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The rate of formation of microcrystalline hydroxyapatite from amorphous calcium phosphate has been studied at constant temperature as a function of the pH of the mediating solution; the effect of temperature on this conversion was observed in systems maintained at pH 8. The extent of reaction, determined by X-ray diffraction analysis of per cent crystallinity, was verified by titrimetric measurements. The rate of formation of crystalline material is characterized by a sigmoid plot of weight per cent crystallinity vs. time. The transformation kinetics, which can be described by a "first-order" rate law, is a function only of the pH of the mediating solution at constant temperature. A solution-mediated, autocatalytic mechanism is proposed which fits the experimental observations and explains the metastability of the amorphous material. Activation energies are reported for both the transformation process and the onset of nucleation.

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