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www.mineralgallery.co.za Minerals A-E |
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Bornite |
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Name
Bornite
Chemistry
Cu5FeS4, Copper Iron Sulfide
Uses
As a specimen and for extracting copper.
Color
Brown to black with a typical purplish-bluish tarnish, a reddish bronze color on freshly broken surfaces.
Hardness
3
Specific gravity
4.9 - 5.4
Crystals are rare and thus bornite is usually known as a massive mineral ore.
It does however get used in the mineral trade as a curiosity called "peacock ore". A favorite among children, the bornite chunks sold as peacock ore or alternatively peacock copper have a rich bouquet of colors. The colors are from an iridescent tarnish that forms on bornite upon exposure to air. The tarnish is made of assorted copper oxides or hydroxides that form a mere atoms thin layer over the bornite. The thickness of the layers is close to the wavelength of light. When light waves bounce between the bornite surface and the top of the tarnish layer they will leave with the wavelengths of various colors. This effect is the same as the rainbow effect that occurs with oil on water. In the case of bornite, the tarnish will have a purplish, violet or blue color. Because bornite is often intergrown with chalcopyrite which tarnishes to more greens and yellows, the peacock ore may have many colors ranging from purple to blue to green to yellow.
Bornite at Tsumeb
Cu5UeS4 Forms massive areas of a copper-to pinchbeck-brown color which tarnish rapidly, the purplish blue characteristic of bornite. Crystals have very rarely been found. Some bornite contains up to 1.5 kg of silver per ton.
Borrnite is also a rather rare mineral at Tsumeb. In contrast to the upper levels it is found in greater abundance at greater depth. Massive pieces of about 1 cm in size occur with germanite
