Synthesis of microcrystalline hydroxylapatite and influence of the growth kinetics on the crystal growth morphology

Microsized hydroxylapatite (Ca5(PO4)3(OH)) was synthesized under mild hydrothermal conditions (T=220°C, autogenic pressure) by modifying the procedure proposed by Perloff and Posner (1956). The method involves the hydrolysis of a secondary calcium phosphate previously obtained (monetite, CaHPO4) into a tertiary calcium ortophosphate (hydroxylapatite). Crystals grown in this way were measured by X-ray powder diffraction using a D5000 Siemens diffractometer. They correspond to the pure hydroxylapatite (PDF 00-009-0432, Hodge et al.1938), are morphologically monoclinic and show habit modifications strongly dependent on the growth rate and supersaturation of the system. In fact, smaller crystals exhibit an undeniable monoclinic morphology showing well developed forms belonging to some of the pinacoids expected from the ab-initio calculations about the equilibrium shape of the hydroxylapatite (Aquilano et al., 2014). The morphology becomes gradually more complicated when crystal size increases, because of the recurring twinning of the crystals. At a first glance, the biggest ones can be misinterpreted as hexagonal crystals because of their morphology, but a careful observation of the relative extension of the forms, of the orientation of the closing facets with respect to the “prisms” and of their surface character, will banish all doubts about their monoclinic symmetry. The final growth morphology differs significantly from the calculated equilibrium shape, but some of the expected forms can be found again, with a change in the relative size due to the growth regime in the presence of the solvent. This observation fits in with the hypothesis that the twinning of monoclinic hydroxylapatite polymorph could mime both structure and morphology of the hexagonal one engendering a mess of conjectures, found in literature, about twinning and symmetry of pure hydroxylapatite. Perloff A., Posner A.S. 1956. Preparation of pure hydroxyapatite crystals. Science, 124, 583-584. Hodge H.C., LeFevre M.L., Bale W.F. 1938. Chemical and X-ray diffraction studies of calcium phosphates. Industrial and Engineering Chemistry, 10/3, 156-161 Aquilano D., Bruno M., Rubbo M., Massaro F. R., Pastero L. 2014. Low Symmetry Polymorph of Hydroxyapatite. Theoretical Equilibrium Morphology of the Monoclinic Ca5(OH)(PO4)3. Crystal Growth and Design, Article ASAP,