A marcasite "dollar"

The mineral Marcasite, sometimes called white iron pyrite, is iron sulfide (FeS₂). Marcasite is often mistakenly called pyrite, but marcasite is lighter and more brittle. Specimens of marcasite often crumble and break up due to the unstable crystal structure, and it is this crystal structure that is the main difference between marcasite and pyrite. Though marcasite has the same chemical formula as pyrite, it crystallizes in a different crystal system, thereby making it a separate mineral. Pyrite used as a gem is improperly termed marcasite. This is wholly incorrect, as marcasite is never used as a gem.

Marcasite is a secondary mineral -- it forms by chemical alteration of a primary mineral such as chalcopyrite. On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs." Perhaps the most recognizable form of marcasite is its occurrence in radiating discs ("sand dollars").

Marcasite may go through a condition known as "pyrite decay", in which a specimen slowly disintegrates into a white powder. Little is known about this detrimental condition. It only effects certain marcasite specimens at random, while other specimens remain unaffected. When a specimen goes through pyrite decay, the sulfur atoms free themselves and form an acid that attack other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals to prevent this "disease" from spreading.

Other minerals often found associated with marcasite are galena, sphalerite, fluorite and calcite.

See also

• List of minerals