Formula: (Mg,Fe)SiO3 Orthorhombic Description: Orthopyroxenes are a group of minerals ranging from pure MgSiO3 (enstatite) to pure FeSiO3 (ferrosilite). Bronzite and hypersthene are orthopyroxenes mid-way between these two end members. As the particular orthopyroxene mineral described is not always …
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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 …
Olivine
Formula: (Mg,Fe)2SiO4 Orthorhombic Description: Olivine is here used to refer to a solid solution series of minerals ranging from the magnesium end member (forsterite) to the iron end member (fayalite). Olivine is widespread in basalts and gabbros, …
Northupite
Formula: Na3Mg(CO3)2Cl Isometric Description: A rare carbonate mineral found in dry lake sediments and other unusual sedimentary environments. DODGE COUNTY: A grain 0.6 cm. long found in the matrix of the Mayville oolitic iron formation was tentatively …
Nickeline
Formula: NiAs Hexagonal Description: Nickeline, also referred to as “niccolite”, is a rare mineral found in high temperature hydrothermal deposits, especially those in mafic igneous rocks. Other nickel minerals, pyrrhotite and chalcopyrite are commonly associated. ASHLAND …
Nepheline
Formula: (Na,K)AlSiO4 Hexagonal Description: Weathered surface of the same specimen, showing gray pits and hollows where the nepheline has weathered out faster than the surrounding minerals. Green areas are lichen. From the Stettin area, Marathon County, Wisconsin. (Image …
Neotocite
Formula: (Mn,Fe,Mg)SiO3.H2O Amorphous Description: Neotocite forms black to brown resinous masses from the alteration of manganese silicates. DODGE COUNTY: Neotocite is tentatively identified as microscopic light yellow brown grains in the oolites of the Mayville iron formation …
Natrolite
Formula: Na2Al2Si3O10.2H2O Orthorhombic Description: Natrolite is a member of the zeolite family that occurs most frequently in veins and as amygdule fillings in basaltic rocks. BAYFIELD COUNTY: Thin veins of coarsely fibrous radiating clusters of white natrolite …
Muscovite
Formula: KAl2(AlSi3)O10(OH,F)2 Monoclinic Description: Muscovite is a common mica, forming silvery to pale-colored flakes most commonly schists and granite pegmatites. It can survive weathering, and form sparkling grains in sands and sandstone. Muscovite. From the mineral collection …
Montmorillonite
Formula: (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2.nH2O Monoclinic Description: Montmorillonite with pen for scale. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 794. Courtesy of the U.S. Geological Survey Denver Library Photographic Collection. (Photo by …