Replacement Mechanisms Among Iron Sulphide Minerals

Abstract

Authigenic pyrite is common in both modern and ancient marine sediments. In natural aquatic systems, pyrite formation depends upon oxidation state, pH, and sulphate and iron concentrations, so pyrite is an indicator of geochemical environments. Berner (1970, 1984) described the genesis of sedimentary pyrite by three stages. In stage (a), bacterial reduction converts sulphate to aqueous sulphide species. Stage (b) reacts sulphide and dissolving detrital iron minerals to form amorphous Fe(HS)2 (Rickard, t989) which subsequently transforms to amorphous FeS and then to mackinawite (Fe9Ss). Stage (c) is a series of replacement reactions producing pyrite from mackinawite through progressively more sulphurrich phases. Although these stages have been studied often, our understanding of stage (c) is far from complete. Experimental studies have indicated that zerovalent sulphur species, such as polysulphides ($2-), are essential in forming pyrite through replacement reactions such as, 2FeS + S 2--, FeS2 + Sn-t (I) (e.g., Schoonen and Barnes, 1991). However, sufficient zero-valent sulphur species are not always observed in pyrite-forming environments. An alternative reaction may be by iron loss from the precursor iron sulphide to form pyrite, such as 2FeS + 2H + ~ FeS2 + Fe 2+ + H2 (2) but there is no experimental evidence in support of this reaction (e.g., Schoonen and Barnes, 1991). Sulphidation reactions, like (1), have positive changes in molar volumes (+At/s) of these solids. A + V~ results in armoring of the original surface by the secondary phase, thereby impeding further direct interaction between the original core phase and aqueous solutions. Replacement reactions are favored by -A~?~ causing increased permeability for the efficient exchange of atoms among participating solid phases and aqueous solution. A -Al?s generates void spaces as a replacement reaction proceeds, and further promotes interaction among the solid surfaces and aqueous solutions. Replacement reactions among iron sulphide minerals must also follow these principles. Therefore, the scarcity of zero-valent sulphur species in pyrite-forming environments plus the +AI?~ makes such sulphidat ion reactions geochemically unrealistic.