Dehydration-Driven Glass Formation in Aqueous Carbonates

Amorphous carbonates,
in their liquid and solid (glassy)
forms,
have been identified to play important roles in biomineralization,
volcanism, and deep element cycling. Anhydrous amorphous calcium and
calcium–magnesium carbonate (ACC and ACMC05, respectively)
are structural glasses that exhibit a glass transition upon being
heated. We report a significant effect of the water content on glass
formation. The results yield a parametrization 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 isothermally cross the glass transition. This
work 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.