Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using the evolutionary crystal structure prediction method uspex, we systematically explore the $\mathrm{MgO}\text{−}{\mathrm{CO}}_2$ and $\mathrm{CaO}\text{−}{\mathrm{CO}}_2$ systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While ${\mathrm{MgCO}}_3$ is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known ${\mathrm{CaCO}}_3$, and previously unknown ${\mathrm{Ca}}_3{\mathrm{CO}}_5$ and ${\mathrm{CaC}}_2{\mathrm{O}}_5$. ${\mathrm{Ca}}_3{\mathrm{CO}}_5$ polymorphs are found to contain isolated orthocarbonate ${{\mathrm{CO}}_4}^{4-}$ tetrahedra, and are stable at relatively low pressures $\left(>11\mathrm{GPa}\right)$, whereas ${\mathrm{CaC}}_2{\mathrm{O}}_5$ is stable above 33 GPa and its polymorphs feature polymeric motifs made of ${\mathrm{CO}}_4$ tetrahedra. Detailed analysis of the chemical stability of ${\mathrm{CaCO}}_3,{\mathrm{Ca}}_3{\mathrm{CO}}_5$, and ${\mathrm{CaC}}_2{\mathrm{O}}_5$ in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. Instead, ${\mathrm{MgCO}}_3$ is the main host of oxidized carbon throughout the lower mantle.

• Received 11 July 2017
• Revised 11 May 2018

DOI:https://doi.org/10.1103/PhysRevB.98.014108