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Aggregate of Marcasite crystals

The Mineral marcasite

Marcasite is the lesser known companion of the famous mineral Pyrite. Marcasite has the same chemical formula as Pyrite, but crystallizes in a different crystal system, thereby making it a scientifically distinct mineral species. Aggregates of iron sulfide (FeS2) where the crystal structure cannot be determined without complex analysis may be wrongly labeled by dealers. Some Marcasite specimens are labeled as Pyrite, and some Pyrite specimens as Marcasite. Pyrite used as a gemstone is improperly termed in the jewelry industry as 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 will slowly disintegrate 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 powder that attacks other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals, to prevent this condition from spreading.
Chemical Formula FeS2
Composition Iron sulfide
Color Pale brass-yellow to nearly white. Tarnishes to a dark brass-like brown.
Streak Dark brown to black, with slightly green tinge
Hardness 6 - 6.5
Crystal System Orthorhombic
3D Crystal Atlas
(Click for animated model) 
Crystal Forms
and Aggregates
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 known as dollars.
Marcasite also forms as pseudomorphs over other minerals and fossils, resulting in bizarre shapes and forms.
Transparency Opaque
Specific Gravity 4.8 - 4.9
Luster Metallic
Cleavage 2,2 - Prismatic
Fracture Conchoidal
Tenacity Brittle
Other ID Marks Tarnishes to a dark brass-like brown.
In Group Sulfides; Simple Sulfides
Striking Features Hardness, color, heaviness, and streak
Environment Mainly in sedimentary deposits and low temperature ore veins, also in skarn metamorphic deposits.
Rock Type Sedimentary, Metamorphic
Popularity (1-4) 2
Prevalence (1-3) 2
Demand (1-3) 2

Marcasite ON EBAY

 -  Flat disc of radiating Pyrite or Marcasite.

There are several good European localities, specifically Vintířov in Bohemia, Czech Republic; Olkusz, Poland; and the Herja Mine, Baia Mare, and Kapnick, Maramures Co., Romania. Marcasite is also one of the many minerals in Panasqueira, Portugal, and was found in good crystals in the Teutonia Mine in Misburg, Hanover, Germany. It is also found in the chalk cliffs on the shore of France and England, specifically at Cap Blanc-Nez, Pas-de-Calais, France and at Kent, England (at Dover and Folkestone).

The U.S. has excellent localities for this mineral. The best and most abundant sources are in the midwest, specifically Joplin, Jasper Co., Missouri; the Sweetwater Mine, Ellington, Reynolds Co., Missouri; Picher, Ottawa Co., Oklahoma; and Baxter Springs and Treece, Cherokee Co., Kansas. Other important localities are the Rensselaer Quarry, Pleasant Ridge, Jasper Co., Indiana; and Shullsburg, Lafayette Co., Wisconsin. In Canada it is found at the Lafarge Quarry, Dundas, Wentworth Co., Ontario.

Galena, Dolomite, Sphalerite, Pyrite, Pyrrhotite, Bornite, Chalcocite, Quartz

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, usually attracted to magnets.

marcasite PHOTOS
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