Water-Controlled Crystallization of CaCO3, SrCO3, and MnCO3 from Amorphous Precursors

Calcium carbonate is the most abundant biomineral, whose amorphous form is stabilized in nature by a variety of organic additives and water. It is used to manipulate the morphology of new materials and to make strong inorganic/organic hybrid materials. However, the crystallization pathways (e.g., nucleation and growth, two-step nucleation pathways involving disordered, amorphous, or dense liquid states preceding the appearance of crystalline phases) remain often unclear. We have synthesized three amorphous carbonates, CaCO3 (ACC), SrCO3 (ASC), and MnCO3 (AMnC), that do not require any stabilization by additives to study their crystallization kinetics and mechanisms in the presence of water. The evolution of the carbonate concentration during crystallization was monitored potentiometrically with a pH electrode. The crystallization of ASC proceeds extremely fast, whereas the transformation of AMnC is relatively slow. ACC is an intermediate case between these extremes. The kinetic data were interpreted by a mathematical model based on a dissolution-recrystallization reaction. For high water concentrations, the dissolution rate (and for lower concentrations, the crystallization rate) determines the reaction kinetically. For all three carbonates, the crystallization rate increases with increasing water content. A comparison with the Pearson hardness of the cations indicates that the hydration energy and the binding strength of the hydration shell pose the main kinetic barrier for recrystallization.