Marcasite crystals, a selection of specimens of this crystallized mineral species. Marcasite is a mineral, also known as marcassite in French. Here you’ll find a number of collector’s items presented with a description, one or more photos and sometimes a video. These marcasites are for sale. It’s an iron disulfide with the chemical formula FeS2, and an orthorhombic crystal system, polymorph of pyrite, which has the same chemical composition, but crystallizes in the cubic system.