The illite in several cyclothemic Pennsylvanian shales and clays has been separated on the basis of particle size into 2M1 and IMd polytypes. K-Ar dates show the 2M1 component to be considerably older than the Pennsylvanian. The K-Ar age of the IMd component is less than half the age of Pennsylvanian sedimentation. The low age may be due to preferential Ar loss because of the small particle sizes involved or to reorganization and K-fixation in montmorillonite and degraded micas in post-Pennsylvanian time.

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