Illite crystallinity an expandability: XRD and HRTEM studies of Gaspe Peninsula mudstones and slates

摘要

In mixed-layer illite/smectite (I/S) containing >80% illite, the presence of an expandable component and changes in its abundance are difficult or impossible to detect by X-ray diffraction (XRD) using conventional polar interstratification compounds. Although high-grade diagenetic and very low-grade metamorphic (VLGM) samples do not expand in response to ethylene glycol treatment, the illite crystallinity of illitic materials improves significantly with increasing diagenetic/metamorphic grade. Previously, peak broadening for anchizonal to epizonal phyllosilicates was thought to be related entirely to stacking faults. The present study reexamines the contribution of expandable components to IC in shale and slate samples of Gaspe Peninsula in the high-grade diagenetic and VLGM zone using XRD patterns and high-resolution transmission electron microscopy lattice-fringe images after treatment with n-alkylammonium cations. These reveal the presence of expandable components including Rl- and R3-ordered structures and "expandable illite".