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. …
Lafayette County
Malachite
Formula: Cu2(CO3)(OH2) Monoclinic Description: Malachite is a common alteration product of other copper-bearing minerals. Its bright green color makes it not only easily recognized, but also highly sought after by mineral collectors. It occurs most notable …
Marcasite
Formula: FeS2 Orthorhombic Description: Marcasite helectite, New Diggings area, Lafayette County, Wisconsin. Sample is about 8 cm long. (Photo by W. Cordua.) Marcasite is common in Wisconsin, often found in well-formed crystals in cavities and veins in the …
Melanterite
Formula: FeSO4.7H2O Monoclinic Description: Melanterite forms fibrous to stalactitic masses, concretions and crusts due to the oxidation of sulfides such as marcasite and pyrite. It is commonly intermixed with other sulfates including jarosite, copiapite gypsum, alunogen, …
Meteorites
Formula: Varies Description: Mifflin L5 chondrite meteorite fragment showing internal brecciation. (Image courtesy John Valley; unauthorized reproduction is prohibited.) CLARK COUNTY: Two stony meteorites with a combined weight of over 200 pounds fell at 6:15 P.M. on …
Millerite
Formula: NiS Hexagonal Description: Millerite is a low temperature hydrothermal mineral found in vugs in carbonate rocks and as an alteration product of other nickel minerals. Millerite typically forms long brassy fibers making very aesthetic specimens. …
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 …
Pearls
Description: Pearls are hard nodular precipitates formed inside the shells of mollusks such as clams. They consist of layers of calcium carbonate, either in the form of calcite or aragonite held together by a horny …
Potassium Feldspar
Formula: KAlSi3O8 Monoclinic or Triclinic Description: Potassium feldspar refers to any of the three polymorphs microcline, orthoclase or sanidine. The distinction between these minerals can be made with certainty only after detailed X-Ray analysis. This has not …
Psilomelane
Formula: Mixture of hydrate manganese-barium oxides Description: Psilomelane is used as a blanket term to denote a variety of hard massive mixtures of manganese oxides. Most, but not all, psilomelane is the mineral romanechite (BaMn2+Mn4+8O16(OH)4 ). The …