Formula: KAl3(SO4)2(OH)6 Hexagonal Description: Alunite is an inconspicuous grayish massive mineral formed by the alteration of pyrite and other sulfides during weathering or solfatara activity. It is probably widespread in Wisconsin but has so far been reported …
Rock County
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 …
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 …
Gypsum
Formula: CaSO4.2H2O Monoclinic Description: Gypsum is a relatively common mineral in a number of environments. It is found in sedimentary rocks, either as a primary precipitate from sea or saline lake waters or as a alteration …
Illite
Formula: K0.65Al2(AlSi)4O10(OH)2 Monoclinic Description: Illite refers to a group of clay minerals formed by weathering or hydrothermal alteration of other aluminum-rich minerals. It occurs intermixed with kaolin and other clay minerals. Its identification requires X-Ray analysis. …
Jarosite
Formula: KFe3(SO4)2(OH)6 Hexagonal Description: Yellow clear jarosite grains on matrix. Individual grains are about 0.05 mm across. (Dan Behnke specimen and image.) Jarosite occurs as earthy pale yellow to brown coatings, crusts and granular masses formed by the …
Kaolinite
Formula: Al2Si2O5(OH)4 Triclinic Description: Kaolinite with pen for scale. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 689. Courtesy of the U.S. Geological Survey Denver Library Photographic Collection. (Photo by Andrew …
Lepidocrocite
Formula: FeO(OH) Orthorhombic Description: Lepidocrocite is a polymorph of goethite and is often found associated with it. The two minerals are very similar in physical properties, and it is likely that the mineral is more common …
Marcasite
Formula: FeS2 Orthorhombic Description: Marcasite helectite, New Diggings area, Lafayette County, Wisconsin. Sample is about 8 cm long. (Photo by W. Cordua.) Marcasite is common in Wisconsin, often found in well-formed crystals in cavities and veins in the …
Pearls
Description: Pearls are hard nodular precipitates formed inside the shells of mollusks such as clams. They consist of layers of calcium carbonate, either in the form of calcite or aragonite held together by a horny …