A series of smectites was investigated to reveal the thickness distribution of crystallites. The Fourier decomposition technique of Bertaut-Warren-Averbach (MudMaster program) was applied to XRD reflections of <0.2 μm, glycolated Na-clays. It was shown that the thickness distribution is lognormal and the mean thickness ranges from 5.7 to 12.3 nm (3.4 – 7.3 layers). At higher humidities, characteristic of TEM sample preparation, the mean thicknesses decrease, but the differences in mean thickness between samples are preserved. Beidellites have the thickest crystallites. The relationships between the mean thickness, volume-weighted mean thickness and the parameters of the lognormal distribution were established. Calculation of these parameters is possible, using the area and the maximum intensity of the 001 reflection. The influence of the fluctuations of d-spacing on the peak width was shown. Smectite crystallites, dispersed into individual layers during infinite osmotic swelling and rebuilt through coagulation, recover their original mean thickness and thickness distribution.

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A B S T R A C T:Mixed-layer illite-smectites (I-S) have a stable charge on the illite interlayer equal to 0-89lOre(OH)2, as shown by electron microscope measurements of mean fundamental particle thickness, using both Pt-shadowing and high-resolution techniques. This has been verified by independent measurements of total surface area and CEC. Typically, the smectite interlayer charge is close to 0.4/O10(OH)2 but clays evolving in K-deficient environments may exhibit higher values. In the course of illitization, the silicate layer composition becomes more restricted than the smectite composition, AI TM and AI vl increasing while Fe and Mg decrease. These substitutions lead to the composition of non-expandable illite differing distinctly from that of muscovite and phengite, close to FIX0.89AlvssFe0.05Mgo.toSi3.zoAlo.80 . HRTEM data show that XRD systematically underestimates %S layers in I-S due to the small size of coherent scattering domains. An experimental curve is proposed for correcting XRD data. The above results were obtained for I-S formed from volcanic materials in different diagenetic and hydrothermal environments. The illite interlayer charge value close to 0.9 reconciles well the available data on I-S produced by wetting and drying of K-smectites.