HRTEM study of individual bone apatite nanocrystals reveals symmetry reduction with respect to P63/m apatite

In this study we present the first crystal structure model for bone apatite based on the analysis of individual nanocrystals by high resolution transmission electron microscopy (HRTEM). Crystallographic image processing of the obtained HRTEM images from different projections indicates symmetry reduction with respect to P63/m stoichiometric apatites and the presence of threefold symmetry along the c axis. Based on HRTEM observations and the measured Ca/P = 2 ratio we propose a structural model with phosphate-to-carbonate substitution and O vacancies localized along c axis, which explains the observed loss of 63 screw axis parallel, and the shift of mirror plane perpendicular to the c axis. Also, the presence of non-equivalent (010) surfaces has been proven. These results on the atomic structure of bone apatite nanocrystals contribute to the understanding of their biochemically controlled nucleation processes. •HRTEM based crystal structure model for bone apatite nanocrystals is presented.•Loss of sixfold symmetry along the c axis in bone apatite is deduced.•Structural model with phosphate-to-carbonate substitution and O vacancies localized along c axis is proposed.•Non-equivalent (010) surfaces are demonstrated.