Formula: FeO(OH) Orthorhombic Description: Stalactitic goethite from Montreal Mine in Iron County, Wisconsin. (Image courtesy W. S. Cordua.) Goethite is a common iron mineral. It often forms by weathering of other iron-rich minerals, thus is a …
Ashland County
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; …
Greenalite
Formula: Fe2-3Si2O5(OH)4 Monoclinic Description: Greenalite is a fine-grained mineral found in unmetamorphosed to slightly metamorphosed iron formations. It is likely more widely occurring than indicated, but must be specifically searched for via x-ray or optical analysis. ASHLAND …
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 …
Hematite
Formula: Fe2O3 Hexagonal Description: “Needle Ore” hematite, longest needle is about 20 cm long. From Montreal Mine in Iron County, Wisconsin. (Image courtesy W. S. Cordua.) Hematite is an extremely common mineral. It can form as a …
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 …
Ilmenite
Formula: FeTiO3 Hexagonal Description: Black opaque ilmenite with clear quartz, greenish chlorite and high-relief apatite. In rock thin section, viewed in plane-polarized light. Ilmenite grains are about 2 mm in longest dimension. Opelt Quarry, Clark County, Wisconsin. (Photo …
Kyanite
Formula: Al2SiO5 Triclinic Description: Kyanite schist outcrop near Powell in Iron County, Wisconsin. (Photo courtesy Tod Roush.) Kyanite is formed by the metamorphism of aluminum-rich rocks, and is often associated with biotite, muscovite, almandine, staurolite, and sillimanite. It …
Laumontite
Formula: CaAl2Si4O12.4H2O Monoclinic Description: Laumontite is a member of the zeolite family and occurs most frequently in veins and amygdule fillings in basaltic rocks. The mineral forms as the rocks are subjected to heating during relatively …