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Abstract--The Kfibler Index (KI) is defined as the full width at half-maximum height (FWHM) of the 10-,~ X-ray diffraction peak of illite-smectite interstratified (I-S) clay minerals. The only parameters controlling the Ktibler Index are assumed to be the mean number of layers (N) in the coherent scattering domains (CSD), the variance of the distribution of the number of layers of the CSD, the mean percentage of smectite layers in I-S (%S), and the probability of layer stacking (Reichweite).The Kiibler-Index measurements on air-dried (KIAD) and ethylene-glycolated (KIEG) 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 %S. Isolines allow a direct and rapid determination of N and %S from KI measurements.The method allows quantification of the metamorphic anchizone limits by considering mean thickness of fundamental particles in MacEwan crystallites. The transition from diagenesis to the anchizone and from the anchizone to the epizone of low-grade metamorphism corresponds to thicknesses of 20-and 70-layer fundamental particles, respectively.

L'analyse par diffraction des rayons X révêle que la kaolinite, ciment des grès de Messaoud, est en fait de la dickite. Toutefois, les minéraux kaoliniques des passées fines et des siltstones sont de la vraie kaolinite. Il existe une nette différence granulométrique entre les cristaux de dickite, 20 à 60 ц, et les cristaux de kaolinite, 2 p, environ. L'étude de la distribution de la dickite et de la kaolinite, en fonction des divers ciments, illite et quartz secondaires, et des divers horizons RI et R 2, confirme l'hypothèse de néoformation pour la dickite et d'héritage probable pour la kaolinite. Aucune manifestation hydrothermale n'ayant été décelée dans ces grès, il est impossible d'attribuer à la dickite une telle origine, même si les pressions (475 kg/cm2) et les températures (100° С environ) actuelles peuvent rappeler de telles conditions.

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The main reason for the initial determinations of illite crystallinity (IC) was to support the exploration for liquid and gaseous hydrocarbons. The application in 1960 of the Weaver Sharpness Ratio to core materials of a borehole from eastern France indicated that it was not a reliable tool for identifying well-crystallized illite. This ratio was later replaced by the Full Width at Half-Maximum (FWHM), the value of which decreases regularly and consistently towards greenschist facies. The use of FWHM allowed a precise definition of the anchimetamorphic zone between the upper diagenesis and the epimetamorphism. Afterwards, analysis of weak-tointermediate diagenetic sequences showed that illite crystallinity decreases together with the amount of swelling interlayers in mixed-layer clay minerals. Technological improvements, such as computing and modelling of X-ray diffraction patterns, increased the analytical precision relative to measurements of the plain FWHM. Consequently, illite crystallinity went back to its initial use, namely detection of the transitions between diagenesis, anchi- and epi-metamorphism in smectitefree lithologies, where it can be used as a stratigraphic and mineralogic marker of alteration stages.