Illite "crystallinity" revisited

The Kubler Index (KI) is defined as the full width at half-maximum height ( FWHM) of the 10-Angstrom X-ray diffraction peak of illite-smectite interstr atified (I-S) clay minerals, The only parameters controlling the Kubler ind ex are assumed to be the mean number of layers (N) in the coherent scatteri ng domains (CSD). the variance of the distribution of the number of layers of the CSB, the mean percentage of smectite layers in I-S (%S), and the pro bability of layer stocking (Reichweite). The Kubler-Index measurements on ah-dried (KIAD) and ethylene-glycolated (K IEG) samples were compared to N and %S using the NEWMOD computer program to simulate X-ray diffraction patterns. Charts of KIAD versus KIEG corrected for instrumental broadening were made and isolines were mapped for constant N and CLS. Isolines allow a direct and rapid determination of N and RS fro m KI measurements. The method allows quantification of the metamorphic anchizone limits by con sidering mean thickness of fundamental particles in MacEwan crystallites. T he transition from diagenesis to the anchizone and from the anchizone to th e epizone of low-grade metamorphism corresponds to thicknesses of 20- and 7 0-layer fundamental particles, respectively.