Formula: (K,Na,Ca,Ba)4-7(Al4-7Si12-9)O32.12H2O Monoclinic Description: White phillipsite-K crystals on matrix. From Nickel Quarry, Oberwiddersheim, Hungen, Volgelsburg, Hesse, Germany. Specimen is 4.5 x 4.5 x 2 cm in size. (Photo courtesy Dan and Diana Weinrich Minerals; unauthorized reproduction is …
Douglas County
Plagioclase
Formula: CaAl2Si2O8 Triclinic Description: Plagioclase is a family of feldspar minerals forming a solid solution that ranges, relatively continuously, from pure sodium aluminum silicate (the end member called albite) to pure calcium aluminum silicate (the end member …
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 …
Prehnite
Formula: Ca2Al2Si3O10(OH)2 Orthorhombic Description: Prehnite in vug in basalt, with dark marcasite crystals. Buckley quarry, Douglas County, WI. field of view about 6 cm. across. (Photo by W. Cordua) Prehnite is most often found as a vein or …
Pumpellyite Group
Formula: Ca2(Fe2+,Mg)Al2(SiO4)(Si2O7)(OH)2.H2O Monoclinic Description: Pumpellyite is used to denote a group of closely related minerals that form during relatively low grade burial metamorphism. It is often associated with prehnite, epidote and chlorite. It is most common …
Quartz
Formula: SiO2 Hexagonal Description: Quartz is the most abundant mineral. It is an essential component of granitic igneous rocks, of sedimentary rocks such as sandstone and chert and of metamorphic rocks such as quartzites. It is a …
Saponite
Formula: Ca0.25(Mg,Fe)3(Si,Al)4O10(OH)2.nH2O Monoclinic Description: Saponite is a clay mineral most often found as an amygdule filling in volcanic rocks. DOUGLAS COUNTY: Saponite is reported as occurring in the amygdules of basaltic volcanic rocks exposed along the Black …
Siderite
Formula: FeCO3 Hexagonal Description: Siderite forms as a sedimentary precipitate, in hydrothermal veins, in metamorphic rocks formed from such protoliths, and, rarely, in pegmatites. In sedimentary environments it forms in extensive beds of banded iron formations …
Silver
Formula: Ag Isometric Description:Native silver is found in hydrothermal veins or, more rarely, as a supergene mineral. It is associated with gold, and a variety of sulfides such as galena, pyrite and acanthite. Major silver mineralization …
Tetrahedrite-tennantite
Formula: (Cu,Fe,Ag,Zn)12(Sb,As)4S13 Isometric Description:Tetrahedrite forms a solid solution series with tennantite and freibergite in moderate to low temperature hydrothermal deposits.DOUGLAS COUNTY: Irving (1882) reports finding tetrahedrite in “small quantities in traverse veins in copper-bearing rocks”. This occurrence has …