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 CaCO