Structures and stability of calcium and magnesium carbonates at mantle pressures

TL;DR

This study uses advanced computational methods to predict new stable structures of calcium and magnesium carbonates at mantle pressures, revising their phase diagrams and implications for Earth's deep carbon cycle.

Contribution

It identifies previously unknown stable carbonate structures at high pressures, updating the phase diagrams of CaCO₃ and MgCO₃ relevant to Earth's mantle.

Findings

Discovered a new stable CaCO₃ structure between 32-48 GPa.

Predicted a pyroxene-type CaCO₃ structure stable above 67 GPa.

CaCO₃ is more stable than MgCO₃ above 100 GPa.

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.