SOLID-STATE NMR ELUCIDATION OF THE ROLE OF MINERALIZERS IN THE THERMAL-STABILITY AND PHASE-TRANSFORMATIONS OF KAOLINITE

The effect of lithium nitrate mineralizer on the thermal stability and phase transformations of kaolinite has been studied using Li-7, Al-27 and Si-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) in tandem with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and TEM. As the temperature is raised, lithium nitrate melts, wets the surface of the kaolinite particles, and Li+ diffuses into the bulk of the crystals. The mineralizer retards the dehydroxylation of kaolinite by 15-20-degrees-C. Al-27 MAS NMR indicates that metakaolinite originating from mineralized kaolinite contains less five-coordinated and slightly more four-coordinated Al. This promotes the transformation of metakaolinite into high-temperature phases which contain only four- and six-coordinated Al. Thus at 920-degrees-C, mineralized samples (MS) contain gamma-alumina type spinel but non-mineralized samples (NMS) do not. At higher temperatures, MS and NMS contain different amounts of spinel phase, mullite and cristobalite. Si-29 MAS NMR shows that cristobalite crystallizes from segregated amorphous silica. The process is reversed between 1300 and 1400-degrees-C in MS (but not in NMS) and cristobalite vitrifies. This is probably caused by the lowering of the liquid temperature and subsequent rapid quenching of the melt.