Effect of chemico‐mineralogical composition on color of natural and calcined kaolins

A total of six washed French and Algerian kaolins were studied. Kaolinite, halloysite, muscovite, feldspars, anatase, rutile, gibbsite, goethite, and todorokite were present. The thermal behavior of the samples was studied and the transformation heats were determined and quantified by differential thermal analysis. Calcined samples from 900°C to 1400°C are studied by X‐ray diffraction, the results show that the crystallite sizes of mullite rises as the temperature rises. The calcined samples showed an inverse correlation of L* and the crystallite sizes of mullite due to the incorporation of chromophore elements (Fe³⁺, Ti⁴⁺, and Mn²⁺) in its structure. Muscovite and rutile phases decreased lightness and increased chromaticity. The reduction state of Fe³⁺/Fe²⁺ and Mn⁴⁺/Mn²⁺ at 1400°C enhanced lightness leading to the diminution of the b* parameter. The CIELAB color parameters were significantly affected with mineralogy and chemical compositions of the samples. Lightness of the natural kaolins is decreased (L* < 59) when organic matters beside manganese and iron oxides are present. L* was not affected when only iron (Fe²⁺) is present in the kaolin; however, the chromaticity is increased (b* > 22). Whiteness and tint indices (W₁₀, T_w,10) revealed that only one kaolin could be considered white (limits of CIE Colorimetry, 1986), though upon calcination, this number is enhanced to two. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 499–505, 2014