Chemistry of illite-smectite inferred from TEM measurements of fundamental particles

Abstract

Abstract Mixed-layer illite-smectites (I-S) have a stable charge on the illite interlayer equal to 0·89/O 10 (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/O 10 (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, Al IV and Al VI 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 FIX 0·89 Al 1·85 Fe 0·05 Mg 0·10 Si 3·20 Al 0·80 . 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.