Structural Heterogeneity and Crystallinity Indices of Natural Kaolinites

To overcome the existing uncertainty in the interpretation of kaolinite “crystallinity” indices (KCLs), such as HI (Hinckley, 1963), IK (Stoch, 1974; Stoch and Sikora, 1966), QF (Range and Weiss, 1969), AGFI (Aparicio and Galán, 1999; Aparicio et al., 2006), and WIRI (Chmielová and Weiss, 2002). Their values obtained for a representative collection of 30 kaolinite samples were compared with the results of modeling the corresponding X-ray diffraction patterns. It is shown that all the studied samples comprise a mixture of almost defect-free high-ordered kaolinite (HOK) and defective low-ordered kaolinite (LOK) phases. The HOK content shows correlation with the crystallinity index values described by different regression equations. The correlation is most prominent for HOK and the Hinckley index (HI), which is described by the quadratic equation HOK (%) = 12.236 HI² + 25.464 HI ‒ 1.2622 with the correlation factor R² = 0.993. The obtained equations can be used to find HOK and LOK concentrations in natural kaolinites. Comparison of the structural parameters of defective kaolinites obtained by modeling their XRD patterns with those of Expert System (Plançon and Zacharie, 1990) showed that the latter sometimes predicts: (1) one-phase highly defective kaolinites, whereas their diffraction pattern modeling establishes a mixture of HOK and LOK phases; and (2) in two-phase samples, the content of the low-defect phase (ldp) is greater than 100%.