The crystal structures of Fe-bearing MgCO3 sp 2- and sp 3-carbonates at 98 GPa from single-crystal X-ray diffraction using synchrotron radiation

Abstract

The crystal structure of MgCO₃-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp ² to the sp ³ type of bonding. We have now determined the crystal structure of iron-bearing MgCO₃-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg_0.85Fe_0.15)CO₃ single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg_2.53Fe_0.47)C₃O₉ in the space group C2/m that contains tetra-hedrally coordinated carbon, where CO₄ ^4- tetra-hedra are linked by corner-sharing oxygen atoms to form three-membered C₃O₉ ^6- ring anions. The crystal structure of (Mg_0.85Fe_0.15)CO₃ (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.

Keywords: carbonates; crystal structure; high-pressure single-crystal X-ray diffraction; magnesite-II; sp2-carbonates; sp3-carbonates.