Phase Transformations in Calcium Phosphate Crystallization

Calcium phosphates are widespread in geochemistry, in biomineralization, and as biomaterials, and thus they have solicited great interest among researchers. In this chapter, prenucleation phenomena, amorphous phases, and crystallization processes from amorphous precursors are discussed for the calcium phosphate system. The focus is almost exclusively placed on pathways resulting in apatite formation, which is the mineral most employed in biomineralization. Phosphate speciation in solution is strongly pH dependent, and this in turn calls for extreme care when designing experiments to study calcium phosphate crystallization. This notion is illustrated by comparing in situ crystallization data obtained at two different pH values, one where the major species in the initial solution is phosphate and one where hydrogen phosphate dominates. Drastically different behavior is seen and discussed. Prior to crystallization, an amorphous calcium phosphate is typically observed, and it is argued that our current understanding of the structure of these phases is far from complete. Finally, prenucleation phenomena occurring before the formation of the first condensed phase are reviewed, and it is highlighted that this area in particular merits further attention, especially in the undersaturated concentration domain.