U+1F738 Alchemical Symbol for Marcasite

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The mineral marcasite, sometimes called "white iron pyrite", is iron sulfide (FeS₂) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures contain the disulfide S₂²⁻ ion, having a short bonding distance between the sulfur atoms. The structures differ in how these di-anions are arranged around the Fe²⁺ cations. Marcasite is lighter and more brittle than pyrite. Specimens of marcasite often crumble and break up due to the unstable crystal structure.

On fresh surfaces, it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs".

In the late medieval and early modern eras, the word "marcasite" meant all iron sulfides in general, including both pyrite and the mineral marcasite. The narrower, modern scientific definition for marcasite as specifically orthorhombic iron sulfide dates from 1845. Jewellery where pyrite is used as the gemstone is called marcasite jewellery; a term which pre-dates the scientific definition, using the original sense of the word. Marcasite in the scientific sense is not used as a gem due to its brittleness.