A comparison of the dissolution behavior of troilite with other iron(II) sulfides; implications of structure

Further knowledge as to the nature of the structure of a terrestrial sample of troilite, FeS stoichiometric iron(II) sulfide] is revealed by a combination of XPS studies and dissolution studies in acid. The XPS analysis of a pristine troilite surface (the sample being cleaved under high vacuum) is compared to that of a surface polished in an inert atmosphere and a surface after reaction in deoxygenated acid. Further comparison is made with polished and acid-reacted surfaces of pyrrhotite (Fe_1-xS) and pyrite (FeS₂). The pristine troilite S2p spectrum comprises mainly monosulfide 161.1 eV, within the reported range of monosulfide, together with evidence of an unsatisfied monosulfide surface state arising from S-Fe bond rupture. Small, higher oxidation state sulfur contributions, including a disulfide-like state are also present, which suggest the presence of defects due to some nonstoichiometry. The dissolution studies showed that the troilite, in addition to dissolving in acid as an ionic solid to produce H₂S, also exhibits some oxidation of sulfur in the surface layers. In addition, a study of the dissolution behavior of troilite under the influence of cathodic applied potential supported the existence of a proportion of the sulfur within troilite needing reduction before dissolution forming HS⁻ or H₂S can occur. A significant increase in the dissolution rate was observed with application of −105 mV (SHE), but further stepped decreases in potential to −405 mV and −705 mV resulted in a decreased rate of dissolution, a response typical of an ionic solid. The results of the studies emphasise the viewing of iron(II) sulfides as a continuum. Pyrrhotite has been reported previously to dissolve in acid both oxidatively (like pyrite) and nonoxidatively (like troilite) on the same surface. Dissolution studies using troilite, in Ar-purged acid, indicate that dissolution of this material may not be uniformly nonoxidative. XPS evidence of restructuring of the surface of troilite to pyrrhotite and the surface of pyrrhotite towards a FeS₂ type structure, after exposure to Ar-purged acid, is presented.