The Transformation of Amorphous Calcium Phosphate into Crystalline Hydroxyapatite

It has been shown that at medium and high supersaturations in the neutral and basic pH range the formation of hydroxyapatite (1–3) and octacalcium phosphate (4, 5) is usually preceded by the precipitation of a cryptocrystalline solid phase (amorphous calcium phosphate, ACP) which is metastable for a given length of time. Amorphous calcium phosphates have been recognized (6) as a class of highly hydrated (T) salts, their chemical composition depending on the solution environment prevailing during precipitation. The mechanism of the conversion of ACP into crystalline calcium phosphate is not yet clear. It has been suggested (2,8,9) that the principal mechanism of conversion is the dissolution of ACP before the onset of hydroxyapatite precipitation. However, kinetic pH and turbidity curves (4, 5) and electron micrographs (4, 10–12) showing that ACP spheres are intimately associated with the formation of crystalline material are at variance with the preceding conception. An alternative conception (11), suggesting that ACP is transformed during the period of metastability and serves as a template for the nucleation of the secondary, crystalline precipitate, is being examined in our laboratory.