Illitization of smectite as seen by high-resolution transmission electron microscopy

Abstract

High-resolution transmission electron microscopy observations have been performed on a sample which represents an early stage of the transformation of smectite into illite (by hydrothermal alteration). Pure smectite (with uncollapsed layers of 1.25 nm thick), pure illite and interstratified illite-smectite have been distinguished. Detailed observations suggest the existence of two transformation mechanisms : (i) dissolution of smectite and crystallization of illite; (ii) solid-state transformation of one smectite layer into one illite layer. Interstratified illite-smectite minerals are of random type and probably formed by a solid-state mechanism. Pure illite crystals, generally of 1 M polytype, are mainly produced by dissolution-crystallization