Formula: (Ca,Na)(Mg,Fe)4Al(Si7Al)O22(OH,F)2 Monoclinic
Description: Hornblende as a mineral name has been recently discredited. Here it is used as a field name for any dark amphibole for which the exact composition has not been determined. In the past black amphiboles were often identified as hornblende without x-ray or microprobe verification. The names used in the original literature are those which are used here. Hornblende is a very common mineral found in many geologic environments. It is found in many intrusive igneous rocks ranging in composition from granites to diorite to gabbros to syenites. It occurs as phenocrysts in several varieties of extrusive igneous rocks, such an andesite. It occurs as a detrital heavy mineral in sedimentary rocks. It forms in a number of gneissic rocks formed at moderate grades of metamorphism. Amphibolites are common metamorphic rocks consisting in large part of dark amphibole. Igneous and metamorphic rocks containing hornblende are widespread throughout Wisconsin’s Precambrian shield.
ASHLAND COUNTY: Large hornblende crystals are found in a mafic pegmatite dike found about 2 miles north of Mellen in the SW Sec. 29 T.45N R.2W. The hornblende is associated with plagioclase, biotite and zircon (Wilcox, 1936).
— Hornblende is common in the amphibolite exposed at Pete’s Landing SE SE Sec. 3 T.41N R.1E (Sims et al., 1985).
— Hornblende is common in the granodiorite phases of the Mellen granite pluton, such as found in Sec. 32-36 T.45N R.3W and Sec. 1 and 2 T.44N R.3W (Katzman, 1968).
— Hornblende occurs along shear zones in a quarry in anorthositic gabbro at NW NW Sec. 4 T.44N R.3W (Cordua, 1994 field notes).
CHIPPEWA COUNTY: Hornblende is common as small needle-like grains with plagioclase and biotite exposed along the Chippewa River near Jim Falls in the E 1/2 Sec. 30 T.30N R.7W (Myers and Maercklin, 1974).
— Hornblende is common as small grains in amphibolites and cross-cutting diorite dikes in outcrops along the Chippewa River at Holcombe Dam (SW Sec. 28 T.32N R.6W) (Myers, 1974).
— Hornblende is common as small grains in amphibolites exposed along the Chippewa River near Cornell in the NE Sec. 18 T.31N R.6W (Myers et al., 1974).
CLARK COUNTY: Hornblende is common as small needle-like grains with plagioclase in amphibolite and gneiss in the Opelt Quarry, south of Neillsville.
EAU CLAIRE COUNTY: Hornblende is common as small grains with plagioclase in amphibolite at Little Falls at the Hwy. K bridge across the Eau Claire River (SW NW Sec. 19 T.27N R.8W) (Myers et. al., 1974).
— Small needle-like crystals of hornblende occur in gneisses and amphibolite in outcrops in Big Falls County Park along the Eau Claire River in the NW SE Sec. 13 T.27N R.8W (Cummings, 1984; Myers, 1974). Some of the amphibolite has been used to make bookends, tie tacks and pendants. (Lovvik, 1976).
FLORENCE COUNTY: Hornblende is a major constituent of amphibolite and hornblende schist in the Quinnesec Formation throughout the county. It is in grains up to 0.5 mm in diameter, associated with oligoclase, biotite and quartz (Dutton, 1971).
— Large platy hornblende crystals are common in the Hoskins Lake Granite, as exposed in NE Sec. 34 T.38N R.19E south of Niagara (Banks and Cain, 1969).
IRON COUNTY: Hornblende is abundant in gneissic and schistose rocks along the west branch of the Montreal River (Van Hise and Irving, 1892).
JACKSON COUNTY: Hornblende occurs in various rocks at the Jackson County Iron Mine in the SE Sec. 15 T.21N R.3W near Black River Falls. In the iron formation it ranges from ferrohornblende to hastingsite to ferro-tschermakite in composition. Most commonly it occurs with garnet, cummingtonite, plagioclase, epidote, scapolite, quartz and ilmenite. The hornblende may form rims and pseudomorphs after the garnet. Hornblende is also found in garnet-free assemblages associated with actinolite or cummingtonite (Kimball and Spear, 1984).
MARATHON COUNTY: Hornblende crystals up to 1 cm. occur as relict phenocrysts with plagioclase and chlorite in meta-andesite at various sites such as in Big Sandy Park and along the Rib River east of Athens (West 1/2 Sec. 9 T.29N R.3E) (LaBerge and Myers. 1983).
— Radiating crystal clusters up to 1 cm. across occur in outcrop along the Eau Claire River north of Callo (LaBerge and Myers, 1983).
— Hornblende is a component of the Hogarthy hornblende granite exposed in group of quarries in the SE SE Sec. 18 and NE NE Sec. 19 T.29N R.10E, north of Hatley (Inst. Lake Superior Geol., 1973).
— Hornblende occurs in coarse quartz monzonite along the north side of the road in the SE SE SE Sec. 34 T.30N R.9E (Van Schmus, 1980).
— Ferrohornblende occurs in quartz syenite at the old Technical Institute in Wausau (NE NE Sec. 35 T.29N R.7E) associated with K Feldspar, fayalite, hedenbergite, fluorite and titanite (LaBerge and Myers, 1983).
MARINETTE COUNTY: Hornblende is common in the Marinette quartz diorite south of Niagara where it occurs with plagioclase, prochlorite and biotite. Good outcrops occur in NE Sec. 22 and SW Sec. 15 T.38N R.20E as well as along the railroad tracks in NE Sec. 18 T.38N R.20E. Hornblende makes up nearly 40% of rock at SW SE Sec. 21 T.37N R.18E (Prinz, 1965).
— Large platy hornblende grains, partly replaced by biotite, are found in the Newingham granodiorite on the north side of U.S. Hwy. 8, west of Pembine (center north 1/2 Sec. 5 T.36N R.20E (Banks and Cain, 1969).
— Abundant in hornblendite outcrops west of Niagara (Gore, 1968).
— Hornblende is common in the Athelsthane Quartz Monzonite exposed in NW SW sec. 10 T.35N. R.20E. (Inst. Lake Superior Geology, 1973).
OCONTO COUNTY: Hornblende is common as small grains with plagioclase, biotite, clinopyroxene, cummingtonite, and epidote in the metavolcanic rocks of the Waupee Formation, near Mountain. (Lahr, 1973, Read and Wein, 1962).
PORTAGE COUNTY: Hornblende is common in granites, amphibolites and gneisses exposed along the Wisconsin River in the Steven’s Point region. Some key localities are:
— At Conant’s Rapids NE SW Sec. 8 T.23N R.7E (Maas, et al., 1980).
— At Steven’s Point Dam, in Sec. 15 T.23N R.8W (Maas et al., 1980).
— In migmatite at the Linwood Township Quarry, NE SE Sec. 15 T.23N R.7E (Maas et al., 1980).
— At Biron Dam, NE NW Sec. 34 T.23N R.6E (Maas et al., 1980).
— In outcrop in SW NE Sec. 10 and along County C NE SE Sec. 32 both in T,24N R.7E (Anderson and Cullers, 1987).
PRICE COUNTY: Hornblende crystals up to 1 cm. long are found with almandine and magnetite in hornblende schist near Sailor Lake, NW SW Sec. 20 T.39N R.2E (Hotchkiss, 1915).
— Hornblende grains up to 2 cm. long occur along the east shore of Blockhouse Lake in Sec. 11, 15 and 16 T.40N R.1E east of Park Falls (Hotchkiss, 1915).
RUSK COUNTY: Hornblende is found with tonalite in outcrops south of Big Falls Dam in the SW SW Sec. 35 T.36N R.5W (Sims and Peterman, 1980).
WAUPACA COUNTY: Euhedral hornblende crystals up to 5 cm. across occur in granitic rocks associated with uraninite-bearing veins at the Uranium claims in Sec. 17-20 T.25N R.13E near Dupont (WGNHS files).
— Coarse hornblende occurs in granitic rocks in the SW Sec. 17 and the NE Sec. 20 T.25N R.12E (WGNHS files).