Polycrystalline apatite synthesized by hydrothermal replacement of calcium carbonates

Aragonite and calcite single crystals can be readily transformed into polycrystalline hydroxyapatite pseudomorphs by hydrothermal treatment in a (NH4)(2)HPO4 solution. Scanning electron microscopy of the reaction products showed that the transformation of aragonite to apatite is characterised by the formation of a sharp interface between the two phases and by the development of intracrystalline porosity in the hydroxyapatite phase. In addition, electron backscattered diffraction (EBSD) imaging showed that the c-axis of apatite is predominantly oriented perpendicular to the reaction front with no crystallographic relationship to the aragonite lattice. However, the Ca isotopic composition of the parent aragonite, measured by thermal ionization mass spectrometry was inherited by the apatite product. Hydrothermal experiments conducted with use of phosphate solutions prepared with water enriched in O-18 (97%) further revealed that the O-18 from the solution is incorporated in the product apatite, as measured by micro-Raman spectroscopy. Monitoring the distribution of O-18 with Raman spectroscopy was possible because the incorporation of O-18 in the PO4 group of apatite generates four new Raman bands at 945.8, 932, 919.7 and 908.8 cm(-1), in addition to the nu(1)(PO4)symmetric stretching band of apatite located at 962 cm(-1), which can be assigned to four O-18-bearing PO4 species. The relative intensities of these bands reflect the O-18 content in the PO4 group of the apatite product. By using equilibrated and non-equilibrated solutions, with respect to the O-18 distribution between aqueous phosphate and water, we could show that the concentration of O-18 in the apatite product is linked to the degree of O-18 equilibration in the solution. The textural and chemical observations are indicative of a coupled mechanism of aragonite dissolution and apatite precipitation taking place at a moving reaction interface. (C) 2011 Elsevier Ltd. All rights reserved.