Abstract: Samples of altered volcanic materials, interbedded with dominantly carbonate sediments, were collected from Ordovician, Silurian, and Devonian strata in Michigan, Illinois, Kentucky, Pennsylvania, Virginia, and Alabama. These altered volcanics, originally called metabentonite and more recently K-bentonite, occur in thin seams commonly only a few inches thick and are generally believed to be altered Paleozoic volcanic ash falls because they contain typical igneous minerals in the heavy suite and retain textures and structures characteristic of ash deposits.
Petrographic, differential thermal, and X-ray analyses were made of these samples to determine the clay mineralogy. Mixed-layer illite-montmorillonite occurs in the majority of samples. Illite is present in all samples and chlorite occurs in a few samples. This is in general accord with findings of previous studies. The mixed-layer clay is believed to have been deposited as montmorillonite in a marine environment and subsequently altered largely to illite through fixation of K in the interlayer position.
K-bentonites from Michigan and Illinois contain a constituent which has a 7.6 Å d-spacing. It is suggested that this material may be of regional significance because its occurrence was confined to the Upper Mississippi Valley K-bentonites in samples of this study.