Accessory clay-size minerals occur as disseminated particles and as fillings of pockets and fissures in the Lake Superior iron ores, the oxidized iron-formation, and the associated altered dikes. Dickite and kaolinite are the most abundant clay minerals in the Marquette, Gogebic, and Iron River districts of Michigan and are accompanied by lesser amounts of nacrite, talc, pyrophyllite, 1M and 2M 1 muscovite, lizardite, clinochrysotile, Al-serpentine, dioctahedral and trioctahedral chlorite, dioctahedral and trioctahedral montmorillomte, palygorskite, and regular interstratifications of chlorite-montmorillonite, as well as the non-clay minerals apatite, alunite, gypsum, calcite, and rhodochrosite. This assemblage is suggestive of a hydrothermal environment. In contrast only kaolinite, 1M muscovite, chlorite, nontronite, montmorillonite, talc, chamosite, apatite, gibbsite, and alunite have been identified from the Mesabi, Cuyuna, and Vermilion districts of Minnesota. The distinctive differences in the assemblages of minerals and in the types of kaolin minerals present in the Michigan and Minnesota areas are believed to be indicative of differences in the environment of formation of the clay-size components in the two areas. The intimate association of the clays with the iron ore suggests that these differences may also extend to the origin of the ore itself.