Bornite is a copper iron sulfide mineral (Cu5FeS4).  It's one of several economically significant copper ore minerals (others include chalcocite and chalcopyrite).  On fresh, unweathered surfaces, bornite has a metallic copper-orange appearance.  Fresh surfaces tarnish relatively quickly.  Early-formed bornite tarnish is iridescent, with blues and purples and reds and greens, resulting in the nickname “peacock ore”.  As the tarnish thickens, more blues and purples stand out.  Late-stage bornite tarnish is a dark purplish-blue.  The tarnish material is actually covellite (CuS).  With weathering, oxidation, and breakdown, bornite converts to covellite and chalcocite.


Bornite is moderately soft (H=3), has no cleavage, and is noticeably heavy for its size.


Bornite (field of view ~1.8 cm across).  Photo by Nicole Byrd.



Bornite (all but the yellow brassy areas - that's chalcopyrite) (field of view ~3.1 cm across) from Butte, southwestern Montana, USA.  The bronzy-gray areas are purplish on the actual specimen (photographing faithful colors on metallic minerals can be difficult).



Photo gallery of bornite



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