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