FRAMBOIDAL PYRITE FORMATION VIA THE OXIDATION OF IRON (II) MONOSULFIDE BY HYDROGEN SULPHIDE

Abstract

Pyrite framboids have been synthesised as the product of the oxidation of FeS (mackinawite) by H2S in aqueous solutions at pH 6, Eh > -250 mV and temperatures above 60°C in the absence of detectable O2. Under these conditions at Eh < -250 mV single crystals of pyrite are formed. The reaction may proceed by dissolution of solid FeS and subsequent reaction of an aqueous FeS cluster complex with H2S. The involvement of an aqueous FeS phase facilitates transport and reaction, and allows the development of framboidal textures from dispersed fine-grained FeS reactant. The results suggest that the framboidal texture results from rapid nucleation in environments where pyrite is strongly supersaturated. At the limits of the pyrite stability field, where pyrite supersaturation is low, rapid pyrite nucleation is inhibited and single crystals form. The results demonstrate that pyrite framboids can form in the absence of molecular O2, magnetic (e.g., greigite) intermediates or biological intervention.