Synthesis of calcium orthocarbonate, Ca2CO4-Pnma at P-T conditions of Earth’s transition zone and lower mantle

We show, by single-crystal diffraction studies in laser-heated diamond-anvil cells, that Ca₂CO₄ orthocarbonate, which contains $CO44−$ tetrahedra, can be formed already at ~20 GPa at ~1830 K, i.e., at much lower pressures than other carbonates with sp³-hybridized carbon. Ca₂CO₄ can also be formed at ~89 GPa and ~2500 K. This very broad P-T range suggests the possible existence of Ca₂CO₄ in the Earth’s transition zone and in most of the lower mantle. Raman spectroscopy shows the typical bands associated with tetrahedral $CO44−$-groups. DFT-theory based calculations reproduce the experimental Raman spectra and indicate that at least in the athermal limit the phase assemblage of Ca₂CO₄ + 2SiO₂ is more stable than 2CaSiO₃ + CO₂ at high pressures.