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Marcasite
Chemical Formula	FeS2
Species	Sulfides
Crystal System	Orthorhombic
Mohs Scale	6
Specific Gravity	4.875-4.887
Color	Tin-white on fresh surface, pale bronze-yellow, darkening on exposure, iridescent tarnish
Streak	Dark-grey to black.
Luster	Metallic
Diaphaneity	Opaque
Cleavage	Distinct/GoodDistinct on . in traces.
Fracture	Irregular/Uneven
Crystal Habit：Crystals typically tabular on {010}, curved faces common; stalactitic,reniform, massive; cockscomb and spearhead shapes due to twinning on .
Geological Setting：Most frequently found in sedimentary rocks and coal beds, as a replacement mineral forming fossils, it is a mineral of low-temperature, near-surface, environments, forming from acid solutions. Pyrite, the more stable form of FeS^2, forms under conditions of higher temperatures and lower acidity or alkaline environments.

The mineral marcasite, sometimes called white iron pyrite, is iron sulfide (FeS₂) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide S₂^2- ion having a short bonding distance between the sulfur atoms. The structures differ in how these dianions are arranged around the Fe²⁺ cations. Marcasite is lighter and more brittle than pyrite. Specimens of marcasite often crumble and break up due to the unstable crystal structure.

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."

In marcasite jewellery, pyrite used as a gemstone is termed "marcasite". That is, marcasite jewellery is made from pyrite not from marcasite. In the late medieval and early modern eras the word "marcasite" meant both pyrite and marcasite (and iron sulfides in general). The narrower, modern scientific definition for marcasite as orthorhombic iron sulfide dates from 1845. The jewellery sense for the word pre-dates this 1845 scientific redefinition. Marcasite in the scientific sense is not used as a gem due to its brittleness.

Occurrence

Marcasite can be formed as both a primary or a secondary mineral. It typically forms under low-temperature highly acidic conditions. It occurs in sedimentary rocks (shales, limestones and low grade coals) as well as in low temperature hydrothermal veins. Commonly associated minerals include pyrite, pyrrhotite, galena, sphalerite, fluorite, dolomite and calcite.

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.

As a secondary mineral it forms by chemical alteration of a primary mineral such as pyrrhotite or chalcopyrite.

Alteration

Marcasite reacts more readily than pyrite under conditions of high humidity. The product of this disintegration is iron(II) sulfate and sulfuric acid. The hydrous iron sulfate forms a white powder consisting of the mineral melanterite, FeSO₄·7H₂O.

This disintegration of marcasite in mineral collections is known as "pyrite decay". When a specimen goes through pyrite decay, the marcasite reacts with moisture and oxygen in the air, the sulfur oxidizing and combining with water to produce sulfuric acid that attacks other sulfide minerals and mineral labels. Low humidity (less than 60%) storage conditions prevents or slows the reaction.