Well-established statistical parameters allow an assessment of the reliability of the structural model. In contrast, single-crystal X-ray diffraction is a powerful and unique tool that can provide accurate structure refinements under these conditions (Boffa Ballaran et al., 2013 ). Powder diffraction data obtained at pressures around 100 GPa generally do not yield accurate structure determinations, and typically do not allow unambiguous assignment of the space group or site occupancies. While structure-prediction techniques are undoubtedly useful for preliminary surveys of phase stabilities, they provide a range of possible new phases, derived under constraints such as unit-cell contents.
Models based on density functional theory (DFT) (Oganov et al., 2008 ) and interpretation of X-ray diffraction data and IR spectra imply that MgCO 3-II contains carbon in a tetrahedral coordination (Boulard et al. It is generally accepted that magnesite (MgCO 3) transforms to MgCO 3-II at 80–115 GPa (Isshiki et al., 2004 Boulard et al., 2011 ,2015 Maeda et al., 2017 ). A reliable structural characterization is, however, a prerequisite for determining phase stabilities and to understand, for example, why the p, T-phase diagram of MgCO 3 is relatively simple compared to the dense phase diagram of CaCO 3 (see summary in Bayarjargal et al., 2018 ). In addition, theoretical modelling predictions imply potential structural analogues of CO 4 4−-bearing carbonates and silicates, and thus carbonates with tetrahedrally coordinated carbon may be important to understanding the complex geochemistry of Earth's mantle.Ĭarbonates with tetrahedrally coordinated carbon are not well characterized, despite their potential significance, as structural studies have to be carried out under high-pressure conditions and are therefore challenging. Recent discoveries of novel compounds that contain tetrahedral CO 4 4− units ( e.g., Merlini et al., 2015 Cerantola et al., 2017 ) increase the relevance of such studies, as the new high-pressure phases may be stable at conditions prevalent in the deep part of Earth's lower mantle. 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.Ĭarbonates and their high-pressure behaviour have attracted significant interest because of their potential role as carbon-bearing phases in the deep Earth. The crystal structure of (Mg 0.85Fe 0.15)CO 3 (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well.
We laser-heated a synthetic (Mg 0.85Fe 0.15)CO 3 single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg 2.53Fe 0.47)C 3O 9 in the space group C2/ m that contains tetrahedrally coordinated carbon, where CO 4 4− tetrahedra are linked by corner-sharing oxygen atoms to form three-membered C 3O 9 6− ring anions. We have now determined the crystal structure of iron-bearing MgCO 3-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. The crystal structure of MgCO 3-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.
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