Novel Calcium sp3 Carbonate CaC2O5-I4̅2d May Be a Carbon Host in Earth’s Lower Mantle

CaC2O5-I4̅2d was obtained by reacting CO2 and CaCO3 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 CO44– groups form pyramidal [C4O10]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. CaC2O5-I4̅2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO2 under ambient conditions. As polymorphs of CaCO3 and CO2 are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC2O5-I4̅2d, which therefore needs to be considered instead of end-member CaCO3 in models of the mantle mineralogy.