Greigite formation in aqueous solutions: Critical constraints into the role of iron and sulphur ratios, pH and Eh, and temperature using reaction pathway modelling

Greigite forms as an intermediate phase along the pyrite reaction pathway. Despite being considered metastable, it is observed in numerous shallow natural systems, suggesting it could be a unique proxy for diagenetic and environmental conditions. We use thermodynamic reaction pathway modelling in PHREEQC software, to un-derstand the role of iron and sulphur ratios, pH and Eh, and temperature on the formation and retention of greigite in aqueous solutions. With newly available experimental thermodynamic properties, this work identifies the chemical boundary conditions for greigite formation in aqueous solutions. Greigite precipitation is likely favourable in anoxic and alkaline aqueous solutions at or below 25 degrees C. Our numerical experiments show that greigite is closer to saturation in iron-rich solutions with minor sulphur input. Greigite precipitation in strongly alkaline solutions suggest polysulfides and ferric iron-bearing minerals may be favourable reactants for its for-mation. Greigite precipitates at iron and sulphur concentrations that are over two orders of magnitude greater than iron sulphide-hosted natural porewaters. This disparity between model and field observations suggest microenvironments within bulk solutions may be important for greigite formation and retention. These con-straints suggest greigite is more likely to form alongside pyrite in shallow, non-steady state aqueous solutions.

Automated summary

Greigite forms as an intermediate phase along the pyrite reaction pathway and may serve as a unique proxy for diagenetic and environmental conditions. Thermodynamic modeling reveals the optimal conditions for greigite formation in aqueous solutions, including anoxic and alkaline environments at or below 25 degrees C. Discrepancies between model and field observations suggest the importance of microenvironments in the formation and retention of greigite alongside pyrite.