Aluminum-27 and Silicon-29 Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Kaolinite-Mullite Transformation

The ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of Al in tetra- and pentacoordination in dehydroxylated kaolinite. The ²⁹Si NMR signal analysis of samples heated above 400°C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the ²⁷Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the modification of the coordination of Al, which changes from the tetra- or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C for 36 h produces the same transformation in the coordination of Al ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystallinity are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200°C, the 3:2 mullite NMR spectrum.