Marcasite (Iron Sulfide)

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The name marcasite comes from a French word whose origin is unclear. This mineral has the same chemical makeup as pyrite but its crystal structure is different. The term marcasite sometimes is used for small pieces of polished and faceted pieces of jewelry, which actually are made of pyrite. Marcasite itself is not used as a gem. Sometimes marcasite specimens disintegrate into a white powder. The cause of this "decay" is not known.

Facts

Chemical Composition:Iron Sulfide (FeS2)
Mineral Class:Sulfides
Crystallization:Orthorhombic
Crystal Habit:Commonly well crystallized with tabular forms of crystals most common; also commonly finely crystalline and massive; occasionally found in fossils that have become mineralized; sometimes resembles pyrite, which is much more common in Ohio.
Specific Gravity:4.8 - 4.9
Hardness:6 - 6 1/2
Color:Pale bronze-yellow; tin-white on fresh fractures; tarnishes to dark brassy color or brown.
Transparency:opaque
Luster:Metallic
Streak:Grayish black
Occurrence:<img width="195" height="172" title="Map of marcasite range" alt="Map of marcasite range" src="images/naturalHistory/minerals/marcasitemap.gif" />

Notes

There are noteworthy deposits of marcasite in North America, South America and Europe. The mineral has been reported from 8 counties in Ohio. In comparison to its chemical relative, pyrite, marcasite is relatively rare in Ohio. It is found primarily in the Devonian-age Ohio Shale. In the western part of the state it is found, but rarely, as small crystals and granular aggregates in cavities and fractures of dolostone. In central and eastern Ohio it is found, but uncommonly, as nodules and aggregates of crystals in black shale and coal seams.

Resources

  • Carlson, Ernest H., ed. Minerals of Ohio; Ohio Division of Geological Survey, Columbus, OH; Bulletin 69; 1991.
  • Pough, Frederick H. A Field Guide to Rocks and Minerals; Houghton Mifflin, Boston, MA; 1976.
  • Sorrell, Charles A. Rocks and Minerals; Golden Press, New York, NY; 1973.

See Also