Formula: (Fe,Mg,Mn)Al2(SiO4)O(OH)2 Monoclinic Description: Chloritoid forms small green platy to micaceous grains in rocks that have been subjected to moderate grades of regional metamorphism. It may also form by hydrothermal alteration of rocks. It occurs with muscovite, …
Forest 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 …
Copper
Formula: Cu Isometric Description: Native copper occurs as veins and cavity fillings in hydrothermally altered basaltic igneous rocks. It is also a product of near-surface supergene alteration of other copper minerals, such as chalcopyrite. Copper can …
Covellite
Formula: CuS Hexagonal Description: Covellite with pen for scale. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 430. Courtesy of the U.S. Geological Survey Denver Library Photographic Collection. (Photo by …
Digenite
Formula: Cu9S5 Hexagonal Description: Digenite is a black metallic mineral often found with and mistaken for chalcocite. Both minerals occur in moderate to low temperatures hydrothermal deposits and in supergene enrichment zones. FOREST COUNTY: Digenite occurs sparingly in …
Enargite
Formula: Cu3AsS4 Orthorhombic Description: Enargite is a rare hydrothermal mineral found associated with other copper minerals. FOREST COUNTY: Small grains of enargite occur with bornite and digenite replacing chalcopyrite at the Crandon deposit near Little Sand Lake. (Lambe …
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 …
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 …
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; …