Size and charge characteristics of kaolinitic soils in SE Queensland

Some kaolinitic soils derived from basaltic material contain 2:1 minerals as a discrete phase and/or interstratified with kaolin as determined by X-ray diffraction (XRD) and by chemical analysis of DCB-treated clays, while others show little or no evidence of 2:1 minerals. Mineral impurities were not detected by electron diffraction in the latter group. There is good agreement between the interpretation of XRD traces and the SiO₂/Al₂O₃ ratio given by chemical analysis. Some kaolin is seen by electron microscopy to be of very small particle size, and two distinct morphological types have been recognized. High surface area measurements are consistent with fine particular size. These profiles contain kaolin with higher than normal CEC and permanent negative charges which cannot be accounted for by the presence of 2:1 clay minerals alone and which are attributed to a degree of isomorphous substitution. This substitution and/or mineral impurities may have limited kaolin crystal growth during neoformation.