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 do have in common that they 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”. Jewelers’ meaningIn marcasite jewellery , pyrite is called “marcasite” – that is, marcasite jewellery , not from the mineral marcasite. Marcasite in the scientific sense is not used as a gem due to its brittleness. 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 orthorhombic iron sulfide dates from 1845. The jewelers’ sense for the word “marcasite” pre-dates this 1845 scientific redefinition.