Using ab initio methods, the position and the orientation of a CO32- impurity in a hydroxyapatite lattice is studied. The CO32- ion is oriented in a plane that makes an angle of 7 degrees with the crystallographic c axis. The carbon atom and two oxygen atoms occupy positions close to the axis, whereas the third oxygen is oriented almost perpendicular to the symmetry axis. For the optimized structure, the vibrational characteristics are in reasonable agreement with the experimental data. The influence of experimentally observed variations of the lattice parameters on the geometry of the CO32- impurity is negligible. A study of the potential energy surface indicates that the six symmetry-related equilibrium positions are connected by rotation about the crystallographic c axis.