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We show, by single-crystal diffraction studies in laser-heated diamond-anvil cells, that Ca 2 CO 4  orthocarbonate, which contains CO 4  4  −  tetrahedra, can be formed already at ~20 GPa at ~1830 K, i.e., at much lower pressures than other carbonates with  sp  3 -hybridized carbon. Ca 2 CO 4  can also be formed at ~89 GPa and ~2500 K. This very broad  P-T  range suggests the possible existence of Ca 2 CO 4  in the Earth’s transition zone and in most of the lower mantle. Raman spectroscopy shows the typical bands associated with tetrahedral CO 4  4− -groups. DFT-theory based calculations reproduce the experimental Raman spectra and indicate that at least in the athermal limit the phase assemblage of Ca 2 CO 4  + 2SiO 2  is more stable than 2CaSiO 3  + CO 2  at high pressures.

Synthesis of calcium orthocarbonate, Ca 2 CO 4 - Pnma at P-T conditions of Earth’s transition zone and lower mantle