A CRYSTALLINITY INDEX FOR SOIL KAOLINS AND ITS RELATION TO PARENT ROCK, CLIMATE AND SOIL MATURITY

Summary To sub‐divide the large group of kaolinitic soil clays a new empirical index of crystallinity was obtained from the X‐ray diffraction patterns of eight standard kaolins. The index used is the ratio of the intensity above background of the peak at 4.46Å to the intensity above background at about 2.43Å (43.2° 2θ CoKα) and ranges from 4.8 for halloysite to 83 for a well‐ordered triclinic kaolinite. For the kaolins from a group of soils from S.W. Nigeria, the ratio lies between 4.3 and 13.7 and can be tentatively related to the presence of weatherable minerals in the soil and/or the length of the dry season.