MAS NMR evidence for the presence of silicon in the alumina spinel from thermally transformed kaolinite

Georgia kaolinite dehydroxylated at 950 degrees C for 24 h, then leached with KOH at 90 degrees C for 1 h, shows a well-resolved Si-29 MAS NMR resonance at -77.5 ppm, within 2 ppm of the calculated position for Si incorporated in gamma-alumina spinel, From a knowledge of the total SiO2 in the leached sample and its partitioning between the spinel and residual grain boundary phases, a maximum of 3.9 wt% is estimated to be present in the spinel.