BORNITE
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).