Native copper

Copper
Native copper from Ray mine, Arizona (specimen 5.25 x 4 x 1 cm)
General
Category	Native metal
Formula (repeating unit)	Cu
Strunz classification	01.AA.05
Dana classification	1.1.1.3
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fm3m
Unit cell	a = 3.615 Å; Z = 4
Identification
Color	Pale rose on fresh surface, quickly darkens to copper-red; in reflected light, pale rose
Crystal habit	As cubes, dodecahedra, and as tetrahexahedra; rarely as octahedra and complex combinations. Commonly flattened on {111}, elongated along [001]. Also as irregular distortions, in twisted, wirelike shapes; filiform, arborescent, massive
Twinning	On {111} to produce simple contact and penetration twins and cyclic groups
Cleavage	None
Fracture	Hackly - jagged
Tenacity	Highly malleable and ductile
Mohs scale hardness	2½–3
Luster	Metallic
Streak	Copper-red
Diaphaneity	Opaque
Specific gravity	8.95
Solubility	Soluble in nitric acid
Other characteristics	Tarnishes to black or green in air.
References

Native copper is an uncombined form of copper which occurs as a natural mineral. Copper is one of the few metallic elements to occur in native form, although it most commonly occurs in oxidized states and mixed with other elements. Native copper was an important ore of copper in historic times and was used by pre-historic peoples.

Native copper occurs rarely as isometric cubic and octahedral crystals, but more typically as irregular masses and fracture fillings. It has a reddish, orangish, and/or brownish color on fresh surfaces, but typically is weathered and coated with a green tarnish of copper(II) carbonate (also known as patina or verdigris). Its specific gravity is 8.9 and its hardness is 2.5–3.

The mines of the Keweenaw native copper deposits of Upper Michigan were major copper producers in the 19th and early 20th centuries, and are the largest deposits of native copper in the world. Native Americans mined copper on a small scale at this and many other locations, and evidence exists of copper trading routes throughout North America among native peoples, proven by isotopic analysis. The first commercial mines in the Keweenaw Peninsula (which is nicknamed the "Copper Country" and "Copper Island") opened in the 1840s. Isle Royale in western Lake Superior was also a site of many tons of native copper. Some of it was extracted by native peoples, but only one of several commercial attempts at mining turned a profit there.

Another major native copper deposit is in Coro Coro, Bolivia.