Structural changes on thermal decomposition of the clay mineral halloysite (A12Si207 9 4H20 ) with progressive heating up to 1400"C have been followed by a combinat.ion of 29Si and 27A1 so[id state NMtL 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 principaUy associated with the formation of ,/-AlzO 3 and results in phase separation of alumina-rich and silicafich regions on a scale of < 5 nm. On further heating initially an extremely alumina-rich mullite folms. This progressively reacts with further silica, increasing the silica-content of the mullite, finally producing 3 : 2 muUite with the residual silica crystallising as cristobalite.

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