Formula: (Mg,Fe)2Al3(AlSi5O18) Orthorhombic
Cordierite is found in many medium-grade metamorphic environments. It is often found in the high temperature-low pressure zones in contact metamorphic halos. In Wisconsin, however, it most often is found in schist associated with regional metamorphism where it usually forms non-descript bluish grains easily confused with plagioclase. It readily alters to a yellowish mixture of micaceous minerals. Because of difficulties in its recognition, it is probably more widespread in the state than the reported localities would suggest.
ASHLAND COUNTY: Reported as sparsely developed in the mica schist of the Tyler Formation west of Penokee Gap where it is found with muscovite and biotite (Van Hise and Leith, 1911).
FLORENCE COUNTY: Reported as a minor metamorphic component of the metasedimentary rocks with biotite, garnet, quartz and plagioclase along the northwestern margin of the Dunbar Dome. A specific reported locality is in drill core samples from the Bass Lake area in T.38N R.17E (Neilsen, 1984, 1986).
LINCOLN COUNTY: Large porphyroblasts of cordierite are found with euhedral staurolite porphyroblasts and garnet in “greywacke schist” near Merrill such as in the SE Sec. 7 and the NE Sec. 18 T.31N R.7E (Weidman, 1907b).
MARATHON COUNTY: Found in schist with staurolite and garnet as exposed along the Rib River west of Hamburg in Sec. 14 T.30N R>4E (Weidman, 1907b).
RUSK COUNTY: Makes up as much as 20% of the matrix of the quartz-eye schist at the Flambeau deposit Sec. 9 T.34N R.6W, south of Ladysmith (May, 1977).
— Occurs with anthophyllite in metamorphosed iron formation at the Eisenbrey massive sulfide deposit, north of Ladysmith (NW sec. 17 T.35N. R.6W.) May form poikiloblasts. (May, 1996).