A mechanism for aragonite to post-aragonite transition in MCO (M = Ca, Sr and Ba) carbonates: evidence of a hidden metastable polymorph.

To advance in the understanding of the Earth's carbon cycle, it is necessary to determine thermodynamic boundaries and kinetic barriers associated with the pressure-induced polymorphic sequence of alkaline-earth carbonates. Following a symmetry-based strategy within the martensitic approximation, we propose a two-step mechanism mediated by a hexagonal 6/ structure for the aragonite to post-aragonite transformation in the MCO (M = Ca, Sr, Ba) crystal family. The calculated transition pressures and activation energies, from ∼7 to 42 GPa and ∼0.3 to 0.6 eV, respectively, are low enough to allow this transformation to occur under mantle conditions. Our analysis reveals that the intermediate hexagonal structure is the early one proposed by Holl , , 2000, , 467-473 for high pressure BaCO, and later considered as metastable. Phonon calculations inform that this 6/ structure is in fact unstable at zero pressure. Remarkably, our molecular dynamics calculations showed that this instability smoothly leads to a dynamically stable 6 structure, which we confirm is actually the phase observed by Holl This finding allows us to reconcile previous controversial data and contributes to clarifying the role of carbonates in the Earth's interior.