Pyrite-pyrrhotine redox reactions in nature

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 + Fe 2 + + 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. K E Y W O R D S : pyrite, pyrrhotine, redox reactions, iron sulphides. THE common iron sulphide minerals are pyrite (cubic FeS2), marcasite (orthorhombic FeS2), and pyrrhotine (monoclinic and hexagonal Fel_xS). They are found as accessory minerals in the three major categories of rocks: igneous, sedimentary, and metamorphic. In addition they are often major minerals in hydrothermal ore deposits. The origin of the iron sulphides and their behaviour during geochemical processes has been the subject of much research but the redox aspect of their reactions has been relatively neglected. The objective of this paper is to emphasize the value of considering iron sulphide reactions as redox reactions by presenting a general redox equation and demonstrating its Copyright the Mineralogical Society application to provide a better understanding of the occurrence and origin of iron sulphide minerals. The approach adopted here stems from study of the complex relationships exhibited by iron sulphides in metasediments. Iron sulphide reactions. The relationship between pyrite and pyrrhotine can be expressed by the general redox equation: 2FeS ~ FeS 2 + Fe 2 + + 2e-. (1) The formula of pyrrhotine is simplified for the present discussion and FeS/wi l l be called pyrite. It is the distribution of FeS2 relative to pyrrhotine that is of concern here and not the distinction between pyrite and marcasite, which have a similar distribution. Pyrrhotine is reduced compared to pyrite and their relative stabilities in nature depend on oxidation reactions, i.e. electron transfer reactions which may or may not involve oxygen. A schematic fugacity fugacity diagram for F e S O phases following the method of Holland (1959) and Ohmoto and Kerrick (1977) is presented in fig. 1. This emphasizes that pyrite coexists with iron oxides indicative of relatively high oxygen fugacity, whereas pyrrhotine coexists with iron oxides indicative of relatively low oxygen fugacity (Barnes and Kullerud, 1961). The relationships illustrated in fig. 1, based on thermodynamic calculations, are confirmed by observation of coexisting iron sulphide and iron oxide minerals in nature. Although the numerical value of the fugacities vary with temperature, there is no significant change in the geometry within the P T range of interest in the outer crust of the Earth. A version of this diagram was used by Froese (1971) who emphasized the value of fixing oxygen fugacity when describing sulphide-silicate phase relations. Barnes and Kullerud (1961) explained how the pyrrhotine + pyrite + magnetite equilibrium assemblage fixes sulphur and oxygen fugacity at a given temperature; the useful reference point representing this special assemblage is marked by an asterisk in fig. 1. The effect of pH is not of major concern here;