Pyrite synthesis was examined in the laboratory, and the chemical, crystallographic and textural results were compared with naturally occurring pyrite in marine sediment. The path of pyrite formation begins with reaction of hydrogen sulfide and dissolved iron. The first product is either amorphous iron sulfide or mackinawite with composition FeS (sub 0.9) . In the presence of limited oxygen, this can change to hexagonal pyrrhotite, FeS (sub 1.1) . Further reaction of either of the above phases with elemental sulfur will produce greigite, Fe 3 S 4 and, finally, pyrite, FeS 2 . The greigite assumes the morphological character of spherules. Pyrite framboids were found to form only on spherical nuclei. Thus, both chemical and textural evidence is provided for the pathway of pyrite formation.