29Si and 27Al MAS NMR study of the thermal transformations of kaolinite from North China
Abstract:The thermal transformations of kaolinite have been studied using 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NM R), X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TG). The experimental results show that this sample is a pure kaolinite which contains ~3% carbonaceous material as impurity. 27Al and 29Si MAS NMR spectra show that the microstructural evolution of the thermal transformation of kaolinite at 450-1050°C is similar to that of the kaolinite-mullite reaction series previously published by many authors. 29Si MAS NMR spectra of mullite at 1190 and deg;C display two resonances at ~-87 and-92 ppm, corresponding to sillimanite-type geometry around Si and the typical Si environment of mullite, respectively. At 1350°C, the splitting of (hk0) and (kh0) of mullite occurs, indicating that the primary mullite transforms into orthorhombic mullite. Simultaneously, the resonance at ~-92 ppm splits into two signals at ~-90 and -94 ppm. 27Al MAS NMR spectra of the mullite consist of three signals centred at ~-4, 45 and 60 ppm, corresponding to octahedral, distorted tetrahedral and tetrahedral Al sites, respectively.
Document Type: Research Article
Publication date: 2003-12-01