Potassium-bentonites of Ordovician age in Iowa consist of 1Md mica interstratified with about 30 percent expandable layers. Accessory sand-size grains are euhedral apatite, euhedral zircon, sanidine, biotite, hornblende, and pyrite. Clay beds just above the bentonites contain similar mixed-layer clay along with the detrital 2M mica. The volcanic origin of the bentonites is clear.

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