Elucidation of the crystal structure of oxyapatite by high-resolution electron microscopy

High-resolution electron-microscopy (HREM) images from different hydroxyapa tite (OHAp) samples showed p3 projection symmetry along [001] instead of th e p6 projection symmetry compatible with the space group P6(3)/m of OHAp. I mage processing was used to establish without ambiguity that threefold symm etry dominates the images along [001]. OHAp crystals decompose in the trans mission electron microscope and it is concluded that the threefold symmetry observed corresponds to an early step in the decomposition process, the de hydration of OHAp to oxyapatite (OAp). A structural model for OAp where eve ry second O atom along the 6(3) axis in OHAp is removed has the maximal spa ce-group symmetry <P(6)over bar>. This is compatible with the p3 projection symmetry observed. Atomic shifts in this OAp model compared to the OHAp st ructure were estimated using the HREM images and geometric optimizations of the atomic structure. No refinements of the atomic coordinates against dif fraction data were possible but the simulated HREM images of this crude mod el fit well with the experimental images.