THE H INCKLEY INDEX FOR KAOL IN ITES

ABSTRACT: The (02,11) X-ray diffraction band from a low-defect kaolinite from Cornwall (Hinckley index HI = 1-22) was examined to determine the defect structure. No combination of interlayer translations and admixing of dickite layers accurately modelled the observed diffraction pattern. Calculated iffraction patterns which gave a good agreement with the shape, position, and intensity of the observed peaks, uniformly had inter-peak intensities which were too weak. By treating the kaolinite as a mixture of low-defect (HI = 1.76) and moderate-defect (HI = 0.29) kaolinites, the agreement between the observed and calculated patterns was improved substantially. The existence ofa mixture of two kaolinites was also found for a number of low-defect samples (HI> 0.4) from Georgia and Cornwall, and may be of even wider occurrence. The HI, which is very sensitive to the inner-peak intensities, does not estimate the types or abundances ofvarious tructural defects (the classical "crystallinity"), but is related directly, in a non-linear fashion, to the proportions of the two kinds of kaolinite which are present in the sample. It is well known that phyllosilicates, and particularly those with low or zero layer charge, contain a variety of structural defects, and the kaolinites have been studied the mos