BaFe[CO3]2, a new double carbonate: Synthesis, structural characterisation, and geostability implications for high and low PT

Abstract

For the first time, a new ordered double carbonate, BaFe(CO3)2, was synthesized by a solid-state reaction between BaCO3 and FeCO3 at high pressure-temperature conditions (PT; 3 GPa and 700 °C). This finding adds a new so-far unknown member to the group of norsethite-structured carbonates, which is often used as a geochemical analogue for dolomite. Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) reveal that BaFe(CO3)2 decomposes between temperatures of 450 °C and 880 °C, which is similar to norsethite (BaMg(CO3)2), but substantially higher than of pure FeCO3. The formation of BaFe(CO3)2 through solid-state reaction is evidently slower than that of BaMg(CO3)2 and BaMn(CO3)2 under the same experimental conditions. Using synthetic BaFe(CO3)2 powder as starting material, the reaction in the presence of CO2-bearing solution in contrast to water as a flux medium indicates the BaFe(CO3)2 instability at high PT conditions. The crystal structure and composition of the double carbonate were obtained using single crystal X-ray diffraction (XRD), Raman spectroscopy, and electron probe (EP) analysis. Two different types of the new double carbonate were identified: The experimentally derived phase belong to crystal structures that are related to the R3¯m and R3¯c space groups, respectively. The Raman spectra indicated that the difference is caused by a superstructure parallel to the c-axis. The charge and spin states of Fe in the new phases is determined using Nuclear forward scattering of synchrotron radiation. Furthermore, the thermodynamic properties of the double carbonate at standard conditions are estimated and a phase diagram is constructed depicting the stability field in low-temperature aqueous environments with these results being compared to natural aqueous solutions.