Transformation Kinetics of Protiated and Deuterated Amorphous Magnesium Carbonate Studied Using In Situ Total Neutron and X-ray Scattering

Amorphous magnesium carbonate (AMC) is recognized as a precursor material during the formation of crystalline magnesium carbonates, which are important materials, e.g., for next generation cements and CO2 sorbents, but its amorphous-to-crystalline transition is poorly understood. In this study, we combined in situ X-ray and neutron diffraction and total scattering to determine the activation energy of the amorphous-to-crystalline transition for AMC in water and identify the reaction mechanism. The calculated activation energy Ea of 41 ± 13 kJ/mol for the AMC to nesquehonite (MgCO3·3H2O) transformation matches previous studies indicating a dissolution–reprecipitation mechanism. Ea for AMC to hydromagnesite (Mg5(CO3)4(OH)2·4H2O) of 116 ± 19 kJ/mol is considerably higher and combined with our peak area analysis of total scattering data, we conclude that a second-order solid state transition is likely. Remarkably, we observed 2 orders of magnitude longer lifetime of deuterated AMC compared to protiated AMC in liquid.