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; …
Marathon County
Grayite
Formula: (Th,Pb,Ca)PO4.H2O Hexagonal Description: Grayite is a rare member of the rhabdophane group found in rare element pegmatites. Minerals such as grayite often show up as anomalous “specks” that are recognized only by master mineral collectors, and …
Grossular
Formula: Ca3Al2(SiO4)3. Isometric Description: Grossular is a member of the garnet family found as pale white, green or tan crystals, most often in marbles, calc-silicate and other metamorphic rocks formed from calcium rich protoliths. MARATHON COUNTY: Small …
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 …
Hisingerite
Formula: Fe2Si2O5(OH)4.2H2O Monoclinic Description: Hisingerite is common as an alteration product in many iron-rich rocks. Although reported in only one locality in the state, it is likely present in a number of other places. Its earthy …
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 …
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. …
Ilmenite
Formula: FeTiO3 Hexagonal Description: Black opaque ilmenite with clear quartz, greenish chlorite and high-relief apatite. In rock thin section, viewed in plane-polarized light. Ilmenite grains are about 2 mm in longest dimension. Opelt Quarry, Clark County, Wisconsin. (Photo …
Ixiolite
Formula: (Ta,Nb,Sn,Fe,Mn)4O8 Orthorhombic Description: Massive black ixiolite crystal in matrix (tip of red arrow). From Okhmyl’k Mt., Voron’i Tundry, Kola Peninsula, Murmanskaja Oblast’ Northern Region, Russia. Ixiolite crystal is 0.15 mm across. (Photo courtesy Dan and Diana …