Formula: FeCr2O4 Isometric Description: Chromite occurs as small grains in ultramafic rock associated with serpentine, talc, magnesite and brucite. It is an important ore of chromium when found in quantity. CLARK COUNTY: A minute black metallic grain in …
Marinette County
Clinozoisite
Formula: Ca2Al3(Si2O7)(SiO4)O(OH) Monoclinic Description: Common in low-grade metamorphic rocks, particularly as an alteration product of plagioclase. ASHLAND COUNTY: Formed as an alteration product of plagioclase in the gabbros near Mellen, where it forms with epidote, zoisite, calcite …
Cummingtonite
Formula: (Mg,Fe)7Si8O22(OH)2 Monoclinic Description: Cummingtonite is a member of the amphibole family. It commonly forms elongated to fibrous green to brown grains in metamorphosed iron-rich rocks. ASHLAND COUNTY: Cummingtonite is found in the Ironwood Iron Formation where it …
Epidote
Formula: Ca2(Al2Fe3+)(Si2O7)(SiO4)O(OH)2 Monoclinic Description: Epidote is a common mineral formed during low grades of metamorphism and hydrothermal activity. It is particular common in metamorphosed basalts and gabbros where it replaces plagioclase, pyroxene and olivine. It can also …
Ferrimolybdite
Formula: Fe2(MoO4)3.n(H2O) Orthorhombic Description: Ferrimolybdite forms yellow earthy to fibrous coatings as a result of the oxidation of molybdenite. FLORENCE COUNTY: Found at the Payant-Chrissman prospect pits in the north central part of sec. 33 T.38N. R.19E, …
Fluorite
Formula: CaF2 Isometric Description: Fluorite crystals, from Sturgeon Bay in Door County, Wisconsin. Field of view is about 4 cm. (Image courtesy W. S. Cordua.) Fluorite often forms coarse cleavable masses or cubic crystals. It can take …
Gold
Formula: Au Isometric Description:Gold is usually found in hydrothermal deposits where it forms at a wide range of temperatures and geological settings. Gold may form in exhalative deposits formed on the sea floor associated with Archean …
Graphite
Formula: C Hexagonal Description: Graphite with U.S. quarter for scale. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 589. (Image courtesy U.S. Geological Survey Denver Library Photographic Collection; …
Grunerite
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 …
Hornblende
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 …