The crystal structure, crystal chemistry and surface properties of kaolinite in kaolin clays have been studied. The crystallinity index was lower for hard kaolin than for soft kaolin. The floatability of hard kaolin was lower than that of soft kaolin. Hard kaolin was the dominant kaolin in bauxite. A new type of collector for hard kaolin was studied in order to selectively separate kaolin from diaspore in bauxite. The surface zeta potential variation of the hard kaolin as a function of pH was almost the same as that of the soft kaolin. The IEP of the hard kaolin varied from 2.5 to 3.8. The SiO(2) (wt%) content of the several kaolins showed negative correlations with IEP, but Al(2)O(3) (wt%) content showed positive correlation with IEP. The crystallinity index of kaolinite was not the major factor determining IEP. In the broad range of pH, the negative potential of the hard kaolin was lower than that of the soft kaolin. At the point (pH = 7.3+/-) of zero net proton charge at the edge of the kaolinite, TFe (wt%) showed negative correlation with surface zeta potential of the kaolin, but the crystallinity index of kaolinite showed a positive correlation. (C) 2003 Elsevier Ltd. All rights reserved.