Pyrite
The iron sulfide mineral pyrite has been called
fool's gold because of its pale, brass yellow color and
glistening metallic luster. It is the most widespread and
abundant sulfide mineral in rocks of all ages. The cubic,
dodecahedral, and octahedral crystals and the fine grain
masses may be distinguished from gold by their higher Mohs
hardness of 6 to 6.5 and their lower specific gravity of
4.9 to 5.02; pyrite has a greenish black streak, conducts
electricity, and generates a weak electric current when
heated. Its darker colored isometric crystals distinguish
pyrite from the chemically identical marcasite, which
has orthorhombic crystals. Pyrite forms large bodies in
moderate to high temperature hydrothermal deposits and in
contact metamorphic ore deposits, is an accessory in many
igneous rocks, and is common in sedimentary beds and
metamorphosed sediments. It is mined as a source of sulfur
and iron as well as of the impurities gold and copper.
Pyrite is readily oxidized to create sulfuric acid, both
commercially and naturally; in the latter case it helps
form the enriched zone of ore deposits, especially of
copper; oxidation also creates limonite, which forms the
gossan, or iron capping, or sulfide deposits.