Solid-state NMR elucidation of the role of mineralizers in the thermal stability and phase transformations of kaolinite

Abstract

The effect of lithium nitrate mineralizer on the thermal stability and phase transformations of kaolinite has been studied using7Li,27Al and29Si 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 °C.27Al 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 hightemperature phases which contain only four- and six-coordinated Al. Thus at 920 °C, mineralized samples (MS) containγ-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.29Si MAS NMR shows that cristobalite crystallizes from segregated amorphous silica. The process is reversed between 1300 and 1400 °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.