In the calcined low-Fe clay, Al was present in approximately equal proportions of 4-fold coordination (37.7%) and in the mid-range resonance (39.7%), with a smaller amount also present in 6-fold coordination (22.6%). These percentages correspond to average values from the two field strengths' datasets in Figure , and the values lie within the range reported by previous studies (Table ) . Differences between sets of reported values can arise from a combination of several factors: 27 Al MAS NMR measurement parameters; the somewhat subjective nature of peak-fitting; and, most notably, differences in starting clays and calcination conditions . clays (Table ) show that, even when multi-operator variation is eliminated, there is inherent variation between clays. are unknown, as these were all industrially sourced.