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.
Perhaps the most famous case of a polymorph pair is diamond and graphite. Though both are composed entirely of pure carbon, diamond has a cubic structure with strong bonds in 3 dimensions. Graphite, by contrast, forms in layers with only weak bonds between layers. As a result of their structural differences, diamond has a hardness of 10 on the Mohs scale, while graphite rates only 1. Diamond is the ultimate abrasive, while graphite is a superb 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.
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.
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