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

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Binck, Jannes ; Laniel, Dominique ; Bayarjargal, Lkhamsuren ; Khandarkhaeva, Saiana ; Fedotenko, Timofey ; Aslandukov, Andrii ; Glazyrin, Konstantin ; Milman, Victor ; Chariton, Stella ; Prakapenka, Vitali B. ; Dubrovinskaia, Natalia ; Dubrovinsky, Leonid ; Winkler, Björn:
Synthesis of calcium orthocarbonate, Ca2CO4-Pnma at P-T conditions of Earth's transition zone and lower mantle.
In: American Mineralogist. Bd. 107 (2022) Heft 3 . - S. 336-342.
ISSN 1945-3027
DOI: https://doi.org/10.2138/am-2021-7872

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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.