Marcasite is formed by precipitation from acidic waters in surface or near-surface environments. it is commonly found in sediments, sedimentary rocks, and hydrothermal deposits in many parts of the world. Marcasite has historically been used as a source of sulfur.

Marcasite can be said to be similar to pyrite in appearance and physical properties, and it has the same chemical composition. However, the two minerals differ in crystal structure. Pyrite crystallizes in the isometric system, while marcasite is orthorhombic.

The most important difference between pyrite and marcasite is their difference in stability in the surface and near-surface environment. Marcasite is much more reactive than pyrite, and it alters at a much more rapid rate. Marcasite will tarnish rapidly when exposed to the weather and will even tarnish in the specimen drawers of a classroom.

When stored in a location with moderate humidity, marcasite specimens can alter to form ferrous sulfate minerals. In the presence of moisture or humidity, these sulfate minerals can produce small amounts of sulfuric acid that can damage specimen notecards, specimen boxes, and adjacent specimens. The reaction can discolor the wood or cause rusting of the drawers of a specimen cabinet. Marcasite specimens should be stored where humidity can be controlled and where any alteration will not cause damage.

Occurrence of Marcasite

Marcasite has an economic significance when it is found in coal. It contributes to the sulfur dioxide emissions during the combustion of coal. When coals are analyzed for their sulfur content, they are often leached in the lab with nitric acid. The amount of sulfide mineral in the sample is estimated by calculation based upon the amount of dissolved iron in the nitric acid leach. The result is reported as “pyritic sulfur” – which disregards that some of the iron might have been contributed by marcasite. Most coal seams contain very little marcasite, but in some coal seams marcasite can be the dominant sulfide mineral and primary source of sulfur.

Marcasite can form in organic-rich clays and peats during their sedimentation or during diagenesis. The organic debris in these sediments produces a slightly acidic environment that is conducive to the formation of sulfide minerals. Pyrite is the more common sulfide to form, but this is also a common environment of marcasite.

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