Single-crystal X-ray diffraction study on neighborite (NaMgF3) from Gierdingselva, Norway

The sample of neighborite, NaMgF3, studied in this paper has been found in miarolitic cavities of a mildly peralkaline (sodic) granite pluton in the Permian "Oslo Region" paleorift area, at Gjerdingselva, Lunner, Oppland, Norway. Neighborite forms cube-like, colourless or pale-yellow to brownish crystals up to I mm, often with distinct polysynthetic twinning striations, sometimes apparently untwinned. As representative of the different morphologies, two grains have been selected to collect diffraction data by a single-crystal P4 diffractometer (MoK alpha radiation). The crystal named NGH1 shows no striation and only a negligible presence of twinning was detected. Its crystal structure was refined (R-1 = 0.026) in the space group Pbnm with cell parameters a = 5.352 (1), b = 5.485 (1), c = 7.663 (2) angstrom. A second grain (NGH2) exhibits evident striations and other signs of twinning by optical and X-ray diffraction tests; the quite poor refinement of its crystal structure is not reported. The crystal structure of neighborite is discussed in terms of its deviation from the ideal cubic symmetry and compared with results published for synthetic NaMgF3. Neighborite in the Gjerdingselva ekerite is a low-temperature hydrothermal mineral which probably crystallized around or below 300 degrees C. The twinning most likely originated during growth due to internal stresses.