Structural changes in precipitates and cell model for the conversion of amorphous calcium phosphate to hydroxyapatite during the initial stage of precipitation

► We found structural and compositional peculiarities of amorphous calcium phosphate (ACP) in the early stage of precipitation. ► We proposed a cell model for the hydroxyapatite (HA) crystallization. ► The ACP to HA conversion has been found to commence immediately after the ACP precipitation.