Effect of the shape of fundamental particles on XRD characteristics of illitic minerals

Abstract

Chemical and electron-microscope data (Pt-shadowing and HRTEM) on monomineral illitic samples are used to show that XRD peak positions and peak intensities related to the swelling characteristics of the samples strongly depend on the size of the mixed-layer crystals, while the Kubler index (the half-height width of the 001 reflection) of the samples recorded in Na-form is related to the thickness of fundamental particles. The ability of fundamental particles to associate face-to-face into mixed-layer crystals depends on geometric factors: particles of a given thickness are more prone to form mixed-layer crystals inducing more smectitic XRD characteristics if they have flat surfaces, a large diameter and are not elongated. The variability of particle morphologies detected in natural samples explains the lack of a good correlation between the Kubler index and the parameters reflecting the swelling ability of illites such as the percentage of smectite layers or the Ir index of Srodon (1984). In swelling clays, the thickness of coherent scattering domains, calculated from the Kubler index by means of the Scherrer equation, is larger than the mean thickness of the fundamental particles