Formula: Ca2(Mg,Fe)5Si8O22(OH)2 Monoclinic Description: Actinolite is a common member of the amphibole family. It typically forms during low-grade metamorphism of mafic igneous rocks (such as basalt and gabbro) or carbonate-rich rocks such as limey mudstones. ASHLAND COUNTY: Van …
Marathon County
Aegirine
Formula: NaFeSi2O6 Monoclinic Description:Aegirine (also called acmite) is a sodium-rich member of the pyroxene family. Aegirine grades into other pyroxenes such as augite or hedenburgite as a result of solid solution series. Aegirine is most common in …
Agrellite
Formula: NaCa2Si4O10F Triclinic Description: Rock sample showing agrellite prisms up to 1 cm long in granitic rock. (Photo by Gordon Medaris.) Agrellite is a very rare mineral found in mafic, sodium-rich and silicon-poor igneous and metamorphic rocks. MARATHON …
Allanite
Formula: (Ce,Ca,Ce,La,Nd,Y)2(Al,Fe)3(Si2O7)(SiO4)O(OH) Monoclinic Description:Allanite refers to a group of minerals found as an accessory in a number of different rocks such as syenite, granite and gneiss. It may also be a component of granite pegmatite. It …
Almandine
Formula: Fe3Al2(SiO4)3 Isometric Description: Almandine is a common garnet found in a many metamorphic rocks such as schist, gneiss, amphibolite and iron formation. It is commonly associated with micaceous minerals such as muscovite, biotite and chlorite and …
Anatase
Formula: TiO2 Tetragonal Description: Anatase is a relatively rare mineral found in veins and cavities in schists, gneisses, granites, syenites and other related igneous rocks. It is also a product of weathering of other titanium minerals. Tyler …
Andalusite
Formula: Al2SiO5 Orthorhombic Description: Andalusite is found in phyllites, schists and gneisses and associated quartz veins. It indicates relatively low pressure metamorphism of aluminum-rich rocks, such as shales. ASHLAND COUNTY: Andalusite occurs as “chiastolite” in fine-grained schists outcropping …
Andradite
Formula: Ca3Fe2(SiO4)3 Isometric Description: Andradite is a member of the garnet group found in calcareous metamorphic rocks, especially marbles and skarns. FOREST COUNTY: Small grains of andradite, usually with chlorite rims occur in outcrops of the McCaslin …
Anhydrite
Formula: CaSO4 Orthorhombic Description: Anhydrite is a relatively common mineral usually formed as a chemical precipitate from saline water in limestone, dolostone, and salt beds. Its presence is obscured by its tendency to dissolve or be altered …
Anorthoclase
Formula: (Na,K)AlSi3O8 Triclinic Description: Anorthoclase (unpolished) showing “moonstone” schiller. Stettin, Marathon County County, Wisconsin. (Photo from Bill Schoenfuss, Moonlight Photography.) Anorthoclase is a high-temperature solid solution series found in certain volcanic and plutonic rocks. Polished anorthoclase showing “moonstone” …