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Covellite (latest Cretaceous to earliest Tertiary, 62-66 Ma; Leonard Mine, Butte, Montana, USA) 5 | by James St. John
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Covellite (latest Cretaceous to earliest Tertiary, 62-66 Ma; Leonard Mine, Butte, Montana, USA) 5

Covellite 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.


Covellite is an attractive, deep metallic blue mineral having the formula CuS - copper sulfide. It's principally a secondary sulfide mineral, formed by the breakdown of pre-existing copper-bearing sulfides, but also occurs in a massive or crystalline form as a primary mineral. Covellite has a metallic luster, an intensely deep blue or purplish blue color, a dark gray streak, and is quite soft (H = 1.5 to 2). Well-formed crystals are hexagonal plates with a somewhat micaceous appearance.


The wonderful crystalline covellite sample shown above is from Montana's Butte Mining District. In this area, covellite 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


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Taken on August 14, 2011