ROLE OF IRON REDUCTION IN THE CONVERSION OF SMECTITE TO ILLITE IN BENTONITES IN THE DISTURBED BELT, MONTANA

The mixed-layer illite-smectite(I/S) components of the Cretaceous bentonites from the Sweetgrass Arch have 0 to 25% illite layers and no detectable structural Fe2+, whereas the samples from the Disturbed Belt have 25-90% illite layers, and all contain Fe2+. A positive correlation exists between the percentage of structural iron that is Fe2+ and the amount of fixed interlayer K in the I/S. The higher percentage of illite layers in the samples from the Disturbed Belt is attributed to reactions related to elevated temperatures caused by burial beneath thrust sheets. The increase in Fe2+/Fe3+ with increasing percentages of illite layers is tentatively attributed to a redox reaction involving the oxidation of organic matter. -from Authors