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.