THE CRYSTALLIZATION OF FLUORAPATITE IN THE PRESENCE OF HYDROXYAPATITESEEDS AND OF HYDROXYAPATITE IN THE PRESENCE OF FLUORAPATITE SEEDS

The kinetics of growth of crystals induced by hydroxyapatite (HAP) see d crystals in supersaturated solutions of fluorapatite and of fluorapa tite (FAP) seed crystals in supersaturated solutions of hydroxyapatite have been studied using the constant composition method. The reaction s were investigated at relative supersaturations ranging from sigma(FA P) = 0.99 to 12.0 at pH 6.5 and for HAP, sigma(HAP) = 3.6 to 12.6 at p H 7.4. In FAP-supersaturated solutions, this phase was nucleated at th e HAP surfaces and underwent growth at a rate more than three times gr eater than that on FAP seed crystals of equivalent surface area. Trans mission electron microscopy (TEM) and electron diffraction imaging cle arly demonstrated that the new needle-like HAP phase originated at the FAP surface with the 002 planes of HAP growing on FAP. In contrast, t he mutual orientation of FAP crystallization on HAP seed crystals coul d not be established. Interfacial energies of FAP, HAP, and octacalciu m phosphate (OCP) microcrystals against aqueous solutions were obtaine d by using a thin-layer wicking technique. The interfacial energy valu es measured in pure aqueous solutions were 18.5, 9.0, and 4.3 mJ m(-2) for FAP, HAP, and OCP, respectively. The much smaller value for OCP a s compared with the other phases may explain why this phase has been s o frequently implicated as a possible precursor to the formation of ap atite, especially in biological mineralization reactions. (C) 1997 Aca demic Press