Formula: Al2O3 Hexagonal Description: Corundum is most famous for its rare colored varieties: the bright red ruby and the deep blue sapphire. More often, it is found as drabber blue to blue-gray crystals. Corundum forms in rocks …
Dodge County
Delvauxite
Formula: CaFe4(PO4,SO4)2(OH)8.4-6H2O Amorphous Description: Delvauxite is a poorly described mineral generally found as amorphous brown botryoidal crusts and masses in iron ores. DODGE COUNTY: Delvauxite (called “borickite”) was tentatively identified as wine-red to red brown material in …
Dolomite
Formula: CaMg(CO3)2 Hexagonal Description: Dolomite forms the main mineral in the bluff-forming rocks along the Mississippi River, such as Granddad Bluff near La Crosse. (Image courtesy W. S. Cordua.) Dolomite is an abundant mineral in Wisconsin. It …
Evansite
Formula: Al3(PO4)(OH)6.6H2O Amorphous Description: Evansite is a poorly described mineral forming white to ocherous opalline crusts associated with other phosphates formed by surface weathering. DODGE COUNTY: Evansite was tentatively identified by x-ray analysis as microscopic anhedral grains …
Gahnite
Formula: ZnAl2O4 Isometric Description: Green gahnite crystal on quartz. From Waterloo quarry, Dodge County, Wisconsin. Field of view is 1 mm left to right. (Image courtesy Dan Behnke, specimen collected by Tom Buchholz.) Gahnite is an uncommon …
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 …
Goethite
Formula: FeO(OH) Orthorhombic Description: Stalactitic goethite from Montreal Mine in Iron County, Wisconsin. (Image courtesy W. S. Cordua.) Goethite is a common iron mineral. It often forms by weathering of other iron-rich minerals, thus is a …
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 …
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 …
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 …