 - Pnma , Mg 2 CO 4 - P 2 1 / c , Ca 3 CO 5 - Cmcm , Sr 2 CO 4 - Pnma , and Sr 3 CO 5 - Cmcm . This is nearly 50% of the known structures of alkaline-earth orthocarbonates.…”

“…11 Recently, we have also shown the existence of Mg-orthocarbonate, which is stabilized at pressures above 50 GPa. 12 Our results on the stability of the orthocarbonates of the other alkaline-earth metals, in particular Sr 2 CO 4 -Pnma, have been presented in the form of a conference presentation and the corresponding book of abstracts. 13 A recent experimental synthesis has confirmed the existence of the Sr 2 CO 4 -Pnma phase.…”

Sr3[CO4]O Antiperovskite with Tetrahedrally Coordinated sp3-Hybridized Carbon and OSr6 Octahedra

“…In addition to the established orthoanions, it was recently predicted and experimentally shown that carbonates with [CO 4 ] 4– groups may even be recovered at ambient conditions. , Further, stable structures with stoichiometries M 2 [CO 4 ] and M 3 [CO 4 ]O (M = Mg, Ca, Sr, and Ba) have been predicted by DFT-based calculations. − Here, we report the successful outcome of the synthesis of one of these structures using M = Sr. In contrast to earlier syntheses, we obtained a carbonate with an antiperovskite structure, where the oxygen anion is coordinated by six Sr cations.…”

“…Tetrahedral coordination implies the presence of C-sp 3 hybrid orbitals. Initially, such sp 3 carbonates were obtained as high-pressure polymorphs of conventional carbonates at very high pressures ( p > 70 GPa) and high temperatures (a few thousand Kelvin) and were thought to exist only at these extreme conditions. − This situation changed when, instead of transforming carbonates by pressure, novel sp 3 carbonates were formed by reactions. − Guided by density functional theory (DFT)-based predictions, − alkaline-earth oxides were reacted with the corresponding carbonates at high temperatures and Sr 2 [CO 4 ]- Pnma and isostructural Ca 2 [CO 4 ]- Pnma were formed in a laser-heated diamond anvil cell (LH-DAC) at ≈20 GPa. − …”