Formula: (Fe,Mg)7Si8O22(OH)2 Monoclinic
Description: Grunerite is a member of the amphibole family. It occurs in relatively iron-rich rocks that have been subjected to moderate grades of metamorphism.
ASHLAND COUNTY: Grunerite is a widespread contact metamorphic minerals in the Ironwood Iron Formation around mafic intrusions. It occurs as tiny white to greenish acicular or platy crystals. It is associated with magnetite, ankerite, almandine, minnesotaite, cummingtonite, stilpnomelane and iron-rich pyroxenes (U.S.G.S., 1976; Laybourn, 1979). Some of a number of outcrops cited by Laybourn include:
— East bank of Ballou Creek, NW NE Sec. 11 T.44N R.2W.
— The Birkshire Mine on Mt. Whittlesey, SW SW SE Sec. 9 T.44N R.2W.
— Near Mellen in the SE NW Sec. 13 on the west side of highway 13 and at Penokee Gap, NW NW sec. 14 T.44N R.4W.
— In the Hanna Mining Co. test pit in SW SE Sec. 18 T.44N R.3W.
— Near the Mineral Lake Lookout Station, NE NE Sec. 24 T.44N R.4W.
— Outcrops east of Markle Memorial Drive, SE NE Sec. 14 T.44N R.4W.
— Leighton (1954) described grunerite in the south 1/2 of Sec. 16 and the SE Sec. 17 T.44N R.5W.
FLORENCE COUNTY: Grunerite is found with magnetite, quartz and garnet as a component of iron formation within the Michigamme Slate. Some localities cited by Dutton (1971) include the Dunkel test shaft and adit in Sec. 25 T.40N R.17E and outcrops in the SW NE Sec. 28 T.39N R.18E.
— Grunerite schist within the Qiunnessec Formation is found along the west side of the Little Popple River in the south 1/2 Sec. 3 and the N 1/2 Sec. 12 T.38N R.18E and in the SE Sec. 15 T.39N R.17E. In the schist it forms small subhedral crystals with quartz and hornblende (Dutton, 1971).
— Grunerite is a component of rocks at the Pine River Reservoir, about 5 miles south of Florence (Nielsen, 1964).
IRON COUNTY: Grunerite occurs with magnetite in the pits of the Lucas-Whiteside exploration in Sec. 4 T.41N R.1E, near Butternut (Dutton and Bradley, 1970).
JACKSON COUNTY: Grunerite is a common constituent of the iron formation mined at the Jackson County Iron Mine in the SE Sec. 15 T.21N R.3W it is associated with magnetite, quartz, almandine, cummingtonite, ferroactinolite, and biotite (Jones, 1978).
MARINETTE COUNTY: Coarse grunerite occurs in iron formation at the Duval massive sulfide deposit Sec. 2 and 3 T.35N R.18N and Sec. 28 T.36N R.18E. It is associated with magnetite, pyrrhotite and other amphiboles such as ferroactinolite, cummingtonite and anthophyllite (Hollister and Cummings, 1982).