Digenite 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.

Digenite is a scarce copper sulfide mineral, Cu9S5. It has a metallic luster, dark bluish-gray to blackish color, dark gray streak, and a Mohs hardness of 2.5 to 3. It’s usually massive, but can form pseudocubic crystals. Broken surfaces show conchoidal fracture. Primary digenite occurs in some copper-bearing hydrothermal veins, some intrusive igneous rocks, and in some sulfide exhalative deposits.

The digenite sample shown above is from Montana's Butte Mining District. In this area, digenite 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: Leonard Mine, Butte Mining District, Silver Bow County, southwestern Montana, USA

Photo gallery of digenite: