Outstanding Problems in the Kaolinite‐Mullite Reaction Sequence Investigated by 29Si and 27Al Solid‐state Nuclear Magnetic Resonance: 11, High‐Temperature Transformations of Metakaolinite

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Abstract
Solid‐state 29Si and 27Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X‐ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI‐0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina‐rich but at higher temperatures progressively gains silica, approaching the conventional 3Al2O3· 2SiO2 composition. (4) The spinel phase contains insufficient Si to be detected by 29Si NMR but has a 27Al NMR spectrum consistent with γ‐Al2O3. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.