Abstract
Recently, calcium orthocarbonate (Ca2CO4-Pnma) has been predicted to be stable under P–T conditions of the Earth’s transition zone and the lower mantle. Here, we investigate its stability against the main mantle minerals in the pressure range of 20–100 GPa and temperatures of 1000–2000 K, based on the density functional theory within quasi-harmonic approximation. We found that Ca2CO4 appears in equilibrium with MgO (periclase) in the whole studied P–T range, while its coexistence with MgSiO3 (bridgmanite) is prohibited owing to the following reactions: Ca2CO4 + 2MgSiO3 → 2CaSiO3 + MgCO3 + MgO and Ca2CO4 + MgSiO3 + SiO2 → 2CaSiO3 + MgCO3. Our results revealed that Ca2CO4 can coexist with SiO2 up to 70–90 GPa at 500–2000 K where it reacts to produce carbon dioxide and Ca-perovskite, Ca2CO4 + 2SiO2 → 2CaSiO3 + CO2, typically observed as inclusions in ultradeep diamonds.
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