The Average Structure of a Fine-Grained Nepheline to 900 °C: Disorder of O1, Al-Si, and K-Vacancy

A fine-grained nepheline sample from Egan Chutes, Ontario, with the chemical composition (K1.32▯0.68)(Na6.05Ca0.22)[Al7.77Si8.21O32], where ▯ represents vacancies, was studied through in situ synchrotron powder X-ray diffraction (XRD) data and Rietveld structure refinement from 26 to 900 °C on heating and cooling. The average structure was refined in the space group P63. The satellite reflections in nepheline give rise to order–disorder transitions. The average <Al,Si-O>{4} distances in nepheline indicate a partially ordered Al-Si distribution, especially in the Al2 and Si2 sites at room T before heating. The nepheline structure shows that except for the positional disorder of the O1 oxygen atom, the other atoms are well defined and contain no unusual features. Vacancies, ▯, occur at the K site. Different satellite (s) reflections arise from (1) positional order of the O1 atoms to 299 °C (s1 disappears), (2) K-▯ order to 486 °C (s2 and s3 disappear), and (3) some Al-Si order 900 °C, where some satellite reflections are present. Complete Al-Si order is obtained at room T on cooling.