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

Abstract

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.