Role of Structural Fe(III) and Iron Oxide Nanophases in Mullite Coloration

Abstract

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.

Mullite Phase

“…The results of Djemai et al 2001) and Soro et al (2001) that that added iron promotes the growth of the size of mullite conforms previous observation of Grofcsik and Tamas (1961). Moreover, iron is found to absorb during heating and formation of metakaolin.…”

“…The EPR and Mossbauer spectroscopic results of Djemai et al (2001) and Soro et al (2001) indicated the following role of iron oxide catalyst on mullitization.…”

“…Fig. 21.16 EPR spectra of raw and heated kaolinites (after Djemai et al 2001 The three steps of transformation are presented below.…”

“…The behavior of chemical and mineralogical impurities during the thermal transformation of kaolinite on heating has been studied for electron paramagnetic resonance resonance by Djemai et al (2001). Iron was detected in two different states in progressive heat-treated kaolinite ( Fig.…”

Elimination of discolouration in reformulated bone china bodies

“…Takashima 33 studied the behaviour of Cr 3+ and Fe 3+ ions in the gahnite crystallized from zinc opaque glazes and showed that almost all the Fe 3+ ions migrated into the gahnite (zinc-spinel) phase. Djemai et al 34 postulated that the increased whiteness of mullite samples, which contain iron oxide impurities, by iron incorporation into mullite structure.…”