Marcasite and Pyrite

A mineral is defined both by its chemical composition and its crystal structure. In some cases two different minerals have the same chemical composition, but different crystal structures. Known as polymorphs, these intriguing cases illustrate how the different crystal structures can result in quite different physical properties.

Marcasite Crystals

One of the most well-known examples of polymorphs is diamond and graphite. Although both are made entirely of pure carbon, they differ significantly in structure. Diamond has a cubic arrangement with strong three-dimensional bonds, while graphite consists of layers held together by weak bonds. This structural disparity results in diamond being rated 10 on the Mohs scale for hardness, whereas graphite rates only 1. Consequently, diamond serves as an exceptional abrasive, whereas graphite functions as an excellent lubricant.

Another interesting polymorph pair is marcasite and pyrite. Both minerals are composed of iron sulfide. But where no one could ever confuse diamond and graphite, it can be difficult to tell pyrite and marcasite apart. In fact pyrite is often sold under the name marcasite in the gemstone trade. But despite their apparent similarities, they have some important differences, such that one can be used as a gem material while the other cannot. Discover our extensive range of pyrite for sale.

Pyrite Crystals

Pyrite has a cubic structure, metallic luster and a yellow-gold color that has earned it the nickname "fool's gold". With a hardness of 6 to 6.5 on the Mohs scale, pyrite is hard enough to be used in jewelry. Pyrite is also exceptionally dense, with a specific gravity of 5.0 to 5.2. Only hematite has a higher density.

Marcasite tends to be lighter in color, and is sometimes referred to as "white iron pyrite". Sometime marcasite has a greenish tint, or a multi-colored tarnish that is the result of oxidation. But marcasite has an unstable orthorhombic crystal structure and is liable to crumble and break apart. In some cases marcasite will react with moisture in the air to produce sulfuric acid. For these reasons marcasite is never used in jewelry. When a gemstone is sold as marcasite you can be quite sure that it is actually pyrite.