Formation of aluminum rich 9:1 mullite and its transformation to low alumina mullite upon heating

The formation of aluminum rich mullites Al-4+2x Si2-2xO10-x with x > 2/3 has been studied at annealing temperatures between 700 and 1650 degrees C. Calcination of the amorphous precursor at 700 degrees C yields a mullite with 88 mol% Al2O3 corresponding to an x-value of 0.809. Simultaneously, a gamma-alumina phase is formed. Further increase of the annealing temperature yields an increase in the aluminum incorporation up to 92.1 mol% Al2O3 at 1000 degrees C derived from the refined lattice constants. This is the highest amount of Al observed so far in a mullite except the supposed end member iota-Al2O3 which, however, has not yet been established unambiguously. Above 1000 degrees C, the aluminum content in mullite is reduced This is accompanied by a transformation of the spinel-type phase to a superstructure of a theta-alumina like phase. The final product at 1650 degrees C consists of 34 mol% of a 'normal' mullite with x = 0.32 and 66 mol% corundum.