The mineral marcasite
sometimes called white iron pyrite
, is iron sulfide
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
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. 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.
Two halves of a ball of radiating
marcasite from France.
Marcasite can be formed as both a primary or a secondary
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
As a secondary mineral it forms by chemical alteration of a primary
mineral such as pyrrhotite
. 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."
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"
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
less likely to decay.
Other minerals often found associated with marcasite are pyrite