Layer-stacking sequences and structural disorder in mixed-layer illite/smectite: image simulations and HRTEM imaging

R1- and R3-ordered mixed-layer illite/smectite (I/S) samples with 10, 18, and 30% expandable layers were investigated by high-resolution transmission electron microscopy (HRTEM) at 400 kV. In both R1 and R3 I/S samples, 1M_d stacking dominates. Although a high degree of stacking disorder largely accounts for 1M_d characteristics as seen in X-ray diffraction patterns, HRTEM images indicate that individual 2:1 layers of I/S have slightly different β values. Simulated images show significant differences between illite and smectite interlayers, but they cannot be differentiated in experimental images. All structure images show regions with coherent stacking, most of which are at least two or three times thicker than the predicted fundamental particles of Nadeau and coworkers. This observation supports the suggestion of Ahn and Peacor (1986b) that thin "fundamental particles' are secondary crystallites derived from larger crystals by cleaving at smectite interlayers during sample preparation. -from Authors