In the 27 Al MAS NMR spectrum for the as-received low-Fe clay, the main resonance exhibited an observed chemical shift centred at δobs = 0.5 ppm (Figure ), independently of the magnetic field used. The majority of Al sites were therefore in 6-fold co-ordination, attributed to octahedral Al sites in kaolinite and a minor contribution from octahedral Al sites in the muscovite impurity .