Fe2q HSaq FeSS H qeqn 613

Iron monosulphides then convert to pyrite (FeS2). At Eh below -250 mV (see Box 5.4) and at pH around 6, the conversion of FeS to FeS2 may occur via oxidation of dissolved FeS by hydrogen sulphide, for example:

Alternatively, under less reducing conditions, conversion may involve the addition of sulphur (S) from intermediate sulphur species (e.g. polysulphides, polythionates or thiosulphate (S2O32-)), which are the products and reactants in microenvironmental sulphur redox cycling. The reaction involving S2O23- can be summarized as:

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