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	2+1⁄2–3
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	[1][2][3][4]

Native copper is an uncombined form of copper that 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.