File:Enargite (latest Cretaceous to earliest Tertiary, 62-66 Ma; Butte Mining District, Montana, USA) (18743152648).jpg

Enargite from Montana, USA. (2.2 cm across at its widest)

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.

Enargite is a scarce copper arsenosulfide mineral, Cu3AsS4. It has a metallic luster, dark gray to black color and streak, and a Mohs hardness of 3. This mineral often occurs in a massive or granular form, or in radiating masses. It can form tabular or prismatic or blocky crystals having a pseudohexagonal cross-section. Some crystal faces can have striations. Enargite breaks along several planes of cleavage and also breaks along irregular fracture surfaces. It usually occurs in copper sulfide-bearing hydrothermal veins. It has been mined as an ore mineral of copper, but the arsenic is a "penalty element". Extra money has to be spent during ore processing to remove the arsenic.

The enargite sample shown above is from Butte, Montana. Enargite is one of the three most important copper-bearing ore minerals in the Central Zone of the Butte Mining District. For many decades, Butte was the # 1 producer of copper on Earth. Enargite at Butte occurs in copper sulfide-rich hydrothermal veins (dating to ~62-66 million years) 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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