Structures and stability of calcium and magnesium carbonates at mantle pressures

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.

arXiv comment6 pages, 8 figures

References in corpus (3)

• Ab initio Random Structure Searching
• Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D" layer
• High-pressure phases of silane

Cited by in corpus (7)

• Raman spectroscopy and X-ray diffraction of sp3-CaCO3 at lower mantle pressures
• Novel high-pressure calcium carbonates
• Post-aragonite phases of CaCO${}_3$ at lower mantle pressures
• Ground state structure of polymeric carbon monoxide with high energy density
• Stability of calcium and magnesium carbonates at lower mantle thermodynamic conditions
• Anomalous thermal properties and spin crossover of ferromagnesite (Mg,Fe)CO3
• First-principles anharmonic vibrational study of the structure of calcium silicate perovskite under lower mantle conditions