We report the first calorimetric observations of glass transition temperatures and crystallization rates of anhydrous, amorphous calcium-magnesium carbonate using fast scanning differential scanning calorimetry. Hydrous amorphous Ca _0.95 Mg _0.05 CO ₃ · 0.5H ₂ O (ACMC) solid was precipitated from a MgCl ₂ –NaHCO ₃ buffered solution, separated from the supernatant, and freeze-dried. An aliquot of the freeze-dried samples was additionally dried at 250°C for up to 6 h in a furnace and in a high-purity N ₂ atmosphere to produce anhydrous ACMC. The glass transition temperature of the anhydrous Ca _0.95 Mg _0.05 CO ₃ was determined by applying different heating rates (1000–6000 K s ⁻¹ ) and correcting for thermal lag to be 376°C and the relaxational heat capacity was determined to be C p = 0.16 J/(g K). Additionally, the heating rate dependence of the temperature that is associated with the corrected crystallization peaks is used to determine the activation energy of crystallization to be 275 kJ mol ⁻¹ . A high-resolution transmission electron microscopy study on the hydrous and anhydrous samples provided further constraints on their compositional and structural states. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.