Thermodynamic and Kinetic Crystallization Pathways

As mentioned above, consideration of the thermodynamic equihbrium is not very well suited to describe the crystalhzation pathways observed in biomin-eraUzation and biomimetic minerahzation processes. Instead, it can be stated that these systems are usually far from equiUbrium so that kinetic effects prevail over thermodynamic control, with the consequence that it is still almost impossible to predict the morphology of a crystal with the currently existing thermodynamic equUibriiun models, which are more or less a reflection of Wulff s rule [58]. 

Kinetically driven crystallization often involves an initial amorphous phase that may be non-stoichiometric, hydrated, and susceptible to rapid phase transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble, has a low density of almost half of the crystalline mineral indicating a high hydration [62], and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In aqueous solution, this transformation into vaterite or calcite takes place within seconds or less even if additives are present, as shown by recent SAXS/WAXS measurements of ACC transformation in the presence of a DHBC [63]. 

---