Novel high-pressure calcium carbonates

TL;DR

This study uses crystal structure prediction to identify stable calcium carbonates under high pressure, revealing new compounds and confirming magnesium carbonate as the primary host of oxidized carbon in Earth's mantle.

Contribution

The paper predicts and characterizes new calcium carbonate structures at high pressures, expanding understanding of carbon hosts in Earth's mantle.

Findings

Ca3CO5 and CaC2O5 are stable under high pressure.

MgCO3 remains the main stable carbonate in the lower mantle.

Calcium carbonates are unlikely to exist in the Earth's lower mantle.

Abstract

Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO2 and CaO-CO2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO3, and newly predicted Ca3CO5 and CaC2O5. Ca3CO5 polymorphs are found to contain isolated orthocarbonate (CO4)4- tetrahedra, and are stable at relatively low pressures (>11 GPa), whereas CaC2O5 is stable above 33 GPa and its polymorphs feature polymeric motifs made of CO4 tetrahedra. Detailed analysis of chemical stability of CaCO3, Ca3CO5 and CaC2O5 in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. We conclude that MgCO3 is the main host of oxidized carbon throughout the lower mantle.