Characterization of the intermediate cubic phase formed during the transformation of coprecipitated SiO 2 ‐Al 2 O 3 gel on heating was studied and X‐ray diffraction methods are reviewed and criticized. Coprecipitated gels of different SiO 2 /Al 2 O 3 ratios were prepared; all showed a 980°C exotherm followed by crystallization of the cubic phase and liberation of SiO 2 . Alkali extraction of SiO 2 showed two types present in the 980°‐heated product. One variety is free amorphous SiO 2 and the other, chemically bonded to alumina in the crystalline cubic phase, was isolated and characterized as Si‐Al spinel with the same composition as mullite. Thus, its formation from the gel of mullite composition shows the highest exotherm and the measured density agrees approximately with the theoretically calculated value.
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Akshoy Kumar Chakraborty (1979) studied this question.
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