Kaolinites with various degrees of structural order and iron content were heated and subsequently analyzed via electron paramagnetic resonance. Iron was present in two different states in the heated materials, either as dilute structural Fe 3؉ ions or in concentrated Fe 3؉ phases. During metakaolinization, the environment of dilute Fe 3؉ ions changed, following modifications of the Al 3؉ coordination, and the Fe 3؉ concentration increased. With the breakdown of metakaolinite, the diffusion of Fe 3؉ ions induced their exsolution in superparamagnetic iron-rich domains (Fe 3؉ clusters in ␥-Al 2 O 3 and/or Fe 3؉ oxide nanophases), which produced a decrease in the dilute Fe 3؉ concentration. The subsequent breakdown of ␥-Al 2 O 3 and the formation of mullite made the dilute Fe 3؉ concentration increase again, because of the incorporation of Fe 3؉ ions in the mullite structure.

Six mullite samples, derived from heat-treated natural kaolinites with various iron content, were investigated and compared to synthetic monophasic mullite. They were analyzed by X-ray diffraction, diffuse reflectance spectroscopy, and electron paramagnetic resonance. To quantify mullite coloration, the CIE colorimetric system was used. In contrast to synthetic mullite, samples showed charge transfer bands involving O 2؊ and Fe 3؉ ions as well as ferric crystal-field transitions due to Fe 3؉ ions in iron oxide nanoparticles. Absorption edges showed red shifts. The resulting yellowness, saturation of which increased with the content of iron oxide nanoparticles, is direct evidence for the coloring effect of Fe 3؉ ions.