The Crystallization of Fluorapatite in the Presence of Hydroxyapatite Seeds and of Hydroxyapatite in the Presence of Fluorapatite Seeds.

The kinetics of growth of crystals induced by hydroxyapatite (HAP) seed crystals in supersaturated solutions of fluorapatite and of fluorapatite (FAP) seed crystals in supersaturated solutions of hydroxyapatite have been studied using the constant composition method. The reactions were investigated at relative supersaturations ranging from sigmaFAP = 0.99 to 12.0 at pH 6.5 and for HAP, sigmaHAP = 3.6 to 12.6 at pH 7.4. In FAP-supersaturated solutions, this phase was nucleated at the HAP surfaces and underwent growth at a rate more than three times greater than that on FAP seed crystals of equivalent surface area. Transmission electron microscopy (TEM) and electron diffraction imaging clearly demonstrated that the new needle-like HAP phase originated at the FAP surface with the 002 planes of HAP growing on FAP. In contrast, the mutual orientation of FAP crystallization on HAP seed crystals could not be established. Interfacial energies of FAP, HAP, and octacalcium phosphate (OCP) microcrystals against aqueous solutions were obtained by using a thin-layer wicking technique. The interfacial energy values 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 as compared with the other phases may explain why this phase has been so frequently implicated as a possible precursor to the formation of apatite, especially in biological mineralization reactions.