A shoulder at δobs = 16.0 ppm was observed in the spectrum acquired at B0 = 9.4 T, but not in the spectrum acquired at B0 = 11.7 T. The same phenomenon was observed by , who attributed this difference to the stronger influence of second-order quadrupolar interactions in the lineshape at lower field strengths. A weak resonance at δobs = 69.0 ppm was attributed to 4-fold Al in the muscovite impurity .