Dehydration-Driven Glass Formation in Aqueous Carbonates

Amorphous carbonates, in both their liquid and solid (glassy) forms, have been identified to play an important role in biomineralization, volcanism, and deep element cycling. Anhydrous amorphous calcium and calcium-magnesium carbonate (ACC and ACMC05) are structural glasses which exhibit a glass transition upon heating. Here we report a significant effect of water content on glass formation. The results yield a parameterization enabling prediction of the stability of their liquid and solid amorphous phases as a function of temperature and water content. These results, obtained through novel fast differential scanning calorimetry, demonstrate that hydrous ACC and ACMC05 do indeed exhibit the behavior of structural glasses and that dehydration of these materials by lyophilization is a route that can be used to cross the glass transition isothermally. This presents a viable process for a significantly wider range of geo- and biomaterials. Dehydration controlled formation of glassy ACC therefore constitutes the missing link in the transformation from supersaturated aqueous solutions through an intermediate amorphous glassy state to crystalline CaCO3 polymorphs. These results yield direct implications for the mechanistic interpretation of geological processes and biomineralization.