Pyrite (#72)

A cluster of brassy octahedral crystals to 3/8" on edge, an unusual locality.
North America Garfield County, Utah 1 ½" x 1 ½" x 1"

 

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This is an octahedron, "Oct," of course, means 8, and "hedron" comes from the Greek word for the face of a geometrical solid. An octahedron is like two four-sided pyramids placed base to base, resulting in eight faces. Octahedral pyrite is not rare, but it is not nearly as common as the cube or pyritohedron forms of pyrite. In most cases, it is not clear why a particular specimen has formed with one sort of crystal form instead of another. In this case, it may have something to do with the growth rate, or the chemical properties of the growth environment, or some other factor, but we just don't know. Pyrite is iron sulfide, FeS2, and is therefore not a pure metal. Nevertheless, it is shiny like a metal, Why is that? Pyrite and several other sulfide minerals are semiconductors.

This means that their electrons, in a bulk sample with extremely large numbers of atoms (that is, a typical visible specimen) occupy a set of dense energy levels that are separated from the next allowable (but unoccupied) energy levels by a gap called the band gap. In insulators, this gap is large, so that electrons do not move out of the lower energies. In semi-conductors, this gap is relatively small, so that light can promote electrons across the gap. The immediate return of the electrons to lower energies re-radiates the photons, causing the metallic luster. The size of the gap determines which wavelengths (or energies) of light can promote electrons and which can't, and that determines the "metallic" color of the mineral - gray, silvery, coppery, or brassy, for example.