CHEMISTRY OF I LL ITE-SMECTITE INFERRED FROM TEM MEASUREMENTS OF FUNDAMENTAL

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, AITM and AI vl increasing while Fe and Mg decrease. These substitutions lead to the composition f 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 indifferent 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. According to geochemical calculations, the ill ite-smectite group of clay minerals account for 30 % of the mass of the Earth's sedimentary cover (Garrels & Mackenzie, 1971, p. 245). This enormous mass, combined with a broad range of chemical composition, high surfac