Formula: (Mg,Fe)SiO3 Orthorhombic
Description:
Orthopyroxenes are a group of minerals ranging from pure MgSiO3 (enstatite) to pure FeSiO3 (ferrosilite). Bronzite and hypersthene are orthopyroxenes mid-way between these two end members. As the particular orthopyroxene mineral described is not always identified, all reported orthopyroxene occurrences are grouped together.
Orthopyroxenes are relatively widespread as a primary mineral in gabbroic and ultramafic rocks where they form with augite, olivine and plagioclase. Orthopyroxene may occur in high grade metamorphic rocks. It is essential in the granitic gneisses called charnockites, and may be found in pyroxene granulites and metamorphosed iron formations.
ASHLAND COUNTY: Hypersthene occurs as individual grains or lamina in augite in gabbros of the Mineral Lake and related plutons where it occurs with plagioclase, augite and olivine. The gabbros are widely exposed throughout sec. 20, 21, 28 through 33 T.45N. R.4W. (Leighton, 1954, Klewin et al., 1989).
— Orthopyroxene occurs with plagioclase and augite in gabbro exposed in the quarry south of Loon Lake, NE SW sec. 29 T.45N. R.2W. (Klewin, 1987).
— Orthopyroxene is common in the gabbro and troctolite of the Rearing Pond Intrusion of the Mellen Complex, such as exposed along Spring Brook, SE SW sec. 26 T.45N. R.4W.
— Fe-rich orthopyroxene occurs in the Ironwood Iron Formation where it has experienced intense contact metamorphism from the Mellen intrusives. Generally it is associated with ferroaugite and other pyroxenes, hornblende, fayalite, magnetite and quartz. Such rocks can be found in a roadcut along Hwy 13 (SE NW sec. 13 T.44N. R.3W), in outcrops on the west side of the Bad River through Penokee Gap (NW NW sec. 14 T.44N. R.3W), in the Hanna Mining Company pits (SW SE sec. 18 T.44N. R.3W), and near the Mineral Lake Lookout tower (NE NE sec. 24 T.44N. R.4W.) (Laybourne, 1979).
BAYFIELD COUNTY: Hypersthene occurs as individual grains or lamina in augite in gabbros exposed throughout sec. 1-9, 11-13 and 16-18 T.44N. R.5W. It can also be found in gabbros in sec. 25, 35, and 36 T.45N. R.4W. Coarse orthopyroxene occurs in gabbro pegmatite in the south 1/2 sec. 19 T.45N. R.4W. and SE sec. 11 T.44N. R.5W. (Leighton, 1954).
IRON COUNTY: Orthopyroxene is common in the gabbros and troctolites of the East Mellen Complex as described by Tabet and Mangham (1978). A number of outcrops exist in T.45N. R.1W, such as in SW SW sec. 3, NW NW sec. 10, and SE SE sec. 17. – Orthopyroxene is found in the rocks of the Potato River Intrusion. For example, it is found in picrites exposed in the SE SE sec. 30 and NE NE sec. 31 T.46N. R.1E. and in olivine gabbros exposed in SW SW sec. 2 T.45N. R.1W. and NW NW sec. 32 T.46N. R.1E. (Klewin, 1987, Klewin et al., 1989).
JACKSON COUNTY: Orthopyroxene, patchily rimmed and replaced by chlorite, occurs in the gneisses exposed along the Black River, below the Lake Arbutus Dam, near Hatfield (DuBois, 1981).
LINCOLN COUNTY: Coarse, lustrous cleavable orthopyroxene occurs serpentinized olivine, magnetite and minor plagioclase in outcrops of peridotite in the bed of the Wisconsin River, near the confluence with the Copper River (Irving, 1883b).
SHAWANO COUNTY: Orthopyroxene is a component of the Tigerton anorthosite, where it occurs as small grains interstitial to plagioclase. It is found in numerous outcrops, such as in the SE SE sec. 36 T.28N. R.12E. Other outcrops are described for the county under plagioclase. (Greenberg et al., 1986).