Bornite

Bornite
Lightly iridescent bornite crystal on quartz needles from Kazakhstan (size: 3.6 x 2.2 x 1.2 cm)
General
Category	Sulfide mineral
Formula (repeating unit)	Cu5FeS4
Strunz classification	2.BA.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbca
Unit cell	a = 10.95 Å, b = 21.862 Å, c = 10.95 Å; Z = 16
Identification
Formula mass	501.88 g/mol
Color	Copper red, bronze brown, purple
Crystal habit	Granular, massive, disseminated - Crystals pseudocubic, dodecahedral, octahedral
Twinning	Penetration twins on [111]
Cleavage	Poor on [111].
Fracture	Uneven to subconchoidal; brittle
Tenacity	Brittle
Mohs scale hardness	3 - 3.25
Luster	Metallic if fresh, iridescent tarnish
Streak	grayish black
Specific gravity	5.06 - 5.08
Refractive index	Opaque
Pleochroism	Weak but noticeable
Other characteristics	Magnetic after heating, iridescent
References

Bornite, also known as peacock ore, is a sulfide mineral with chemical composition Cu₅FeS₄ that crystallizes in the orthorhombic system (pseudo-cubic).

Bornite has a brown to copper-red color on fresh surfaces that tarnishes to various iridescent shades of blue to purple in places. Its striking iridescence gives it the nickname peacock copper or peacock ore.

Bornite is an important copper ore mineral and occurs widely in porphyry copper deposits along with the more common chalcopyrite. Chalcopyrite and bornite are both typically replaced by chalcocite and covellite in the supergene enrichment zone of copper deposits. Bornite is also found as disseminations in mafic igneous rocks, in contact metamorphic skarn deposits, in pegmatites and in sedimentary cupriferous shales. It is important as an ore for its copper content of about 63 percent by mass.

At temperatures above 228 °C (442 °F), the structure is isometric with a unit cell that is about 5.50 Å on an edge. This structure is based on cubic close-packed sulfur atoms, with copper and iron atoms randomly distributed into six of the eight tetrahedral sites located in the octants of the cube. With cooling, the Fe and Cu become ordered, so that 5.5 Å subcells in which all eight tetrahedral sites are filled alternate with subcells in which only four of the tetrahedral sites are filled; symmetry is reduced to orthorhombic.