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; …
Vilas County
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 …
Opal
Formula: SiO2.nH2O Amorphous Description: Opal is a mineraloid composed of spherical silica aggregates with various amounts of interstitial water. It forms under a wide range of low-temperature geological conditions. Opal may be deposited by silica secreting …
Staurolite
Formula: (Fe,Mg,Zn)2Al9(SiO4)4(OH,O)2 Monoclinic Description: Staurolite forms during the metamorphism of aluminous rocks at intermediate temperatures and pressures. It can form large euhedral crystals in schist, associated with muscovite, biotite, almandine and kyanite. Some crystals show distinctive cross …