Postaragonite phases of CaCO3 at lower mantle pressures

Abstract

The stability, structure, and properties of carbonate minerals at lower mantle conditions have significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron x-ray diffraction in a diamond anvil cell coupled with direct laser heating of ${\mathrm{CaCO}}_{3}$ using a ${\mathrm{CO}}_{2}$ laser, we identify a crystalline phase of the material above 40 GPa---corresponding to a lower mantle depth of around 1000 km---which has first been predicted by ab initio structure predictions. The observed $s{p}^{2}$ carbon hybridized species at 40 GPa is monoclinic with $P{2}_{1}/c$ symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of ab initio random structure search (AIRSS) and quasiharmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of ${\mathrm{CaCO}}_{3}$. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of ${\mathrm{CaCO}}_{3}$ and we postulate that the mineral is capable of undergoing $s{p}^{2}\ensuremath{-}s{p}^{3}$ hybridization change purely in the $P{2}_{1}/c$ structure---forgoing the accepted postaragonite $Pmmn$ structure.