Formula: Al2Si2O5(OH)4 Triclinic
Description: Kaolinite is a clay mineral formed by weathering or hydrothermal alteration of other aluminum-rich minerals. It occurs intermixed with halloysite and other clay minerals. Its identification requires X-Ray analysis. Many “clay” occurrences within the state are likely partly kaolinite, hence the occurrences listed under other clay minerals (dickite, halloysite, montmorillonite) are also likely kaolinite occurrences. Irvine (1876) described kaolinite as an important economic resource occurring in a belt 50 miles long and 15 miles wide stretching from the Black River to Wisconsin Rapids. Must of this was kaolinized Precambrian rocks (now saprolite) exposed beneath the unconformity with the Cambrian Mt. Simon sandstone. The economic promise of this clay was never fully realized and was likely greatly exaggerated. Many of Irving’s specific localities below for kaolinite may in fact be occurrences of dickite, halloysite or other clay minerals.
BARRON COUNTY: Kaolinite forms matrix material in the Barron quartzite exposed throughout the Blue Hills in Barron County and adjacent Rusk County (Razacky, 1987) .
CHIPPEWA COUNTY: Kaolinite is a component of the saprolite developed on Precambrian trondjhemites exposed beneath the Mt. Simon Sandstone along Duncan Creek in Irvine Park, Chippewa Falls. The saprolite preserves the texture of the Precambrian rocks (Cummings and Scrivner, 1980).
JACKSON COUNTY: Kaolinite deposits exist as saprolite on top of weathered gneisses but underneath sandstone between Black River Falls and Black River Station. The kaolinized rock retains the textures of the gneiss (Irving, 1876).
MARATHON COUNTY: Kaolinite is a constituent of the clay fill in pockets in the pegmatites of the Wausau and Ninemile Plutons in the Wausau area (Falster, 1987).
ROCK COUNTY: Tiny euhedral crystal of kaolinite are found in the pores of the St. Peter Sandstone in the silica sand quarry near Hanover (Odom, Willand and Lassin, 1979).
SAUK COUNTY: Kaolinite is a component of the Baraboo quartzite and overlying slates as seen in drill core. Its formation is ascribed to the in situ breakdown of feldspars (Geiger, 1986).
WOOD COUNTY: Kaolinite is widespread in saprolite zones on Precambrian rocks, particularly along the Wisconsin River in the area of Wisconsin Rapids. The material is locally gritty and stained with iron oxides. Some localities reported by Irving (1876) include SW sec. 5 T.22N. R.6E., along the east side of the Wisconsin River in SW sec. 4 T.22N. R.6E., on the west side of the Wisconsin River in Sec. 24 T.22N. R.5E., in pits in NE sec. 26 T.22N. R.5E., and along the west bank of the Wisconsin River in NW sec. 10 T.21N. R.5E.
— Irving (1876) reports kaolinite developed on top of feldspathic gneisses along the Yellow River “above Dexterville”.
— Kaolinite is the dominant clay mineral in argillite in the quarry at Power’s Bluff (SW NE sec. 32 T.24N. R.4E.) (Brown, 1986).