In mullite–alumina composite precursors, interaction between the silica matrix and the fine γ-alumina texture strongly influences the precursor phase transformation, the nucleation and the crystal geometry both of the mullite and of the α-alumina. The mullite–alumina composite precursor calcined at 1000 °C has a layered structure probably derived from the layered texture of the γ-alumina. The phase transition of this layered texture is retarded by the presence of the silica matrix and a metastable mullite phase is formed before nucleation of α-alumina. By leaching away the silica matrix, the remaining layered texture is readily transformed into very fine, thin α-alumina platelets by calcination at 1000 °C. This seems to be one reason for the appearance of elongated mullite grains in a pure mullite matrix and the platelet shaped α-alumina grains in the mullite–alumina composite prepared from diphasic precursors.