Range of solid solutions of silica in spinel type phase

Al₂O₃–SiO₂ mixed gel of varying compositions has been synthesised by using aluminium nitrate nonahydrate (AR) and tetraethyl orthosilicate (BDH) by coprecipitation technique. Phase evolution studies of these gels have been made by X-ray diffraction technique and ²⁹Si MAS NMR analysis. The lattice parameter of intermediate Al–Si spinel phase has been measured. The calculated data indicate that the composition of spinel phase does not alter irrespective of the composition of mixed gels chosen. Results demonstrate that slightly <28 wt-%SiO₂ is likely to be present as solid solution with γ-Al₂O₃ in Al–Si spinel structure. Mullitisation reactions of mixed gels of compositions on two sides of mullite stoichiometry are described.