Structural Characterization of the Spinel Phase in the Kaolin‐Mullite Reaction Series Through Lattice Energy

Kaolinite undergoes structural transformation on heating. X‐ray photographs reveal the existence of a spinel‐type phase when kaolin is heated at 980°C. The kaolinite decomposes into a spinel phase with the expulsion of silica. A controversy arises as to whether the spinel phase is γ‐Al 2 O 3 or Si‐Al spinel. Calculating the lattice energies of the structures confirms that the spinel phase is γ‐Al 2 O 3 and not Si‐Al spinel, as proposed earlier. The heat involvement in phase transformation, as obtained from experimental observation at 980°C, is also explained in the light of lattice energies.