Reaction pathways in the Fe-S system below 100°C

The formation pathways of pyrite are controversial. Time resolved experiments show that in reduced sulphur solutions at low temperature, the iron monosulphide mackinawite is stable for up to 4 months. Below 100 degrees C, the rate of pyrite formation from a precursor mackinawite is insignificant in solutions equilibrated solely with H2S(aq). Mackinawite serves as a precursor to pyrite formation only in more oxidised solutions. Controlled, intentional oxidation experiments below 100 degrees C and over a wide range of pH (3.3-12) confirm that the mackinawite to pyrite transformation occurs in slightly oxidising environments. The conversion to pyrite is a multi-step reaction process involving changes in aqueous sulphur species causing solid state transformation of mackinawite to pyrite via the intermediate monosulphide greigite. Oxidised surfaces of precursors or of pyrite seeds speed up the transformation reaction. Solution compositions from the ageing experiments were used to derive stability constants for mackinawite from 25 degrees C to 95 degrees C for the reaction: FeS(s) + 2H(+) double left right arrow Fe2+ + H2S The values of the equilibrium constant, log K-FeS varied from 3.1 at 25 degrees C to 1.2 at 95 degrees C and fit a linear, temperature-dependent equation: log K-FeS = 2848.779/T - 6.347, with T in Kelvin. From these constants, the thermodynamic functions were derived. These are the first high temperature data for the solubility of mackinawite, where Fe2+ is the dominant aqueous ferrous species in reduced, weakly acidic to acidic solutions. (C) 2000 Elsevier Science B.V. All rights reserved.