LOW-TEMPERATURE FORMATION OF FLUORAPATITE IN AQUEOUS-SOLUTION

The low-temperature formation of fluorapatite by the reaction of particulate calcium phosphate precursors in aqueous solution was investigated: 6CaHPO4 + 3Ca4(PO4)2O-->(NaF)FAp Variations in the aqueous phase were measured and these correlated with the consumption of reactants and the formation of apatitic phases. The variations in solution pH and fluoride are in accord with a mechanism by which CaF2 precipitates relatively early in the reaction. After this, minimal fluoride is consumed until the formation of an apatitic phase reaches substantial completion. Once this has occurred, fluoride substitutes for hydroxyl and the pH increases. The extent of apatite formation exhibits a strong dependence on the concentration of the NaF solution in which reaction is occurring. Analysis of reaction in saturated CaF2 solution indicates that apatite formation precedes the uptake of fluoride from solution. For reaction in dilute suspension, a low concentration of NaF retards apatite formation whereas a higher one accelerates it. Alternatively, when reaction is carried out at a low liquid-to-solids ratio, apatite formation is accelerated in NaF solution regardless of concentration.