Formula: (Mg,Fe)7Si8O22(OH)2 Monoclinic
Description:
Cummingtonite is a member of the amphibole family. It commonly forms elongated to fibrous green to brown grains in metamorphosed iron-rich rocks.
ASHLAND COUNTY: Cummingtonite is found in the Ironwood Iron Formation where it has been contact metamorphosed by gabbroic intrusions. Here it is associated with minnesotaite, stilpnomelane, grunerite and iron-rich pyroxenes (USGS, 1976).
EAU CLAIRE COUNTY: Found as tiny grains and exsolution lamellae in hornblende in amphibolite at Big Falls County Park along the Eau Claire river in the NW SE Sec. 13 T. 27N R.8W ( Myers et al., 1974; Cummings, 1984).
JACKSON COUNTY: Pale green cummingtonite occurs in a complex assemblage of amphiboles in the iron formation at the Jackson County Iron Mine (SE Sec. 15 T.21N R.3W). Here it occurs with grunerite, riebeckite, hastingsite, ferroactinolite, quartz and biotite (Jones, 1978; Kimball and Spear, 1984).
MARINETTE COUNTY: Cummingtonite occurs in the iron formation at the Duval massive sulfide deposit in Sec. 2 and 3 T.35N R.18E and Sec. 28 T.36N R.18E where it occurs with magnetite, pyrrhotite, and grunerite and other amphiboles (Hollister and Cummings, 1982).
OCONTO COUNTY: Cummingtonite is associated with hornblende and plagioclase in the metamorphosed mafic lava flows of the Waupee Formation near Mountain (Lahr, 1972).
ONEIDA COUNTY: Cummingtonite occurs with talc in the metamorphosed chemical sedimentary rocks at the Lynn massive sulfide deposit (Kennedy et al., 1991).
SHAWANO COUNTY: Cummingtonite occurs as a component of reaction rims surrounding and replacing augite in the Tigerton Anorthosite, such as in outcrop about 1 km. south of Bowlee in the SE SE Sec. 36 T.28N R.12E (Greenberg et al., 1986).