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Chemical Formula | FeS2 | ||
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Composition | Iron sulfide | ||
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Color | Pale brass-yellow to nearly white. Tarnishes to a dark brass-like color or brown. | ||
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Streak | Dark brown to black, with slightly green tinge | ||
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Hardness | 6 - 6½ | ||
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Crystal Forms and Aggregates |
(Orthorhombic) Usually as bipyramidal and tabular crystals, commonly in coxcomb aggregates. Crystals may form curved formations. Marcasite also occurs massive, radiating, mammilary, encrusting, as groups of small crystals, and in radiating discs ("dollars"). | ||
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Transparency | Opaque | ||
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Specific Gravity | 4.8 - 4.9 | ||
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Luster | Metallic | ||
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Cleavage | 2,2 - Prismatic | ||
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Fracture | Conchoidal | ||
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Tenacity | Brittle | ||
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Other ID Marks | 1) Tarnishes to a dark brass-like color or brown 2) Unless carefully protected, disintegrates into a white powder |
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Varieties |  Dollar - Flat disc of radiating Marcasite |
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Polymorphs | Pyrite - Crystallizes in the Isometric system | ||
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In Group | Sulfides ; Simple sulfides | ||
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All About | Marcasite has the same chemical formula as Pyrite, but crystallizes in a different crystal system, thereby making it a separate
mineral. Aggregates of FeS2 where the crystal structure cannot be determined without
complex analyzing material may be wrongly labeled by
dealers. Some Marcasite specimens are labeled as Pyrite,
and some Pyrite specimens as Marcasite. Pyrite used as a gem is improperly termed Marcasite. This is wholly incorrect, as Marcasite is never used as a gem. Marcasite may go through a condition known as pyrite decay, in which a specimen slowly disintegrates into a white powder. Little is known about this detrimental condition. It only effects certain Marcasite specimens at random, while other specimens remain unaffected. When a specimen goes through pyrite decay, the sulfur atoms free themselves and form an acid that attack other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals, to prevent this "disease" from spreading. |
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Striking Features | Hardness, color, heaviness, and streak | ||
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Popularity | 2 | ||
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Prevalence | 2 | ||
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Demand | 2 | ||
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Distinguishing Similar Minerals |
Chalcopyrite - softer (3½ - 4), more intense
yellow Cobaltite - softer (3½), crystallizes in different crystal system (isometric) Pyrite - crystallizes in different crystal system (isometric) Pyrrhotite - softer (3½ - 4½), darker color |
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Commonly Occurs With |
Galena, Dolomite, Sphalerite, Pyrite, Pyrrhotite, Bornite, Chalcocite, Quartz | ||
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Noteworthy Localities |
Marcasite is found, usually associated with Galena, Sphalerite an Dolomite in Joplin, Jasper Co., Missouri, and the surrounding area. Coxcomb aggregates and elongated crystals have been found there. Guanajuato, Mexico, has also provided many fine crystals. | ||
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Picture Icon Links |
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Picture Links | 1. Marcasite crystal aggregate |
