The "Al and "Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of A1 in tetru-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 mod$ication 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 A1 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 crystalliniv 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.
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