Pyrite (#73)

Bright brassy cluster of pyritohedrons to 1" maximum dimension, very showy.
North America Bingham Canyon, Salt Lake County, Utah 10" x 8" x 2 ½"


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Pyrite (FeS2), is found in all kinds of rocks - igneous, metamorphic, and sedimentary - but usually in small to modest amounts. In ore deposits, it is often found in massive aggregates, as you can see from this specimen. In fact, pyrite is the most common of the sulfide minerals (those with a metal combined with sulfur). So you might wonder how many pyrite mines we have in the U.S. The answer is, none now. There used to be pyrite mines (although never very many), but the element of interest was sulfur, not iron. Iron is typically recovered from iron oxides, which occur in larger deposits than pyrite and are more easily mined and refined.

There are more economical sources of sulfur nowadays than pyrite - for example, as a byproduct of natural gas and petroleum refining. What pyrite is mined now is because pyrite is often associated with gold, and may even contain some gold as an impurity.

Many mineral collectors like specimens of pyrite, because many tend to be showy and large and are easily accessible, but pyrite turns out to be a nuisance in many ways. Pyrite is common in coal deposits, but burning coal that contains pyrite releases sulfur, which combines with oxygen to form sulfur dioxide, an air pollutant. The exposure of pyrite to air during mining may also result in acid mine drainage. Because pyrite occurs in many kinds of rocks, care must be taken when mining limestone for concrete. Pyrite that ends up in concrete also oxidizes, resulting in production of acid and lowering of the strength of the concrete, leading to possible failure.