Biotite Group

Brown biotite crystals in granite. Crystals are about 1 cm. long. Wimmer Pit, Marathon County, WI. (Photograph by W. Cordua.)

Formula: K(Mg,Fe)3(Al,Fe3+)Si3O10(OH,F)2 Monoclinic


Biotite refers to a group of common rock-forming minerals forming a series between phlogopite and annite. The name is best used as a field name for dark micas for which the exact composition has not been determined. It is found in a variety of occurrences. It occurs in plutonic igneous rocks ranging from granites to gabbronorite to syenites. Large crystals may form in pegmatites. It can form phenocrysts in lava flows of dacitic or andesitic composition. Biotite is also common in metamorphic rocks where it is a frequent component of schists, gneisses and phyllites. Biotite survives a certain amount of weathering and is found in soils, sediments and sedimentary rocks. Weathered biotite becomes relatively brassy of bronzy in color and has been mistaken for gold. Its lower density, cleavage, and other properties are soon apparent to the careful observer. Finally, biotite can form as a result of hydrothermal processes, especially wall-rock alteration around ore veins. The localities listed below highlight typical or remarkable biotite occurrences in the state.

ASHLAND COUNTY: Biotite is a widespread component of the fine-grained schists, phyllites and slates of the Tyler Formation throughout the region. Some localities given by Van Hise and Irving (1892) include schist near English Lake in the NE Sec. 9 T44N R3W with muscovite, quartz, plagioclase and orthoclase; in schist along the Bad River at Penokee Gap with andalusite (variety chiastolite), muscovite, quartz and pyrite; and in coarse schist exposed in the NW NW Sec. 6 T44N R2W.
— Biotite is found as large crystals in a pegmatite dike about 2 miles north of Mellen in the SW 1/4 Sec. 29 T45N R2W. Associated minerals are plagioclase, hornblende and zircon (Wilcox, 1936).
— Biotite occurs in granite pegmatites exposed in 2001 during road construction along State Hwy. 13 just north of its intersection with State Highway 169 (Sikkila, 2002).

BAYFIELD COUNTY: Locally found as a contact metamorphic mineral in Keweenawan volcanic rocks near their contact with gabbro plutons (Leighton, 1954).

CLARK COUNTY: Biotite is common in schist along Cunningham Creek, east of Highway 73-95, in NW SW Sec. 26 T24N R2W south of Neilsville. Also in augen gneiss at NW NW Sec.26 (Mass and Van Schmus, 1981) – Biotite flakes occur as coarse flakes in pegmatite and smaller flakes in foliated adamellite country rock in a quarry north of Granton (SW NE sec. 35 T.25N. R.1W.) (Myers, 1978; WGNHS outcrop descriptions).

FLORENCE COUNTY: Large biotite flakes are common in the Hoskins Lake granite such as is exposed in the NE 1/4 Sec. 34 T38N R19E southwest of Niagara (Banks and Cain, 1969). It is also common in the Marinette quartz diorite, such as exposed in SW NE Sec. 18 T38N R20E (Sims et al., 1992).

IRON COUNTY: Biotite is a common component of the fine-grained schists, phyllites and slates of the Tyler Formation (Van Hise and Irving, 1892).
— Biotite is common in schists found in the Powell region throughout Sec. 28, 29, 32 and 33 T.42N R4E where it is associated with kyanite, staurolite, almandine and quartz. A particularly good outcrop is at the Highway 182 road cut in the SE 1/4 SW 1/4 Sec.28 (WGNHS Files).

JACKSON COUNTY: Biotite is found as crystals up to 1 cm. across in mica schists, talc schist and iron formation at the Jackson County Iron Mine east of Black River Falls in the SE 1/4 Sec.15 T21N R3W. (Jones, 1978).

MARATHON COUNTY: Large biotite crystals up to 12 cm. in diameter in wall zones and up to 30 cm. in diameter in core zones are found in pegmatite dikes of the Wausau pluton (Falster, 1987). These can occasionally be found exposed in the granite quarries in Sec. 19 and 20 T 28N R7E south of Rib Mountain.
— Biotite is found in the core zone of the Stettin pluton in the SE 1/4 Sec. 2 T29N R6E. It also occurs at the old zircon quarry in nepheline syenite of the Stettin pluton in the SE 1/4 SE 1/4 Sec. 22 T29N R6E (Sood, Myers, and Berlin, 1980).
— Biotite is found in schist xenoliths in the Wausau pluton exposed in SW 1/4 Sec. 10 and in the quartzite quarry on the south end of Mosinee Hill in NE 1/4 NE 1/4 Sec. 27 both in T28N R7E (Myers et. al., 1984).
— Meyers et. al. (1984) note biotite in the nepheline syenite pegmatite and associated rocks in roadcuts along County U 0.5 miles east of the Little Rib River in the SW 1/4 SW 1/4 Sec. 18 T29N R7E.

MARINETTE COUNTY: Large flakes of biotite altering to chlorite and containing tiny zircon inclusions occurs in the Newingham tonalite such as is exposed on the north side of U.S. Hwy. 8 near the center of the N1/2 Sec. 5 T36N R20E west of Pembine (Banks and Cain, 1969).
— Biotite is abundant in Newingham Tonalite such as exposed in road cuts in the NW Sec. 5 T36N R20E and NW NW Sec.2 T36N R19E and a railroad cut in NW NW Sec. 27 T37N R20E (Sims et al., 1992).
— Biotite is common in the Marinette Quartz Diorite exposed widely south of Niagara. Associated minerals are prochlorite, hornblende and plagioclase. Some outcrops are found at NW sec. 22, SW sec. 15, and along the railroad tracks in NE sec. 18 all in T.38N. R.20E. (Prinz, 1965).
— Found as an accessory in the Hoskin Lake granite such as is seen on the west side of U.S. Hwy 141 in the center of Sec. 1 T37N R20E near Pembine (Bamks and Cain, 1969) and at the RR cut in the SW Sec. 7 T38N R20E (Sims et al., 1992).
— Large biotite flakes locally making up to 25% of the rock is found in the Dunbar Gneiss such as outcrops along the east side of County Hwy. U in the center of the west 1/2 Sec. 13 T27N R18E (Banks and Cain, 1969).

ONEIDA COUNTY: Biotite is found with chloritoid, actinolite and stilpnomelane in the volcanic rocks in the wall rock altering zone of the Pelican River massive sulfide deposit in Sec. 29 T39N R9E (Bowden, 1978).
— Biotite occurs in coarse-grained granite exposed on the east side of U.S. route 45, near Monico (SW SE sec. 6 T35N.R11E) (Van Schmus, 1980).

PORTAGE COUNTY: Biotite is common in the gneisses and tonalites exposed at numerous points in T.23N. R.7E. near Steven’s Point. These include outcrops at Conants’ Rapids ( NE SW sec. 8), the Linwood Township Quarry (NW SE sec. 15), and outcrops along Mill Creek (NW NE sec. 22) (Anderson and Cullers, 1987).

PRICE COUNTY: Biotite is abundant at the north quarter post of Sec. 29 T39N R3E (Hotchkiss, 1915).
— Biotite occurs in granite exposed in SW NW NW Sec. 20 T38N R1W near Lugerville and in NW Sec.6 T38N R3E along U.S Forest Service Road 136 (Sims and Peterman, 1980).

RUSK COUNTY: Biotite occurs in tonalite on the east side of Hwy. 27, 13 km. north of Ladysmith (SW NW sec. 22 T.36N. R.6W.) (Sims and Peterman, 1980).
— Biotite occurs in tonalite south of Big Falls Dam, (SW SW sec. 35 T.36N. R.5W.) (Sims and Peterman, 1980). ,p> SAUK COUNTY: Biotite is a common component of the Baxter Hollow granite and associated rocks found near Sampler in the SW 1/4 Sec. 35 T11N R6E (Gates, 1942).

SAWYER COUNTY: Biotite occurs in porphyritic granite at the dam near Radison (NW NW SE sec. 6 T.35N. R.11E.) (Van Schmus, 1980).

SHAWANO COUNTY: Biotite as books several inches across are found in the beryl-bearing pegmatite located along the Embarass River just south of the Tigerton Dells in the NE 1/4 SW 1/4 NE 1/4 Sec. 15 T26N R12E near Tigerton (WGNHS files, 1986).

WAUPACA COUNTY: Biotite, commonly altered partly to chlorite and other minerals is common in pegmatite cutting granite found in road cuts along County G near Keller Lake, NE of Big Falls. Associated minerals include euxenite-(Y), fluorite, K feldspar, bastnaesite group minerals, allanite and various sulfides. (Tom Buchholz, 2002, pers. com.)