Post-Aragonite Phases of CaCO3 at Lower Mantle Pressures*

The properties of carbonate minerals at mantle conditions have significant impact on our understanding of the carbon cycle and the composition of the Earth interior. In recent years, there has been interest in the behavior of carbonates at lower mantle conditions, specifically in their C hybridization. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating using a CO₂ laser, we identify a crystalline phase of CaCO₃ above 40 GPa - corresponding to a lower mantle depth of ∼1,000 km - which is predicted by ab initio random structure search (AIRSS). The observed sp² C-hybridized species at 40 GPa is monoclinic (P2₁/c) and is stable up to 50 GPa, above which its structure cannot be indexed by existing CaCO₃ phases. We investigate with nudged elastic band calculations the reaction mechanisms between relevant phases of CaCO₃ and postulate that the mineral is capable of undergoing sp²-sp³ hybridization change purely in the P2₁/c structure - forgoing the accepted post-aragonite Pmmn structure.