Crystal‐chemical investigation of the “cubic Al–Si spinel” issued from the thermal transformation of kaolinite and halloysite

Since its discovery in 1959 as a transient nanocrystalline product of the thermal transformation aluminum-rich clays, so-called Al–Si spinel has been object numerous studies aimed at determining composition. In this work, samples obtained from fired kaolinite- and halloysite-containing clays were analyzed using X-ray diffraction, leading to conclude single cubic (space group Fd-3m) form, with an unvarying size crystallites 5 nm constant cell volume, typical transition alumina form. Trial-and-error Rietveld fitting procedure revealed crystal structure consisting oxygen close packing disordered cation sublattice that differs known forms alumina. quantitative phase analysis based on model was successfully implemented series halloysite giving first insight transformations 980–1260°C range.