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Chemistry
Marcasite is a crystalline form of iron sulfide. It is a secondary mineral, according to the Kansas Geological Survey, derived from chemical changes to other minerals such as copper iron sulfide, also known as chalcopyrite.
Formation
The Mindat website states that marcasite typically forms as a precipitate in sedimentary rock, such as shale or limestone, and in beds of coal, usually close to the surface in areas of low temperature and acidity.
Location
The mineral is available in many states within the United States, particularly those in the Midwest, where iron is abundant. Marcasite is often found along with sphalerite, galena, pyrite and calcite.