Marcasite

Marcasite
Marcasite with tarnish (8x6 cm)
General
Category	Sulfide mineral
Formula (repeating unit)	FeS2
Strunz classification	2.EB.10a
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnnm
Unit cell	a = 4.436 Å, b = 5.414 Å, c = 3.381 Å; Z = 2
Identification
Formula mass	119.98 g/mol
Color	Tin-white on fresh surface, pale bronze-yellow, darkening on exposure, iridescent tarnish
Crystal habit	Crystals typically tabular on {010}, curved faces common; stalactitic, reniform, massive; cockscomb and spearhead shapes due to twinning on {101}.
Twinning	Common and repeated on {101}; less common on {011}.
Cleavage	Cleavage: {101}, rather distinct; {110} in traces
Fracture	Irregular/Uneven
Tenacity	Brittle
Mohs scale hardness	6-6.5
Luster	Metallic
Streak	Dark-grey to black.
Diaphaneity	Opaque
Specific gravity	4.875 calculated, 4.887 measured
Pleochroism	[100] creamy white; [010] light yellowish white; [001] white with rose-brown tint. Anisotropism: Very strong, yellow through pale green to dark green
References

Crystals typically tabular on {010}, curved faces common; stalactitic,

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 S₂²⁻ ion having a short bonding distance between the sulfur atoms. The structures differ in how these di-anions 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 the mineral marcasite. In the late medieval and early modern eras the word "marcasite" meant both pyrite and the mineral 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.

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.