Illite cyrstallinity revisited: Review of studies over the past 30 years

The main reason for the initial determinations of illite crystallinity (IC) was to support the exploration for liquid and gaseous hydrocarbons. The ap plication in 1960 of the Weaver Sharpness Ratio to core materials of a bore hole from eastern France indicated that it was not a reliable tool for iden tifying well-crystallized illite. This ratio was later replaced by the Full Width at Half-Maximum (FWHM), the value of which decreases regularly and c onsistently towards greenschist facies. The use of FWHM allowed a precise d efinition of the anchimetamorphic zone between the upper diagenesis and the epimetamorphism. Afterwards, analysis of weak-to-intermediate diagenetic s equences showed that illite crystallinity decreases together with the amoun t of swelling interlayers in mixed-layer clay minerals. Technological impro vements, such as computing and modelling of X-ray diffraction patterns, inc reased the analytical precision relative to measurements of the plain FWHM. Consequently, illite crystallinity went back to its initial use, namely de tection of the transitions between diagenesis, anchi- and epi-metamorphism in smectite-free lithologies, where it can be used as a stratigraphic and m ineralogic marker of alteration stages.