marcasite or white iron pyrites, a mineral closely resembling and having the same chemical composition (FeS₂) as pyrite. It differs from pyrite in that it is paler in color, becomes darker upon oxidation, and crystallizes in the orthorhombic system. Twinned crystals resembling cockscombs (cockscomb pyrites) or spearheads (spear pyrites) are of common occurrence. The mineral occurs in marls, clays, and limestones in many parts of the world.

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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