Abstract
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.
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Smith, M.E., Neal, G., Trigg, M.B. et al. Structural characterization of the thermal transformation of halloysite by solid state NMR. Appl. Magn. Reson. 4, 157–170 (1993). https://doi.org/10.1007/BF03162561
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DOI: https://doi.org/10.1007/BF03162561