Quadrupolar effects to the NMR spectra were introduced using SIMPSON to carry out spin-simulations at the same sample spinning speeds as the MAS NMR experiments of 14 kHz (16.4 T & 11.8 T) and 40 kHz (23.5 T). The quadrupolar coupling constants, isotropic shieldings and asymmetry parameters for each Al site were used from the output of the GIPAW NMR calculations. These spectra were obtained without including any broadening in the frequency spectrum and were subsequently broadened using a Gaussian broadening scheme to produce the spectra shown in Fig. . To calculate the 2D isotropic vs. quadrupolar shift, the central transition of the quadrupolar shift for MQMAS spectra was calculated as follows,