Formula: (Mg,Fe)9 ((Si,Al)8O20)(OH)10.nH2O Orthorhombic Description: Pale green to cream corrensite, likely pseudomorphous after a zeolite-group mineral, with white calcite in cavity in basalt. From a rock quarry near Hurley, Iron County, Wisconsin. Edge of quarter for scale. …
Iron County
Dickite
Formula: Al2Si2O5(OH)4 Monoclinic Description: Dickite is a clay mineral having the same formula as kaolin, nacrite and halloysite but a different crystal structure. It is found intergrown with other clay minerals. It requires x-ray diffraction for positive …
Dolomite
Formula: CaMg(CO3)2 Hexagonal Description: Dolomite forms the main mineral in the bluff-forming rocks along the Mississippi River, such as Granddad Bluff near La Crosse. (Image courtesy W. S. Cordua.) Dolomite is an abundant mineral in Wisconsin. 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 …
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 …
Galena
Formula: PbS Isometric Description: Galena crystals surrounded by marcasite, from Shullsburg, Wisconsin. (Image courtesy W. S. Cordua.) Galena is formed in a wide range of hydrothermal environments. It can be found with sphalerite and chalcopyrite in …
Goethite
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 …
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 …