Candidate carbonate phases in the Earth

With the current state of art of experimental facilities, we conducted studies on carbonates at the variable pressures and temperatures existing from the Earth’s crust to the mantle/core region conditions. The quality of the diffraction data (single crystal and powder) allow full structural analysis for all the phases encountered. We present an updated overview of all the possible candidate carbonate phases in the inner Earth, based on model carbonate system and on most-likely composition in the ternary diagram CaCO3-MgCO3-FeCO3. The model BaCO3 studied at high temperatures and high pressures present the sequence of transition from aragonite-type phase to disorder-calcite phase and finally NaCl structure. At high pressure, the aragonitetype transform into the post-aragonite above 8 GPa. A similar scenario is observed in CaCO3, but new highpressure polymorphs denser than aragonite are present above 15 GPa. Dolomite, previously considered unstable at mantle conditions, is demonstrated to be stable above 30 GPa at the high pressures and temperatures existing in the Earth’s interior up to the mantle/core boundary. It transforms to dense polymorphs based on distorted calcite-type structure at intermediate pressures, and, above the Mbar, transforms into new polymorphs featuring ring-carbonate C3O9 groups. This structure is likely the candidate phase for carbon storage in a carbonate in the Earth’s mantle, and may form complete solid solution with CaCO3 and MgCO3. Fe-magnesite, stabilized at high pressures and temperatures, undergoes complex redox reactions, with the formation of Fe3+ bearing carbonates with stoichiometry different from ABO3.