Illite is one of the most frequent clay minerals and is an important constituent of many sedimentary rocks. It was named after the state of Illinois, United States, by Grim, Bray and Bradley in 1937. The type locality of the Late Pennsylvania Fithian cyclothem is also the type locality of the clay mineral illite. In October 2013 I had the chance to actually visit the locality when participating in the 50th Anniversary Annual Meeting of the Clay Minerals Society at the University of Illinois at Urbana-Champaign. The trip was lead by Stephan Altaner and Shane Butler.
The image I made shows the succession of Pennsylvanian mudstone or underclay with carbonate concretions, and of course illite, towards a fossil soil horizon. Besides illite the mudstone also contains chlorite, kaolinite, illite-smectite, quartz and jarosite from pyrite weathering. The bituminous Flannigan coal is about 40 to 50 cm thick. Above it is a thinly bedded black shale and then marls & limestones. The sandstone units above and below the described sequence are not visible in the picture or obscured by the trees.
|Fithian cyclothem with illite-bearing mudstone|
Altaner & Butler, 2013: Geologic Field Trip to the Fithian Illite, Fithian, Illinois Guidebook for field trip held on October 10, 2013 in conjunction with the 50th Anniversary Annual Meeting of The Clay Minerals Society held at the University of Illinois at Urbana-Champaign