Novel Calcium sp 3 Carbonate CaC$_2$O$_5$-$I\bar{4}$2d May Be a Carbon Host in Earth’s Lower Mantle

CaC$_2$O$_5$-$I\bar{4}$2d was obtained by reacting CO$_2$ and CaCO$_3$ at lower Earth mantle pressures and temperatures ranging between 34 and 45 GPa and between 2000 and 3000 K, respectively. The crystal structure was solved by single-crystal X-ray diffraction and contains carbon atoms tetrahedrally coordinated by oxygen. The tetrahedral CO$_4$$^{4−}$groups form pyramidal [C$_4$O10]$^{4−}$complex anions by corner sharing. Raman spectroscopy allows an unambiguous identification of this compound, and the experimentally determined spectra are in excellent agreement with Raman spectra obtained from density functional theory calculations. CaC$_2$O$_5$-$I\bar{4}$2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO$_2$ under ambient conditions. As polymorphs of CaCO$_3$ and CO$_2$ are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC$_2$O$_5$-$I\bar{4}$2d, which therefore needs to be considered instead of end-member CaCO$_3$ in models of the mantle mineralogy