Defects and Short-range Order in Nepheline Group Minerals: a Silicon-29 Nuclear Magnetic Resonance Study

29Si magic-angle spinning nuclear magnetic resonance (NMR) spectra are presented for seven crystalline phases of the nepheline group: natural nephelines from a plutonic environment (Bancroft, Ontario) and a volcanic deposit (Mt. Somma, Italy), kalsilite, synthetic pure Na nepheline, carnegieite, and two samples of orthorhombic KA1SiO4. In all phases, nearly all of the Si sites have four A1 neighbors, indicating nearly complete A1-Si ordering. Excess Si over the 1:1 stoichiometric Si/Al ratio appears to substitute randomly for A1 on an ordered lattice, adding Si sites with 3 and 0 A1 neighbors in a 3:1 ratio. Various types of structural disorder, including A1-Si disorder, that are reported from some x-ray diffraction studies are probably long range in nature and are due to the presence of ordered domains. In naturally occurring nepheline, the relative abundance of T sites with three-fold local symmetry is maintained at the ideal stoichiometric value of 1/4, even when the K/(K+ Na) ratio is substantially lower. This is in agreement with conclusions reached about the average structure from x-ray data. The distinction between the two sites, at least in terms of the local structure that is reflected in 29Si NMR chemical shifts, is lost in a pure Na nepheline sample.