Formula: (Mg2)(Mg3Al2)(Al2Si6O22)(OH)2 Orthorhombic Description: Gedrite is a member of the amphibole group. It forms during medium to high grade regional metamorphism of ultramafic, iron-rich or clay-rich protoliths. JACKSON COUNTY: Gedrite has been reported intergrown with cummingtonite in the …
Jackson County
Glauconite
Formula: (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2 Monoclinic Description: Glauconite is abundant in the Paleozoic sandstones in Wisconsin where it forms rounded dark green sand-sized nodules. At places glauconite may make up the majority of the rock, forming what are commonly called …
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 …
Halloysite
Formula: Al2Si2O5(OH)4 Monoclinic Description: Halloysite is a clay mineral 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. Many “clay” occurrences within …
Hedenbergite
Formula: CaFeSi2O6 Monoclinic Description: Hedenbergite is a dark green to black member of the pyroxene group. It usually forms due to metamorphism of iron-rich or carbonate-rich sedimentary rocks or as a primary mineral in certain syenites and …
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 …
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 …
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 …
Magnetite
Formula: Fe2+Fe3+2O4 Isometric Description: Magnetite is a common mineral found at many localities in Wisconsin. Only localities that where the mineral is present in great abundance or as unusual material are listed here. Magnetite occurs as an …