RIPENING KINETICS OF IRON SULFIDES IN THE PRESENCE/ABSENCE OF A CARBONYL GROUP

The ripening of mackinawite (Fe_1+xS) to pyrite (FeS₂) or greigite (Fe³⁺Fe²⁺₂S₄) was examined at temperatures 20-90°C to better understand the kinetics of low temperature iron sulfide crystallization in the presence or absence of aldehydes. Starting mackinawite was synthesized in a glass, thermostatted reaction vessel from aqueous hydrogen sulfide and ferrous ammonium sulfate. Formaldehyde, an organic switch leading to greigite formation instead of pyrite upon ripening, was added to the reaction vessel to explore changes in kinetics which produce greigite and suppress pyrite production in the presence of carbonyls. Samples of iron sulfides were collected at predetermined intervals and analyzed by powder XRD.

Results show two distinct pathways; if a carbonyl (formaldehyde) is present, mackinawite ripens to greigite. Small amounts of marcasite or pyrite are thought to be the result of O₂ or Fe³⁺ contamination. If a carbonyl was absent, the mackinawite ripened to pyrite with small amounts of smythite, and pyrrhotite produced along the way. Application of the Johnson-Mehl-Avrami equation to better understand the solid-state reaction kinetics suggests that nucleation of the greigite or pyrite begins at the surface of the mackinawite grains. That is consistent with oxidation of the Fe²⁺ by the carbonyl group to form greigite, or oxidation of the S^2- by H₂S to form pyrite as suggested by previous workers.

The confirmation of an organic switch in the formation of pyrite or greigite from monosulfides may have implications for the interpretation of depositional conditions of sedimentary sulfides.