Augite (dark) with plagioclase in anorthositic gabbro. Near Mellen, Ashland County, WI. (Photograph by W. Cordua.)

Formula: (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 Monoclinic


Augite is the most common mineral in the pyroxene family. It has many varieties due to the variation in chemistry possible in its complex coupled solid solution series. Augite is common in gabbroic and ultramafic rocks. It also forms large phenocrysts in many basalts. Less commonly it occurs in granulites and gneisses formed at very high grades of metamorphism. It is most common in Wisconsin in the belt of Keweenawan intrusive and extrusive igneous rocks traceable from St. Croix Falls in Polk County, northward to Big Maintou Falls in Douglas County and eastward to Hurley in Iron County. Diabase dike swarms of this age containing fine-grained augite are common to the south wherever Precambrian rocks are exposed. Only a few of the more typical or interesting of the many occurrences of augite in the state are listed here.

ASHLAND COUNTY: Augite occurs as phenocrysts in the many outcrops of Keweenawan basalts in the county. This was recognized by Van Hise and Irving (1892) who noted particularly good outcrops in the south 1/2 of sec. 14 T.44N. R.4W. and in the NE sec. 13 T.45N. R.1W. More recently Ali (1982) mapped and described prominent outcrops near Gordon, such as SW SE sec. 5 and sec. 8 R.42N. R.10W. and NE NW sec. 13 and SW SW sec. 23 T.42N. R.11W. Coarse augite can be found in quarries at NW NW sec. 4 T.44N. R.3W. and in SE sec. 29 T.45N. R. 2W.
— Coarse augite occurs in numerous gabbroic plutons throughout the county. Leighton (1954) noted abundant outcrops near Mellen in sec. 20, 21, 28, 29, and 30.-33 in T.45N. R.4W. where the augite is associated with plagioclase, magnetite and orthopyroxene. Theses are outcrops in what was later described by Klewin et. al. (1989) as part of the Mineral Lake and Rearing Pond Intrusions with the Keweenawan Mellen igneous complex. Some outcrops noted by Klewin et. al. (1989) are in gabbro and gabbro pegmatite in the NE NE sec.7 T.44N. R. 3W. and in ferrodiorite in the NE NE SW sec. 25 R. 45N. R.4W. Augite also occurs in gabbro and anorhtosite in quarries in the NW NW sec. 4 T.44N. R.3W. and SE SE sec. 29 T.45N. R.2W.
— Ferroaugite occurs occasionally in grains up to 4 cm. across in the metamorphosed Ironwood Iron formation near the margins of the Mellen gabbro complex (Laybourne, 1979). Here is associated with orthopyroxene, grunerite, almandine, magnetite and fayalite. some localities described by Laybourne include: in roadcuts along the west side of highway 13 about 2 miles south of Mellen (SE NW sec. 13 T.44N. R.13W.); in outcrops along Penokee Gap on the west side of the Bad River (NW NW sec. 14 T.44N. R.3W.); in the Hanna Mining Test Pit (SW SE sec. 18 T.44N. R.4W.) at the mineral Lake Lookout Tower (NE NE sec. 24 T.44N. R.4W. ) and in outcrops in SE NE sec. 14 T.44N. R.4W.

BAYFIELD COUNTY: Augite occurs as phenocrysts in the many outcrops of Keweenawan basalts in the county. LaPoint (1976) noted particularly good outcrops in the SW NW sec. 3, NE SE sec. 5 and SW SE sec. 14 T.44N. R.6W. Ali (1982) mapped and described many prominent outcrops such as NE NE sec. 27 T.44N. R.8W., NW NW sec. 8 T.42N. R. 9W, and SE NW sec. 2 T.43N. R.8W.
— Coarse augite occurs in numerous gabbroic plutons throughout the county. Hotchkiss (1915) mapped pyroxene-rich gabbros in the NW sec. 1 T.43N. R.6W. near Cable. Leighton (1954) noted abundant outcrops throughout T.44N. R.5W and in sec. 25, 35, and 36 in T.45N. R.5W. where the augite is associated with plagioclase, magnetite and orthopyroxene. Leighton also described mafic augite bearing pegmatites in the south half of sec. 19 T.45N. R.4W. and SE sec. 11 T.44N. R.5W.

DOUGLAS COUNTY: Augite occurs as phenocrysts in the many outcrops of Keweenawan basalts exposed throughout the county. Grant (1901) noted crystals up to 1 inch in diameter in sec. 16 T.42N. R.15W. WGNHS files indicated augite xls. up to 1 inch in diameter also occur in the NW sec. 28 T.44N. R.14W.
— Augite occurs in gabbro intrusives exposed from place to place in the county. For example subhedral crystals up to 5 cm. long, much altered to chlorite and other minerals occur in gabbro in the quarry in NW sec. 32 T.48N. R.12W., near Amnicon Falls. (Johnson and Mengel, 1969; Cordua, 1991).

FOREST COUNTY: Augite occurs in the Duck Lake Gabbro, an intrusion cross-cutting the Crandon deposit. (Lambe and Rowe, 1989).

IRON COUNTY: Augite is an abundant component of the gabbroic rocks of the Keweenawan Mellen Complex, well-exposed at a number of places in the county. Tabet and Magham (1978) mapped the eastern Mellen Complex, noting augite associated with plagioclase, orthopyroxene, olivine and magnetite. The augite may show schiller. Some augite-bearing outcrops noted in their study are SE NW sec. 2, NW NW sec. 10 and SE SE sec. 17 in T.45N. R.1W.; NE SW sec. 35 T.46N. R.1W. and SE SE sec. 14 and SW SE sec. 22 T.46N. R.1E. Klewin (1987) and Klewin et. al. (1989) mapped in detail the rocks of the Potato River Intrusion of the Mellen Complex and found augite in picrites, troctolites and other gabbroic rocks associated with olivine and plagioclase. For example, augite crystals up to 2 cm. across occur with olivine and plagioclase in SE SE sec. 30 and NE NE sec. 31 T.46N. R.1E. Augite occurs in troctolite in the SW SW sec. 2 T.45N. R.1W. It can be found as large crystals enclosing olivine and plagioclase grains in the NW NW sec. 32 T.46N. R.1E. north of Upson. Augite is also a component of the gabbro in the SE NW sec. 35 and the NW SW sec. 36 T.46N. R.1W.

MARATHON COUNTY: Titanoaugite occurs in olivine gabbro at the NE SE NE sec. 11 T.29N. R.2E. (LaBerge and Myers, 1983).
— Augite is a component of Keweenawan diabase dikes at a number of localities such as in Big Sandy Park in NW SW sec. 19 T.29N. R.9E. (LaBerge and Myers, 1983).
— Poikilitic augite grains as large as 10 cm. by 5 cm., partly replaced by hornblende, occur in metagabbro in sec. 25 T.29N. R.9E. (LaBerge and Myers, 1983).

POLK COUNTY: Augite is abundant as phenocrysts in Keweenawan basalt exposed at many localities in the county. Particularly good outcrops can be found near Dresser, throughout Interstate Park, near Milltown, Luck, Frederic, and Clam Falls. It is prominent in the Dresser Trap Rock Quarry in sec. 5 and 6 T.33N. R.18W. In addition to phenocrysts, gabbro can also be found in the Dresser Quarry within coarse gabbroic lenses. Here longated subhedral platy twin crystals up to 4 cm. long occur with plagioclase. Both the augite and plagioclase are partly replaced by epidote and other secondary minerals (Cordua, 1989b).

SHAWANO COUNTY: Augite is a common component of the Tigerton Anorthosite intrusion. Here it occurs as small grains often replaced by hornblende. A good outcrop occurs at the SE SE sec. 36 T.28N. R.12E., south of Bowler (Greenberg, et. al., 1986).

WOOD COUNTY: Black augite crystals were collected and cut into cabochons from an outcrop near Rudolph (Crowns, 1976). Myers and LaBerge (1971) describe the outcrops as west of Rudolph 1 mile on County C, then south 1 mile on a town road and west 0.8 miles on another town road. Myers and LaBerge state that the locality now is largely covered over.