Mineral Species: Bornite

Chemical Formula: Cu5FeS4

Chemical Name: Copper Iron Sulfide

Mineral Class: Sulfide

Mineral Notes: Bornite is a common copper ore found in high temperature veins and intrusive igneous rocks. The colours 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 atom's thick layer over the mineral. The thickness is close to the wavelength of light and thus when light waves bounce between the Bornite surface and the top of the tarnish layer, they refract giving rise to the various colours.

Description: A very attractive specimen of massive Bornite. The grey surface is coverd by several brightly coloured zones. These range from red to purple and violet. The reverse is covered by a thin layer of bright Chalcopyrite.

Location: Craddock Moor Mine, St Cleer, Cornwall, England.