Detection of non-hexagonal symmetry in an apatite-structure-related mineral (nasonite)

Abstract

An analysis of nasonite by high-resolution electron microscopy (HREM) has shown: (a) a loss of the twofold screw axis in the structure, and (b) a deviation from hexagonal symmetry. These results have been checked with a series of computer calculations which show the absence of artefacts in the imaging process. An X-ray structure determination of the same specimen has been carried out resulting in space group P63/m which is in contradiction with the HREM findings. It is suggested that the differences in results between the HREM and the X-ray diffraction techniques arise from the presence of phase microdomains inside the original crystal, although no crystal domain showing a hexagonal structure was observed using HREM.