Structural changes upon dehydration of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a common transient precursor to biogenic crystalline calcium carbonate, but factors controlling the amorphous to crystalline transformation remain unclear. We present a structural analysis and comparison of hydrated and partially dehydrated, synthetic ACC samples. Thermogravimetric analysis showed total H₂O losses of 46% with heating to 115 C and 75% for heating to 150 C. The ¹H NMR spectra of hydrous ACC, obtained both directly and indirectly, via ¹³C- detection, contain signals from four principal hydrogen environments: translationally rigid structural H₂O, a restrictedly mobile H ₂O environment, fluidlike mobile H₂O that is decoupled from rigid H and C, and hydroxyl. The retention of some restrictedly mobile H₂O and lack of change in X-ray total scattering and absorption spectroscopy data for dehydrated ACC suggest that thermal dehydration does not significantly disrupt the calcium-rich ACC framework. NMR results and thermal analyses of samples dehydrated isothermally for extended periods indicate that the H₂O loss mechanism is kinetically hindered and occurs in three stages: simultaneous loss of fluidlike mobile, restrictedly mobile, and rigid H₂O → loss of restrictedly mobile and rigid H₂O → loss of hydroxyl and trapped rigid and mobile components that cannot be removed without transformation to crystalline calcium carbonate.