English: Bornite from Montana, USA. (public display, Montana Bureau of Mines and Geology Mineral Museum, Butte, Montana, USA)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.

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.

The bornite sample shown above is from Montana's Butte Mining District. In this area, bornite occurs in 62 to 66 million year old copper sulfide-rich hydrothermal veins that intrude the Butte Quartz Monzonite, a pluton of the Boulder Batholith (mid-Campanian Stage, late Late Cretaceous, 76 million years).

Locality: Butte Mining District, Silver Bow County, southwestern Montana, USA

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Photo gallery of bornite:

www.mindat.org/gallery.php?min=727