Synthesis of calcium orthocarbonate, Ca2CO4-<i>Pnma</i> at <i>P-T</i> conditions of Earth’s transition zone and lower mantle
Jannes Binck, Dominique Laniel, Lkhamsuren Bayarjargal, Saiana Khandarkhaeva, Timofey Fedotenko, Andrey Aslandukov, Konstantin Glazyrin, Victor Milman, Stella Chariton, Vitali B. Prakapenka, Natalia Dubrovinskaia, Leonid Dubrovinsky, Björn Winkler
Abstract
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.