MARCASITE
Etymology
Noun
marcasite (countable and uncountable, plural marcasites)
(mineral) The orthorhombic form of iron disulfide, FeS2, occurring as yellow crystals. [from 19th c.]
Synonym: white iron pyrite
(jewellery) Pyrite.
(historical) Any of various metal sulfides, usually iron sulfide minerals. [from 15th c.]
Usage notes
• Marcasite is physically and crystallographically distinct from pyrite, although the two have the same chemical composition.
Source: Wiktionary
Mar"ca*site, n. Etym: [F. marcassite; cf. It. marcassita, Sp.
marquesita, Pg. marquezita; all fr. Ar. marqashitha.] (Min.)
Definition: A sulphide of iron resembling pyrite or common iron pyrites in
composition, but differing in form; white iron pyrites. Golden
marcasite, tin. [Obs.]
Source: Webster’s Unabridged Dictionary 1913 Edition