Marcasite and pyrite are both iron sulfide, and are chemically identical but have different internal crystal structures. Such minerals are called 'dimorphs'. Marcasite occurs in fine crystals in many localities throughout the world. Some of the most distinctive are aggregates in which individual crystals have grown together and only the terminations of the last-formed ones are evident. These resemble the 'comb' of a rooster and have been termed 'cockscomb' marcasite. One of the best localities for such formations is the area around the lead and zinc mines near Joplin, Missouri.