Date Published: March 01, 2020
Publisher: International Union of Crystallography
Author(s): Christine Rincke, Horst Schmidt, Wolfgang Voigt.
The formation of a previously unknown higher hydrated magnesium carbonate, MgCO3·6H2O, was confirmed. Its crystal structure differs from the other known magnesium carbonates significantly, but it exhibits similarities to NiCO3·5.5H2O.
In the MgO–H2O–CO2 system, besides the thermodynamically stable MgCO3 (magnesite), a variety of hydrated magnesium carbonates are known, which can be divided in basic magnesium carbonates, containing OH− ions [Mg5(CO3)4(OH)2·nH2O and Mg(CO3)(OH)2·nH2O], and neutral magnesium carbonates with the composition MgCO3·nH2O (Hopkinson et al., 2012 ▸; Jauffret et al., 2015 ▸). All these phases are of significant relevance in various technological processes, in geological explorations, mineral conversion in the sequestration of CO2 and in biomineralization (Chaka & Felmy, 2014 ▸). Nevertheless, there are many open questions with respect to the conditions of formation, the characterization, the crystal structure and the stability of higher hydrated neutral magnesium carbonates (Hänchen et al., 2008 ▸; Hopkinson et al., 2012 ▸; Rincke, 2018 ▸).
A new neutral magnesium carbonate with the previously unknown high water content of six units per formula, i.e. MgCO3·6H2O, was produced by passing gaseous CO2 through an aqueous suspension of MgO and storing the filtered solution at 273.15 K. The X-ray diffraction pattern and Raman spectra confirmed the formation of the new phase and its transformation to MgCO3·3H2O. MgCO3·5H2O was not found in our study. Obviously, the formation conditions of magnesium carbonate hydrates depend on the concentration of the MgO suspension, the CO2 pressure, the temperature regime and the time of storage. Therefore, it would be useful to carry out further systematic investigations on the chemical kinetics of the formation of the magnesium carbonate hydrates.