Burial diagenesis has resulted in structural and chemical changes in mixed-layer illite/smectite from shales of the Eocene Wilcox Group, Texas. Randomly interstratified illite/smectite high in expandable layers has been converted to regularly interstratified illite/smectite low in expandable layers with increased depth of burial. This change is reflected in the K₂O content and cation exchange capacity (CEC) of the mixed-layer clay-rich fine clay size fraction. Samples high in expandable layers have a low K₂O content and high CEC; samples low in expandable layers have a high K₂O content and low CEC. Infrared spectra suggest a progressive substitution of Al³⁺ for Si⁴⁺ in the tetrahedral sheets of the smectite layers and preferential conversion of iron-poor smectite layers to illite.

Concentrations of extractable cations over a 60 m (200 ft) thick shale sequence indicate that fluid has moved from the center to the margins of the sequence.

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