Pyrrhotite-pentlandite-chalcopyrite-magnetite from the Precambrian of Ontario, Canada. (SDSMT 86, South Dakota School of Mines and Technology, Museum of Geology, Rapid City, South Dakota, USA)

Silvery-bronze = pyrrhotite ~Whitish-brassy = pentlandite Yellow brassy gold = chalcopyrite Black = magnetite

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.

Pyrrhotite is imperfect iron monosulfide (Fe(1-x)S). The atomic structure of pyrrhotite has holes due to an insufficient number of iron atoms, cf. sulfur atoms. Iron monosulfide is a common, but minor, component of many meteorites, but it lacks the atomic-scale “holes” of pyrrhotite, and is called troilite (FeS).

Pyrrhotite is superficially like pyrite in appearance and chemistry, but they are different minerals. Pyrrhotite has a metallic luster, a brownish-brassy or bronzish color, a black streak, no cleavage, and is magnetic. What’s particularly distinctive about pyrrhotite is that it is variably magnetic. The holes in the atomic structure gives pyrrhotite its magnetism. But, there's variation in the number of missing iron atoms from sample to sample, so pyrrhotite ends up having variable magnetism. More holes results in stronger magnetism. Few holes results in weaker magnetism.

Pyrrhotite has economic significance because it often occurs with nickel-, copper-, and platinum-bearing minerals. A great example of this is the Sudbury Mining District in Ontario, Canada, which is the origin of the pyrrhotite sample shown above. The Sudbury area has intrigued geologists for decades, and not just due to the tremendous economic value of the area’s mineral deposits. Sudbury is one of the largest preserved impact structures on Earth. The impact occurred ~1.85 billion years ago, during the late Paleoproterozoic. The Sudbury Impact Structure is no longer circular or subcircular in shape, however - it's been compessed into a stretched-egg shape from an ancient continental collision event.

Age of sulfide mineralization: syn-impact or early post- Impact, 1.85 Ga

Locality: Worthington, Sudbury Mining District, Ontario, Canada

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