TY - JOUR

T1 - Formation of Mg-Orthocarbonate through the Reaction MgCO3+ MgO = Mg2CO4at Earth's Lower Mantle P- T Conditions

AU - Gavryushkin, Pavel N.

AU - Sagatova, Dinara N.

AU - Sagatov, Nursultan

AU - Litasov, Konstantin D.

N1 - Funding Information: This study was funded by the RFBR under research projects #20-03-00774 and #20-35-90043. Crystal structure predictions, calculation of P– T phase diagram, Raman spectra, and electronic density distribution were performed within the project #20-03-00774, while calculations of melting curve—within the project #20-35-90043. Calculations of elastic properties were supported by a state-assigned project of the IGM SB RAS. The computations were performed using resources provided by the Novosibirsk State University Supercomputer Center. Publisher Copyright: © Copyright: Copyright 2021 Elsevier B.V., All rights reserved.

PY - 2021/5/5

Y1 - 2021/5/5

N2 - 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 Vp and Vs 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 AU is larger for orthocarbonate. The obtained Raman spectra of Mg2CO4-Pnma would help its identification in high-pressure experiments.

AB - 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 Vp and Vs 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 AU is larger for orthocarbonate. The obtained Raman spectra of Mg2CO4-Pnma would help its identification in high-pressure experiments.

UR - http://www.scopus.com/inward/record.url?scp=85106397260&partnerID=8YFLogxK

U2 - 10.1021/acs.cgd.1c00140

DO - 10.1021/acs.cgd.1c00140

M3 - Article

AN - SCOPUS:85106397260

VL - 21

SP - 2986

EP - 2992

JO - Crystal Growth and Design

JF - Crystal Growth and Design

SN - 1528-7483

IS - 5

ER -