A Coarse-Grained Nepheline from Nephton, Canada: Structural Variations to 926 °C

A K-rich nepheline sample from Nephton, Canada, with a composition of (K1.83☐0.17)Na6.08[Al7.98Si8.04O32], was studied using in situ synchrotron high-temperature (T) powder X-ray diffraction (XRD) data and Rietveld structure refinement from 25 to 926 °C on heating and cooling on a fine-temperature (T) scale. The average structure was refined in space group P63. The sample contains satellite reflections indicative of a modulated, incommensurate superstructure. The Al and Si atoms are ordered differently in powdered samples from the same large crystal. The nepheline structure contains domains in which the Al and Si atoms are highly ordered, but the average structure of the domains gives rise to a partially disordered Al-Si distribution. On heating to 926 °C, the Al and Si atoms acquire a fully ordered Al-Si distribution, which is quenched on cooling to room T. The complete Al-Si order is deduced from the average {4} distances. Satellite reflections associated with positional disorder of the O1 atom was not observed. A small amount (9%) of vacancies, ☐, occur on the K site. Disorder of K–☐ occurs at 320 °C where some satellite reflections disappear. Some satellite reflections associated with Al-Si order remain to 926 °C. Re-order of K–☐ occurs at 377 °C on cooling, and the associated satellite reflections reappear.