Formula: (Fe,Mg,Mn)Al2(SiO4)O(OH)2 Monoclinic
Description: Chloritoid forms small green platy to micaceous grains in rocks that have been subjected to moderate grades of regional metamorphism. It may also form by hydrothermal alteration of rocks. It occurs with muscovite, chlorite, garnet, staurolite and kyanite. Its small grain size and superficial resemblance to chlorite makes it difficult to identify with certainty in the field, hence is frequently overlooked.
DODGE COUNTY: Fe-rich chloritoid occurs in the argillites interbedded with the Waterloo quartzite exposed in the Waterloo area and adjacent Jefferson County. It is found as small bundles and radiating clusters and is associated with muscovite, andalusite, chlorite, plagioclase and hematite (Brown, 1986, Geiger et al., 1982).
FLORENCE COUNTY: Reported with grunerite in the rocks exposed at the Pine River Reservoir, about 8 kilometers south of Florence (Nilsen, 1964).
FOREST COUNTY: Chloritoid occurs as tiny needles or laths in the McCaslin quartzite at Deer Lookout Tower Hill in T.34N R.16E (Olson, 1964). Olson also reports its presence in quartzite to the south along the McCaslin Range.
MARINETTE COUNTY: Chloritoid occurs as tiny needles or laths in the McCaslin quartzite at Thunder Mountain in T.35N R.18E (Olson, 1964). Murskey et al. (1989) report andalusite form these rocks as well.
ONEIDA COUNTY: Chloritoid is a microscopic component of metamorphosed volcanic rocks at the Pelican River massive sulfide deposits (Bowden, 1978). Associated minerals are biotite, actinolite and stilpnomelane.