By increasing order of stability: 
* Iron(II) sulfide, FeS
* Greigite, FeS (cubic)
* Pyrrhotite, FeS (where x = 0 to 0.2) (monoclinic or hexagonal)
* Troilite, FeS, the endmember of pyrrhotite (hexagonal)
* Mackinawite, FeS (where x = 0 to 0.1) (tetragonal)
* Marcasite, orthorhombic FeS
* Pyrite, cubic FeS (fool's gold)
* Arsenopyrite (mispickel), FeAsS, or Fe(As-S), Fe(III) mixed arseno-sulfide (monoclinic)