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Mullite is an aluminosilicate of the composition Al 2 (Al 2+2x Si 2-2x )O 10-x  with x generally ranging between 0.2 and 0.5. XRD and  29 Si and  27 Al nuclear magnetic resonance (NMR) have been used to investigate the structure of various mullite compositions (x = 0.26, 0.36, 0.42, and 0.69) as well as sillimanite (x = 0) and γ-Al 2 O 3  (x = 1).  27 Al magic angle spinning (MAS) NMR of the central and satellite transitions have been used to determine the chemical shifts and quadrupole interaction parameters for the various AlO 4  and AlO 6  units. The isotropic chemical shifts of the various units are 5.9 ppm for AlO 6 , 69.1 ppm for AlO 4 (T), 55.0 ppm for AlO 4 (T′), and 45.7 ppm for AlO 4 (T*) where AlO 4 (T) denotes the aluminum tetrahedra in the double chains, and AlO4(T′) and AlO4(T*) those next to the oxygen Oc*. Quantitative numbers of the aluminum occupancy of these sites have been determined for the various powder samples. These results are in good agreement with the average structure model of mullites. NMR and XRD proved the presence of impurities of kyanite in natural sillimanite and of γ-Al 2 O 3  in the mullites with x = 0.42 and 0.69.

Study of the Al coordination in mullites with varying Al:Si ratio by 27 Al NMR spectroscopy and X-ray diffraction