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arXiv · 1407.3369

Structures and stability of calcium and magnesium carbonates at mantle pressures

Chris J. Pickard·Richard J. Needs
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Abstract

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO$_3$ and MgCO$_3$) at high pressures. We find a previously unknown CaCO$_3$ structure which is more stable than the aragonite and "post aragonite" phases in the range 32--48 GPa. At pressures from 67 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO$_3$ structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of MgCO$_3$ in the range 85--101 GPa. Our results lead to a revision of the phase diagram of CaCO$_3$ over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of MgCO$_3$. We predict CaCO$_3$ to be more stable than MgCO$_3$ in the Earth's mantle above 100 GPa, and that CO$_2$ is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle.

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Chris J. Pickard, Richard J. Needs. 2014-07-12. Structures and stability of calcium and magnesium carbonates at mantle pressures. https://doi.org/10.1103/physrevb.91.104101

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