DIRECT HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPIC MEASUREMENT OF EXPANDABILITY OF MIXED-LAYER ILLITE SMECTITE IN BENTONITE ROCK

Samples of mixed-layer illite/smectite were investigated from a single bentonite bed zoned with respect to expandability from 90 to 30%. Chips of natural rocks were embedded in a resin, cut with an ultramicrotome, and observed by high-resolution transmission electron microscopy (HRTEM). These observations confirmed the X-ray powder diffraction (XRD) model of mixed-layer clays, i.e., that illite/smectite grains in natural rocks are built of mixed-layer crystals, from 1 to as many as 15 silicate layers thick. These crystals are present either as individual particles or, typically, they form nearly parallel face-to-face groupings called here quasi-crystals. Free fundamental smectite and illite particles as defined by Nadeau and coworkers were essentially absent. Interlayer spacing and crystal thickness were measured. The systematic discrepancy between XRD and TEM measurements of illite/smectite expandability are explained. -from Authors