A dioctahedral member of the true mica group. The end-member formula is KV2vAlSi3O10(OH)2, where v = vacancy. Roscoelite occurs as a 1M polytype and more rarely as 2M1. Roscoelite is found in sedimentary rocks where V has been remobilized from black shales and mafic rocks. Roscoelite is known from the Colorado Plateau district (Colorado, also Utah, Arizona, New Mexico, USA) in U-bearing sandstones. It is also known from Gabon, Western Australia, and Ontario, in the latter occurrence, the V was introduced from metasomatic fluids.