View of outcrop of the Decorah Formation. The green shale layers, now disintegrating to clay, largely consists of the clay mineral illite. The slabs are limey coquinas loaded with bryozoan, crinoid and brachiopod fossils. Road cut near Clay Corners, 8 miles east of River Falls, Pierce County, Wisconsin.

Formula: K0.65Al2(AlSi)4O10(OH)2  Monoclinic


Illite refers to a group of clay minerals formed by weathering or hydrothermal alteration of other aluminum-rich minerals. It occurs intermixed with kaolin and other clay minerals. Its identification requires X-Ray analysis. Virtually all shale and many “clay” occurrences reported in Wisconsin are likely partly or entirely illite. For example, Davis (1966) reports illite as the dominant clay mineral in the Ordovician Willow River Formation widely exposed throughout western Wisconsin. The occurrences listed under other clay minerals (dickite, halloysite, kaolin, montmorillonite) are also likely illite occurrences as well. Only a few of the more typical or unusual occurrences of illite are reported below.

FOND DU LAC COUNTY: “Especially pure well crystallized illite” is found in pockets in dolostone exposed in the Western Lime and Cement Company Quarry at Marblehead (NE sec. 7 T.14N. R.7E.) (Gaudette, 1965).

LAFAYETTE COUNTY: Illite is a component of the shale at the base of the Quimbys Mill Member of the Platteville formation at the Thompson-Temperly Mine at the south edge of New Diggings (SE sec. 27 T.1N. R.1E.). The particular illite polymorph changes from the Md polymorph to the 1M and 2M polymorph progressively toward the mineralized zone. (Heyl, et al., 1970).

MARATHON COUNTY: Greenish illite-montmorillonite is a constituent of the clay fill in pockets in the pegmatites of the Wausau Pluton. (Falster, 1987; Buchholz, 1999b).

PIERCE COUNTY: Illite occurs in the clay layers associated with hematite and goethite exposed in level 1 of Crystal Cave, near Spring Valley. (Blaze Cummingham, 1992, personal communication).

ROCK COUNTY: Tiny platy crystals of illite are found in the pores of the St. Peter Sandstone in the silica sand quarry near Hanover (Odom, Willand and Lassin, 1979).

RUSK COUNTY: Illite is a component of the gossan cap on top of the Flambeau massive sulfide deposit south of Ladysmith (Sec. 9, T.34N. R.6W.) (May, 1977).