marcasite

{{Infobox mineral | name = Marcasite | category = Sulfide mineral | boxwidth = | boxbgcolor = | image = MarcassiteII.jpg | imagesize = 260px | caption = Marcasite with tarnish (8×6 cm) | formula = FeS2 | IMAsymbol = Mrc | molweight = 119.98 g/mol | strunz = 2.EB.10a | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Pnnm | unit cell = a 4.436 Å, b 5.414 Å, c 3.381 Å; Z  2 | color = Tin-white on fresh surface, pale bronze-yellow, darkening on exposure, iridescent tarnish | 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 = 6–6.5 | luster = Metallic | polish = | refractive = | opticalprop = | birefringence = | dispersion = | pleochroism = [100] creamy white; [010] light yellowish white; [001] white with rose-brown tint. Anisotropism: Very strong, yellow through pale green to dark green | absorption = | streak = Dark-grey to black. | gravity = 4.875 calculated, 4.887 measured | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Opaque | other = | references = }} 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 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".