Abstract: The origin in rocks of the common iron sulphides, pyrrhotine, Fe_1-xS and pyrite, FeS₂ and their behaviour during geochemical processes is best considered using the simplified redox reaction: 2FeS ⇌ FeS₂ + Fe²⁺ + 2e⁻.

Thus pyrrhotine is more reduced than pyrite and is the stable iron sulphide formed from magmas except where relatively high oxygen fugacities result from falling pressure or hydrothermal alteration. Pyrite, on the other hand, is the stable iron sulphide in even the most reduced sedimentary rocks where it usually forms during diagenesis through bacteriogenic reduction of sulphate; it is stable throughout the pressure/temperature range endured by normal sedimentary rocks. Pyrrhotine after pyrite or sulphate in metasediments of regional metamorphic origin results mainly from progressive reduction on metamorphism due to the presence of graphite-buffered fluids. Pyrrhotine and/or pyrite may be precipitated from hydrothermal solutions on epigenetic or syngenetic mineralization but pyrrhotine will only be preserved if protected from oxidation to pyrite or to more oxidized species. Exhalative pyrrhotine appears to have been more common in Precambrian times and/or in depositional environments destined to become regionally metamorphosed. FeS can be considered to be the soluble iron sulphide, rather than FeS₂, in reduced aqueous systems although pyrite may precipitate from solution as a result of redox reactions. The relatively soluble nature of FeS explains the observed mobility of iron sulphides in all rock types.