The crystal structures of Fe-bearing MgCO - and -carbonates at 98 GPa from single-crystal X-ray diffraction using synchrotron radiation.

The crystal structure of MgCO-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the to the type of bonding. We have now determined the crystal structure of iron-bearing MgCO-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (MgFe)CO single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (MgFe)CO in the space group 2/ that contains tetra-hedrally coordinated carbon, where CO tetra-hedra are linked by corner-sharing oxygen atoms to form three-membered CO ring anions. The crystal structure of (MgFe)CO (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles.