Relationships between chemico-mineralogical composition and color properties in selected natural and calcined Spanish kaolins

Abstract A total of 21 raw, washed and ground Spanish kaolins were studied. Kaolinite, mica, quartz, feldspars and occasionally anatase and analcime were present. Calcined samples in which the kaolinite had been transformed into metakaolinite (650 °C, 3 h), as shown by X-ray diffraction and observed using SEM were also studied. These samples featured particles with openings and a lateral loss of laminar continuity, generated spaces, joined crystal edges and superficial coatings which appeared to be fused to the surface of the aggregates. Whiteness and tint indices (W 10 , T w,10 ), revealed that only nine kaolins could be considered white (limits of CIE Colorimetry, 1986. 2nd ed. CIE Publication No.15.2. Vienna: Central Bureau of the CIE), though upon calcination, this number is reduced to three. Calcinating also produced coloring and reddening of the samples. The CIELAB color parameters significantly correlated with compositional properties such as mineralogy, the chemical analysis of elements, the free oxide contents (Al 2 O 3 , SiO 2 , Fe 2 O 3 ) extracted with ammonium oxalate and with citrate–dithionite–carbonate, and trace elements. In the non-calcined kaolins: C ab * is positively related to SiO 2 and negatively to the percentage of kaolinite; L* is negatively related to K 2 O; h ab is negatively related to K 2 O and MgO; and W 10 is positively related to the percentage of kaolinite and negatively to the percentages of mica, K 2 O, MgO, MnO and free Fe 2 O 3 . The calcined samples showed correlations of h ab and L* (positive) and of C ab * (negative) with the proportions of both elemental Al and the Al extracted as free oxides. There are nonsignificant differences between the L* of calcined and non-calcined kaolin samples, meaning that both samples fulfill the requirements of high lightness for use in the paper industry.