Role of structural Fe(III) and iron oxide nanophases in mullite coloration

Six mullite samples, derived from heat-treated natural kaolinites with vari ous iron content, were investigated and compared to synthetic monophasic mu llite, They were analyzed by X-ray diffraction, diffuse reflectance spectro scopy, and electron paramagnetic resonance. To quantify mullite coloration, the CIE colorimetric system was used. In contrast to synthetic mullite, sa mples showed charge transfer bands involving O2- and Fe3+ ions as well as f erric crystal-field transitions due to Fe3+ ions in iron oxide nanoparticle s. 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 Fe3+ ions.