The
mineral marcasite, sometimes called
white iron pyrite, is
iron sulfide (FeS
2) 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 S
22- ion having a short bonding distance between the sulfur atoms. The structures differ in how these di-anions are arranged around the Fe
2+ cations. Marcasite is lighter and more brittle than pyrite. Specimens of marcasite often crumble and break up due to the unstable
crystal structure.