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

HRTEM observations on a sample representing an early stage of the transformations of smectite into illite by hydrothermal alteration were able to distinguish pure smectite (with uncollapsed layers 1.25 nm thick), pure illite and interstratified illite-smectite. Two transformations are suggested: 1) dissolution of smectite and crystallization of illite, and 2) 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 1M polytype, are mainly produced by dissolution-crystallization.