Transmission and Analytical Electron Microscopy of the Smectite-To-Illite Transition

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Abstract
The smectite to illite reaction was studied by transmission and analytical electron microscopy (TEM/AEM) in argillaceous sediments from depths of 1750, 2450, and 5500 m in a Gulf Coast well. Smectite was texturally characterized as having wavy 10- to 13-Å layers with a high density of edge-dislocations, and illite, as having relatively defect-free straight 10-Å layers. The structures of smectite and illite were not continuous parallel to (001) at smectite-illite interfaces. AEM data showed that the smectite and illite were chemically distinct although smectite had a more variable composition. Illite formation appeared to have initiated with the growth of small packets of illite layers within subparallel layers of smectite matrix. With increasing depth, ubiquitous thin packets of illite layers increased in size until they coalesced.A model for the transition requires that the structure of smectite was largely disrupted at the illite-smectite interface and reconstituted as illite, with concomitant changes in the chemistry of octahedral and tetrahedral sites. At least partial Na-K exchange of smectite preceded illite formation. Transport of reactants (K, Al) and products (Na, Si, Fe, Mg, H20) through the surrounding smectite matrix may have taken place along dislocations.The smectite-to-illite conversion process for the studied samples does not necessarily appear to have required mixed-layer illite/smectite as an intermediate phase, and TEM and AEM data from unexpanded samples were found to be incompatible with the existence of mixed-layer illite/smectite in specimens whose XRD patterns indicated its presence.