marcasite

—marcasitical /mahr'keuh sit"i keuhl/, adj.

/mahr"keuh suyt'/, n.

1. a common mineral, iron disulfide, FeS₂, chemically similar to pyrite but crystallizing in the orthorhombic system.

2. any of the crystallized forms of iron pyrites, much used in the 18th century for ornaments.

3. a specimen or ornament of this substance.

[1375-1425; late ME < ML marcasita < Ar marqashita < Aram marqashita]

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Iron sulfide mineral that forms pale bronze-yellow crystals; the name cockscomb marcasite refers to the shape of a common form of its crystals.

Marcasite has the same chemical formula as pyrite (FeS₂) but a different internal (atomic) structure. It is less stable than pyrite, being easily decomposed, and is much less common.

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▪ mineral

      an iron sulfide mineral that forms pale bronze-yellow orthorhombic crystals, usually twinned to characteristic cockscomb or sheaflike shapes; the names spear pyrites and cockscomb pyrites refer to the shape and colour of these crystals. Radially arranged fibres are also common.

      Marcasite is found with lead and zinc minerals in metalliferous veins, as at Galena, Ill., U.S., and at Clausthal Zellerfeld and Linnich, Ger. Marcasite, which has the same chemical formula as pyrite (FeS₂) but crystallizes in the orthorhombic instead of the isometric system, alters upon weathering to ferrous sulfate and sulfuric acid. For detailed physical properties, see sulfide mineral (table).

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