Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A mineral with the same composition as pyrite, FeS2, but differing in crystal structure. Also called white iron pyrites.
  • n. An ornament of pyrite, polished steel, or white metal.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A pale mineral, FeS2. Marcasite is physically and crystallographically distinct from pyrite, although the two have the same chemical composition.

from the GNU version of the Collaborative International Dictionary of English

  • n. A sulphide of iron resembling pyrite or common iron pyrites in composition, but differing in form; white iron pyrites.

from The Century Dictionary and Cyclopedia

  • n. As used by the early mineralogists, the crystallized forms of iron pyrites, including more particularly the isometric species now called pyrite.
  • n. In recent use, the orthorhombic iron pyrites, or iron disulphid, FeS2.

Etymologies

Middle English, from Medieval Latin marcasīta, from Arabic marqašīṯā, from Aramaic marqəšitā, from Akkadian marḫašu, from marḫaši, ancient region in the eastern Iranian plateau.
(American Heritage® Dictionary of the English Language, Fourth Edition)

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  • The mineral marcasite, sometimes called white iron pyrite, is iron sulfide (FeS2). Marcasite is often mistakenly confused with pyrite, but marcasite is lighter and more brittle. Specimens of marcasite often crumble and break up due to the unstable crystal structure, and it is this crystal structure that is the main difference between marcasite and pyrite. Though marcasite has the same chemical formula as pyrite, it crystallizes in a different crystal system, thereby making it a separate mineral.

    Marcasite can be formed as both a primary or a secondary mineral.

    As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, such as at Dover, Kent, England, where it forms as sharp individual crystals and crystal groups, and nodules in chalk.

    _Wikipedia

    February 14, 2008