Tetrahedrite with Pyrite (#78)

Aesthetic intergrown clusters of bright brassy pyritohedrons of pyrite to ½" and lustrous black tetrahedrite crystals to ½" on edge, very fine small specimen.
North America Daly Judge Mine, Park City, Wasatch County, Utah 1" x ½" x ½"

 

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Tetrahedrite has the chemical composition (Cu,Fe,Ag,Zn)12Sb4S13, and is one of the most common of the sulfosalts. A sulfosalt is a combination of one or more metals (such as the copper, iron, silver, and zinc in the formula above), sulfur (S), and a semimetal such as antimony (Sb), arsenic (As), or bismuth (Bi). (Cu,Fe,Ag,Zn)12 means that the major metal in tetrahedrite is Cu, but Fe, Ag, and Zn may substitute for Cu in some degree, as long as the total comes to 12.

The name comes from its tetrahedral shape. A tetrahedron is a polyhedron (a solid bounded by flat faces) with four faces - envision a 3-sided pyramid and then add its base, and you have a tetrahedron.

Tetrahedrite forms a solid solution series with both tennantite and freibergite. Tennantite has essentially the same metal atoms as tetrahedrite, but arsenic (As) is substituted for antimony (Sb). Freibergite contains both As and Sb, and silver (Ag) is the dominant metal. "Solid solution series" means that any composition from tetrahedrite to tennantite and from tetrahedrite to freibergite is allowed and may be found in nature.

The Daly-Judge Mine in Park City, Utah was a world-class locality for display-quality crystals of tetrahedrite. The mine has been out of production for over a century, but specimens continue to circulate among mineral collectors and dealers.