A K-bentonite in the Middle Carbonate Group of the Belt Supergroup is evidence for eruptive volcanic activity within or adjacent to the Belt basin during Middle Proterozoic deposition. The texture of this olive-green rock is dominated by a fine-grained groundmass similar to Paleozoic K-bentonites. Throughout this groundmass are a few randomly occurring angular, irregularly shaped clasts composed of quartz. The morphology of these clasts indicates the quartz grains have not been transported. Traces of small euhedral zircons are also present. The mineralogy and chemical composition of the groundmass in this K-bentonite is also similar to Paleozoic K-bentonites but appears to reflect a more evolved mineralogy as a result of its age and thermal history. Unlike Paleozoic K.bentonites, which are characteristically the 1M polytype, authigenic 1M and 2M 1 illite/mica polytypes coexist in this Proterozoic K-bentonite and are separable by size fractionation. The K.Ar ages of the clay fractions range from 807 Ma in the coarser fraction to 606 Ma in the finer fraction. The 807 Ma K-Ar age of 2M 1 mica indicates temperatures in excess of 250 degrees C during this time. The 606 Ma K-AT age of the fine-grained 1M polytype probably reflects later 1M crystallization at temperatures below 250 degrees C. The reaction of 1M to 2M 1 polytype in this K-bentonite did not require major changes in chemical composition, and the dominant controls on the 1M to 2M 1 polytype transformation appear to have been temperature and time.