Structures and stability of calcium and magnesium carbonates at mantle pressures

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO₃ and MgCO₃) at high pressures. We find a previously unknown CaCO₃ structure which is more stable than the aragonite and ``post aragonite'' phases in the range 32--48 GPa. At pressures from 76 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO₃ structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of MgCO₃ in the range 85--101 GPa. Our results lead to a revision of the phase diagram of CaCO₃ over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of MgCO₃. We predict CaCO₃ to be more stable than MgCO₃ in the Earth's mantle above 100 GPa, and that CO₂ is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle.