Pathways of smectite illitization.

Junction probability diagrams show variation in both composition and layer arrangement in mixed-layer clay minerals. These diagrams can represent short-range and long-range ordered, random and segregated interstratifications. Mineralogical analyses of illite/smectite from shale cuttings, bentonites and hydrothermally altered tuffs define characteristic reaction pathways. Shale and bentonite analyses fall along pathways joining smectite and illite on diagrams showing nearest-neighbour (R1) layer arrangements. Transition from random to R1-ordered interstratifications occurs in shale samples containing 60-70% illite layers, and in bentonites containing 55-67% illite layers. Analyses of alteration products, however, fall near a line connecting rectorite and illite, which represents the maximum degree of R1 layer ordering. No mineralogical evidence is available to suggest that these alteration samples formed from a smectite precursor. All samples develop next-nearest (R₂) and thrice-removed (R3) neighbour ordering along similar pathways. Transition to R2 ordering occurs gradually in samples composed of 65-80% illite layers, and samples containing >85% illite layers may show strong R3 ordering. Predominant ordering, fraction of illite layers and best-fit probabilities are given for 29 shale samples, 51 bentonite samples and 62 samples of hydrothermal alteration.-J.M.H.