Named in 1937 by Ralph E. Grim, R. H. Bray, and W. F. Bradley for one of the co-type localities, in Illinois, USA. In the time period the mineral was named, the probable supposition was that the leaching of interlayer K was a consistent feature throughout the mineral's structure and that the alteration was systematic. Grim et al. (1937) noted unusual optical properties of illite as well as a cation exchange capacity of 20-40 meq/100 grams.