Bornite
Copper Iron Sulfide Cu5FeS4
Not a great image, but I couldn't resist the colour of this beautiful mineral.
Taken at the Natural History Museum in Dublin.
"...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.
www.galleries.com/minerals/sulfides/bornite/bornite.htm
