Mackinawite Composition Revised to FeS

A recent statistical review led by mineralogist Rickard has overturned long‑held assumptions about the composition of the sulfide mineral mackinawite. The study concludes that the correct formula is FeS rather than the previously accepted Fe_{1+x}S, where “x” had been used to describe variable iron content ¹.

The revision stems from an examination of halogen fractionation during vapor‑brine phase separation processes observed over decades post‑discovery. These processes appear to preferentially remove excess iron while retaining sulfur, thereby shifting the stoichiometry toward a simple binary compound ².org].