marcasite

1. A mineral with the same composition as pyrite, FeS₂, but differing in crystal structure. Also called white iron pyrites.
2. An ornament of pyrite, polished steel, or white metal.

[Middle English, from Medieval Latin marcasīta, from Arabic marqašīṯā, from Aramaic marqəšitā, from Akkadian marḫašu, from marḫaši, ancient region in the eastern Iranian plateau.]

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A mineral having composition FeS₂ and crystallizing in the orthorhombic system. Marcasite frequently has a radiating structure and may be globular or stalactitic. There is poor prismatic cleavage. The hardness is 6–6.5 on Mohs scale and the specific gravity is 4.89. The luster is metallic and the color pale bronze-yellow to nearly white on a fresh fracture. Marcasite and pyrite are dimorphous; both have the composition FeS₂. Because marcasite is whiter, it is called white iron pyrite.

Marcasite is found in metalliferous deposits associated with lead and zinc ores, as replacement deposits in limestone, and in concretions in clays and shales. The nodular and lenticular masses in coal known as brasses are in part marcasite and in part pyrite. See also Pyrite.

Environment

Sedimentary rocks and low-temperature veins.

Crystals tabular, parallel to a horizontal axis. However, usually grown together in curving or cockscomblike groups. Also in concretionary masses with radiating structure and botryoidal or reniform crusts.

Light brass yellow. Luster metallic; hardness 6-6Ɖ; specific gravity 4.9; fracture uneven; cleavage poor prismatic. Brittle.

Iron sulfide (46.5% Fe, 53.5% S).

The same as pyrite, except that excess sulfur is freed in the strong nitric acid solution, leaving it cloudy. Some solution takes place in cold dilute nitric acid, indicated by the immediate formation of bubbles on the grains.

Likely to be confused only with pyrite, from which it can be distinguished by greater solubility in cold dilute nitric acid. Generally whiter than pyrite on a fresh surface, and, like it, harder than most other sulfide minerals.

The geological conditions that cause iron and sulfur to combine to form the mineral marcasite are critical; normally pyrite is the product. Marcasite is often associated with galena, sphalerite, calcite, and dolomite, as near Joplin, Missouri, and in the Wisconsin lead-zinc region. Marcasite "cockscombs" and spear-shaped intergrowths grow in clays and marls (though many "combs" are often actually distorted pyrite).

Marcasite specimens almost invariably oxidize in collections, freeing sulfur to form an acid that attacks the labels and trays, and speeding the disintegration of the specimens. Often it is intergrown with pyrite, but no truly satisfactory method of preventing the decomposition has yet been found. It has been suggested that the breakdown is due to renewed activity of a geologically ancient infection of anaerobic bacteria and that thorough sterilization, as with Lysol, can halt or slow the destruction. It is an interesting suggestion and should be investigated further by collectors. "Sooty" chalcocite, pyrrhotite, and arsenopyrite are other likely prospects for such experimentation.

Marcasite
Marcasite
General
Category	Mineral species
Chemical formula	FeS2
Identification
Molar mass	119.98
Color	Brass Yellow
Crystal system	Orthorhombic, Pnnm
Cleavage	[010] Indistinct
Fracture	Irregular/Uneven
Tenacity	Brittle
Mohs scale hardness	6-6.5
Luster	Metallic
Streak	Dark-grey to black.
Density	4.875 g/cm3
Ultraviolet fluorescence	None

The mineral marcasite, sometimes called white iron pyrite, is iron sulfide (FeS₂). 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 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 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" from spreading.

Crystal structure of marcasite. Sulfur atoms are red.

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.

See also

• List of minerals

External links

Wikimedia Commons has media related to: Marcasite

• How Minerals Form and Change "Pyrite oxidation under room conditions".

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