Pyrrhotite from Brazil. (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, 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.

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.

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