『Abstract
　Aragonite and calcite single crystals can be readily transformed into polycrystalline hydroxyapatite pseudomorphs by hydrothermal treatment in a (NH4)2HPO4 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 ¹⁸O (97％) further revealed that the ¹⁸O from the solution is incorporated in the product apatite, as measured by micro-Raman spectroscopy. Monitoring the distribution of ¹⁸O with Raman spectroscopy was possible because the incorporation of ¹⁸O 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 ν1 (PO4) symmetric stretching band of apatite located at 962 cm^-1, which can be assigned to four ¹⁸O-bearing PO4 species. The relative intensities of these bands reflect the ¹⁸O content in the PO4 group of the apatite product. By using equilibrated and non-equilibrated solutions, with respect to the ¹⁸O distribution between aqueous phosphate and water, we could show that the concentration of ¹⁸O in the apatite product is linked to the degree of ¹⁸O 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.』

1. Introduction
2. Materials and methods
　2.1. Starting material
　2.2. Hydrothermal experiments
　2.3. Analytical methods
　　2.3.1. Scanning electron microscopy (SEM) and electron microprobe (EMP)
　　2.3.2. X-ray diffraction (XRD)
　　2.3.3. Raman spectroscopy
　　2.3.4. Electron backscatter diffraction (EBSD)
　2.4. Calcium isotope ratios
　　2.4.1. Aragonite by apatite replacement for investigation of the Ca isotopic composition
　　2.4.2. Sample preparation and Ca isotope analysis
3. Results
　3.1. Hydrothermal experiments - textural and chemical observations
　3.2. The behavior of ¹⁸O during the replacement of aragonite by apatite in ¹⁸O-enriched solution
　3.3. Analyses of the calcium isotopic composition of the initial aragonite and of the product of the replacement reaction
4. Discussion
　4.1. Textural relationships, calcium isotopic compositions and the mechanism of the replacement reaction
　4.2. The use of ¹⁸O as a tracer for the replacement reaction
5. Conclusion
Acknowledgements
References