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Pyrite-quartz (latest Cretaceous to earliest Tertiary, 62-66 Ma; Butte Mining District, Montana, USA) 3 | by James St. John
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Pyrite-quartz (latest Cretaceous to earliest Tertiary, 62-66 Ma; Butte Mining District, Montana, USA) 3

Pyrite-quartz 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 about 5400 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.


Pyrite is a common iron sulfide mineral (FeS2). It’s nickname is “fool's gold”. Pyrite has a metallic luster, brassy gold color (in contrast to the deep rich yellow gold color of true gold -, dark gray to black streak, is hard (H=6 to 6.5), has no cleavage, and is moderately heavy for its size. It often forms cubic crystals or pyritohedrons (crystals having pentagonal faces).


Pyrite is common in many hydrothermal veins, shales, coals, various metamorphic rocks, and massive sulfide deposits.


The pyrite-rich specimen shown above is from Montana's Butte Mining District. In this area, pyrite occurs in 62 to 66 million year old 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). The beautiful brassy-gold pyrite crystals have an octahedral morphology (= double pyramid crystals with 8 triangular faces).


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


Photo gallery of pyrite:

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Taken on August 9, 2010