Phase evolution during the formation of stoichiometric hydroxyapatite at 37.4 degrees C.

The mechanism of formation of stoichiometric hydroxyapatite in aqueous solution by the acid-base reaction of the precursor calcium phosphates tetracalcium phosphate [Ca4(PO4)2O, TetCP] and anhydrous dicalcium phosphate (CaHPO4, DCPA) was investigated. Phase evolution during the formation of hydroxyapatite was studied, and could be correlated with the kinetics of reaction. Initial dissolution of precursors is followed by hydroxyapatite nucleation and growth. Consumption of DCPA precedes that of TetCP and results in the formation of nonstoichiometric hydroxyapatite. As a result of continued TetCP dissolution, the pH of the solution increases towards a pH at which hydroxyapatite and Ca(OH)2 co-exist in equilibrium. Hydroxyapatite formation takes place in two stages: initially formed, nonstoichiometric hydroxyapatite subsequently reacts with TetCP and is converted to stoichiometric hydroxyapatite.