We use Raman Spectroscopy to study ancient hydrothermal iron sulfide formation <100 °C.
We show hydrothermal formation of mackinawite and greigite iron sulfide phases.
Greigite is observed at 70–75 °C under anoxic conditions.
We suggest a FeIII-mackinawite intermediate in the mackinawite–greigite transition.
Mackinawite is suggested as the dominant catalyst in ancient pre-biotic chemistry.