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Structures and stability of calcium and magnesium carbonates at mantle pressures


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Authors

Pickard, CJ 
Needs, RJ 

Abstract

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 67 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.

Description

Keywords

cond-mat.mtrl-sci, cond-mat.mtrl-sci

Journal Title

Physical Review B - Condensed Matter and Materials Physics

Conference Name

Journal ISSN

1098-0121
1550-235X

Volume Title

91

Publisher

American Physical Society (APS)
Sponsorship
Engineering and Physical Sciences Research Council (EP/J017639/1)
Engineering and Physical Sciences Research Council (EP/K014560/1)
Engineering and Physical Sciences Research Council (EP/F032773/1)
We acknowledge financial support from the Engineering and Physical Sciences Research Council United Kingdom (EPSRC) of the United Kingdom.