Pressure-Induced Ordering and Mesoporosity in Amorphous Magnesium Carbonate

Amorphous magnesium carbonate (AMC) is an anhydrous analogue of amorphous calcium carbonate, yet its structural stability and response to compressive stress remain poorly constrained. Here, AMC was synthesized by direct precipitation from formamide, yielding an X-ray–amorphous powder that retains trace solvent, as detected by FTIR. Quasi-static compression to ≈25 GPa at room temperature produced structural densification and medium-range order (MRO): powder X-ray diffraction showed broad maxima near the (104) and (113) positions of calcite-group (magnesite-type) carbonates, whereas selected-area electron diffraction remained diffuse, indicating the absence of long-range periodicity. Mesopores with a mean diameter of ≈100 nm formed, plausibly through solvent redistribution and cavitation during compression–decompression. Both the mesoporosity and the MRO persisted for at least 15 months under ambient conditions. In hydration experiments, uncompressed AMC dissolved and reprecipitated as euhedral hydromagnesite, whereas the compressed material was more resistant to alteration. These observations are consistent with pressure-induced densification to a higher-density amorphous state whose local MgO6–CO3 topology approaches, but does not attain, that of rhombohedral magnesite, imparting increased kinetic stability in aqueous environments. The results inform Mg-carbonate phase transformations in geological settings, biomineralization, and the design of mesoporous carbonate materials.