Crystals of marcasite up to 6 mm in size across the matrix (6.0 × 5.0 × 3.0 cm) of crystallized dolomite. Found from Reocín mine, Cantabria, Spain — Photo: Ivar Leidus · CC BY-SA 4.0
Marcasite
Also known as: White iron pyrites.
The orthorhombic polymorph of pyrite — 'white iron pyrite' — paler, more unstable, and once cut as a gemstone.
Intro adapted from Wikipedia (CC BY-SA).
Marcasite is an IMA-recognized mineral species in the sulfide class, with the chemical formula FeS₂.
It crystallizes in the orthorhombic crystal system (space group Pnnm). It is a member of the Marcasite structural group.
Key properties: Mohs hardness 6–6.5; specific gravity 4.887; typically yellow or gray; Grayish or brownish black streak; Metallic luster.
IMA symbol Mrc, grandfathered status — known before the IMA’s founding.
Typically formed under low-temperature highly acidic conditions, both in sedimentary environments (shales, limestones, and low rank coals) and in hydrothermal veins formed by ascending solutions.
It is found associated with Pyrite, pyrrhotite, galena, sphalerite, fluorite, dolomite, calcite.
A word of Arabic or Moorish origin, early applied to pyrite and other substances.
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