Also called white iron pyrites. a common mineral, iron disulfide, FeS 2 , chemically similar to pyrite but crystallizing in the orthorhombic system.
any of the crystallized forms of iron pyrites, much used in the 18th century for ornaments.
a specimen or ornament of this substance.

1375–1425; late Middle English < Medieval Latin marcasīta < Arabic marqashīṭā < Aramaic marqəshītā

marcasitical [mahr-kuh-sit-i-kuhl] , adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
marcasite (ˈmɑːkəˌsaɪt)
1.  a metallic pale yellow mineral consisting of iron sulphide in orthorhombic crystalline form used in jewellery. Formula: FeS2
2.  a cut and polished form of steel or any white metal used for making jewellery
[C15: from Medieval Latin marcasīta, from Arabic marqashītā, perhaps from Persian]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
marcasite   (mär'kə-sīt', -zīt')  Pronunciation Key 
A light yellow to gray, metallic, orthorhombic mineral. Marcasite is a polymorph of pyrite and looks similar to it but has a lower specific gravity, is paler in color, and often has a radiating fibrous structure. Chemical formula: FeS2.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Iron disulfide can occur as either pyrite which forms in the isometric crystal
  system or as marcasite which is orthorhombic.
Marcasite may weather at faster rates than pyrite, sphalerite, and galena.
The breccia is limonite-stained, but pyrite and/or marcasite has only been
  observed in the altered siltstone.
In many cases the shells have been replaced by pyrite and marcasite.
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