Filling the gap between lansfordite and nesquehonite: MgCO3·4H2O, a new magnesium carbonate hydrate

Formation conditions, transformation mechanisms, and transformation product selection of magnesium carbonate hydrate phases at temperatures near the freezing point of water are poorly understood. In this study, carbonated Mg(OH)2 suspensions were aerated at T = 0 °C to obtain solutions from which either MgCO3·6H2O or lansfordite (MgCO3·5H2O) precipitated, with the selected phase depending on the aeration rate. Subsequently, magnesium carbonate hexahydrate was subject to transformation experiments yielding five different products. The product selection depended on the transformation condition. Among the products, a new phase was identified and characterized as a neutral magnesium carbonate hydrate with the chemical composition MgCO3·4H2O. Implications of the synthesis route and analytical results suggest that this tetrahydrate phase forms from MgCO3·6H2O by the release of two weakly bonded water molecules per formula unit, accompanied by a relaxation of the remaining MgCO₃·4H₂O layers without disrupting the edge-sharing octahedral pairs of the parental structure. Density functional theory calculations confirmed the stability of the proposed crystal structure through the examination of its phonon dynamics and subsequent vibrational analysis. Furthermore, the simulated X-ray powder diffraction pattern is in reasonable agreement with the experimental data when accounting for the high crystalline disorder inevitably introduced by the transformation process.