Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions

We report an overview of the crystal structures of carbonates determined ab‐initio with X‐ray single crystal diffraction techniques at mantle conditions. The determined crystal structures of high‐pressure polymorphs of CaCO3 have revealed that structures denser than aragonite can exist at upper and lower mantle pressures. These results have stimulated the computational and experimental research of thermodynamically stable polymorphs. At lower mantle conditions, the carbonates transform into new structures featuring tetrahedrally coordinated carbon. The identification of a systematic class of carbonates, nesocarbonates, cyclocarbonates, and inocarbonates reveals a complex crystal chemistry, with analogies to silicates. They provide fundamental input for the understanding of deep carbonatite melt physical properties. The possible polymerization of carbonate units will affect viscosity, and the reduced polymerization in crystal structures as a function of oxidation state could suggest that also oxidation state may affect the mobility of deep carbonatitic magmas. Finally, we report two high‐pressure structures of mixed alkali carbonates, which reveal that these compounds may form wide solid solutions and incorporate a sensible amount of vacancies, which would allow incorporation of high‐strength elements and therefore play an important role for geochemical element partitioning in the mantle.