Phase transitions in the system MgO–CO2–H2O during CO2degassing of Mg-bearing solutions

This study documents the paragenesis of magnesium carbonates formed during degassing of CO2 from a 0.15M Mg2+ aqueous solution. The starting solutions were prepared by CO2 sparging of a brucite suspension at 25°C for 19 h, followed by rapid heating to 58°C. One experiment was performed in an agitated environment, promoted by sonication. In the second, CO2 degassing was exclusively thermally-driven (static environment). Electric conductance, pH, and temperature of the experimental solutions were measured, whereas Mg2+ was determined by atomic absorption spectroscopy. Precipitates were analysed by X-ray diffraction, Fourier transform (FT) mid-infrared, FT-Raman, and scanning electron microscopy. Hydromagnesite [Mg5(CO3)4(OH)2·4H2O] precipitated at 25°C was followed by nesquehonite [Mg(HCO3,OH)·2H2O] upon heating to 58°C. The yield of the latter mineral was greater in the agitated solution. After 120 min, accelerated CO2 degassing resulted in the loss of nesquehonite at the expense of an assemblage consisting of an unnamed mineral phase: [Mg5(CO3)4(OH)2·8H2O] and hydromagnesite. After 240 min, dypingite [Mg5(CO3)4(OH)2·5H2O (or less H2O)] appears with hydromagnesite. The unnamed mineral shows greater disorder than dypingite, which in turn shows greater disorder than hydromagnesite. In the static environment, there is no evidence for nesquehonite loss or the generation of [Mg5(CO3)4(OH)2·XH2O] phases over the same timeframe. Hence, results indicate that the transformation of nesquehonite to hydromagnesite displays mixed diffusion and reaction-limited control and proceeds through the production of metastable intermediates, and is interpreted according to the Ostwald step rule. Nevertheless, variations in the chemistry of nesquehonite, combined with the established tendency of the mineral to desiccate, implies that its transformation to hydromagnesite is unlikely to follow a single simple sequential reaction pathway.