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A systematic set of samples with compositions in the Ca alpha‐SiAlON (α′) system has been fabricated using pressureless sintering to evaluate the phase relationships on, and near, the Ca α′ plane. A phase behavioral diagram for the Ca α′ plane within the Jänecke prism has been determined and found to be similar to those for the Y and Sm α′ systems. However, distinct differences were also observed; two additional regions (α′+ 27R + liquid) and (α′+ 33R + liquid) have been identified. Investigation of the microstructures revealed elongated Ca α′ grains in the samples prepared by pressureless sintering. The aspect ratio of the elongated grains of α′ increased as the concentration of stabilizing cation was increased. Single crystalline phase Ca α′ samples of comparable density to hot‐pressed material were fabricated successfully using pressureless sintering.

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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