Crystal structures of rare earth elements rich apatite analogues

Abstract

Abstract The crystal structures have been determined for britholite-(Ce) and lessingite-(Ce) from the type localities and a third sample (‘min X’) showing chemical similarities to both britholite-(Ce) and lessingite-(Ce). This sample is from the Ilímaussaq intrusion in Greenland. They are rare earth elements (REE) rich apatite analogues. Based on the X-ray diffraction results they were assigned to the hexagonal system with cell dimensions slightly larger than those of apatite. The three structures have been refined in the space group P63 to R values: 0.033 [britholite-(Ce)], 0.039 [lessingite-(Ce)] and 0.036 (‘min X’). The site occupancy factors for the REE atoms are in good qualitative agreement with results from electron-microprobe analysis. The lowering of the space group symmetry relative to the apatite structure originates in differences in the oxygen positions. There is only analogy between one of the oxygen atoms of the SiO[unk]− and the oxygen atoms in apatite. The two REE sites found on threefold axis which correspond to Ca(1) in apatite are both nine-coordinated, but display significant variations in their REE—O distances. The REE found in a general position is seven-coordinated and similar to Ca(2) in apatite.