Idaho State Mineral
Tetrahedrite is a copper antimony sulfide mineral. Iron, zinc, silver, mercury, bismuth, and lead are sometimes substituted in the structure. The mineral occurs as steel gray to silver to black metallic in color, has a metallic luster and a black streak, and tarnishes to a shade of green. As its name implies, tetrahedrite often occurs in a four-sided crystal, although it also may occur in cubic form. It is an end member of a solid solution series with freibergite and tennantite in moderate- to low-temperature hydrothermal deposits. Tennantite forms when arsenic substitutes for antimony. The tetrahedrite-tennantite series is an important family of ore minerals, though it can be difficult to distinguish between the two minerals.
Idaho is third nationally in silver production. Tetrahedrite, a silver-bearing mineral ore, is the most common source of silver in the state. The main silver-producing tetrahedrite vein deposits in the famous Coeur d’Alene district occur in Precambrian quartzites and argillites. While Idaho does not have an official mineral, tetrahedrite represents the state’s geology, history, and economy well.
This mineral’s name comes from the common tetrahedral form of its crystals. The word tetrahedron comes from the German wordTetraëdrit and the Greek word tetraedros, meaning "four-faced." The mineral was first described in 1845 in Freiberg, Saxony, Germany. A rare, related mineral that can contain up to 18 percent silver is called freibergite, named for the same site.
FORMATION AND OCCURRENCE
Tetrahedrite typically forms in hydrothermal veins or in contact metamorphic deposits of low to medium temperature. It normally occurs as massive, often as well-formed crystals, and sometimes as granular.
Tetrahedrite occurs with galena, quartz, pyrite, chalcopyrite, and other sulfides. Though this mineral typically occurs in the tetrahedron crystal formation it is named for, it is also found in other crystal structures.
Tetrahedrite is one of the most common of the sulfosalt group, a group of complex sulfide minerals with the same general chemical formula. Therefore, only a few localities that have produced well-crystallized material are mentioned here.
Due to tetrahedrite’s tendency to form well-developed crystals, this mineral was one of the first used in the development of the science of crystallography, the study of the formation and structure of crystals. In addition to the mineral’s simple crystal structure, its dark color and metallic luster can produce outstanding specimens, especially when the crystals form on white or light pink minerals. For these reasons, tetrahedrite is prized by mineral collectors.
The mineral is also a resource for silver, copper, and antimony, making tetrahedrite important in mining and mineral exploration.
Chemical Formula: (Cu,Fe)12Sb4S13
Crystal Structure: Cubic
Hardness (Mohs): 3½ - 4
Color: Steel gray to iron-gray
Density: 4.97 g/cm3
Streak: Black, brown to dark red
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press