Everything about Marcasite totally explained
The
mineral marcasite, sometimes called
white iron pyrite, is
iron sulfide (FeS
2). Marcasite is often mistakenly confused with
pyrite, but marcasite is lighter and more brittle. Specimens of marcasite often crumble and break up due to the unstable
crystal structure, and it's this crystal structure that's 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. In jewelry, pyrite used as a
gem is improperly termed "marcasite". True marcasite is never used as a gem, due to its brittle and chemically unstable structure.
Marcasite can be formed as both a primary or a secondary mineral.
As a primary mineral it forms nodules, concretions and crystals in a variety of
sedimentary rock, such as at
Dover,
Kent,
England, where it forms as sharp individual crystals and crystal groups, and nodules (similar to those shown here) in
chalk. It can also be found in low-temperature
hydrothermal veins.
As a secondary mineral it forms by chemical alteration of a primary mineral such as
pyrrhotite or
chalcopyrite. On fresh surfaces it's 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 can't be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs."
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 affects certain marcasite specimens seemingly at random, while other specimens remain unaffected. When a specimen goes through pyrite decay, the marcasite reacts with moisture in the air, the
sulfur combining with water to produce
sulfuric acid that attacks other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals to prevent this "disease" from spreading.
Some research has suggested bacteria may aid and accelerate this process by literally 'eating' the marcasite. What is known is that samples with a rough surface tend to decay faster than those with bright, shiny faces, probably due to the greater surface area to react with water in the air, and also it's clear that samples kept in a dry environment (low
humidity) are less likely to decay.
Other minerals often found associated with marcasite are
pyrite,
galena,
sphalerite,
fluorite and
calcite.
Further Information
Get more info on 'Marcasite'.
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