A TEM-BASED DEFINITION OF 2-1 LAYER SILICATES AND THEIR INTERSTRATIFIED CONSTITUENTS

The layer structure and chemical composition of individual packets of 2:1 layer silicates in illitic materials from various environments hav e been studied with analytical high-resolution electron microscopy usi ng ultrathin sections treated with n-alkylammonium. Individual packets consist of a polar or nonpolar structure, as judged by the response t o treatment with n-alkylammonium. The status of illite-smectite mixed- layers attributed to these materials is found to reflect the compositi on and arrangement of polar and nonpolar 2:1 layer silicates. The natu re of the 2:1 mixed-layer silicate thus cannot be ascribed to a mixtur e of illite and smectite layers. In our opinion, a definition based on TEM reflects the true nature of mixed-layer materials more accurately than one based on XRD: it leads us (1) to distinguish among different types of expandable and nonexpandable components; (2) to document var iations in the density of interlayer charge of the expandable componen ts, which are not detectable by XRD; (3) to distinguish between expand able and nonexpandable illite, which do not respond differently to eth ylene glycol treatment; (4) to distinguish between fundamental particl es and short-range ordered structures; (5) to relate the ordering of t he T-O-T layers to the chemical composition, as measured directly by T EM (XRD study of oriented samples gives information on the interlayer spacing only); and (6) to characterize all types of particles present (not only the coherent sequences, as given by XRD).