Weathering or hydrothermal alteration of muscovite-phengite. Also authigenic alteration of K-spar or recrystallization of smectites in marine sediments. Group name for dioctahedral interlayer-deficient clays.
Not Approved IMA 1998
Maquoketa shale, Gilead, Calhoun County, Illinois, USA. Link to MinDat.org Location Data.
Named in 1937 for the state of Illinois where the mineral was first described.