Structural characterization of the thermal transformation of halloysite by solid state NMR

Structural changes on thermal decomposition of the clay mineral halloysite (Al₂Si₂O₇·4H₂O) with progressive heating up to 1400°C have been followed by a combination of²⁹Si and²⁷Al solid state NMR, X-ray diffraction and transmission electron microscopy. Thermal transformations closely follow those previously outlined for the related mineral kaolinite. Clear evidence is presented here that the major exotherm at 1000°C is principally associated with the formation of γ-Al₂O₃ and results in phase separation of alumina-rich and silicarich regions on a scale of <5 nm. On further heating initially an extremely alumina-rich mullite forms. This progressively reacts with further silica, increasing the silica-content of the mullite, finally producing 3: 2 mullite with the residual silica crystallising as cristobalite.