4 FeS + 3 O2 + 6 H2O → 4 Fe(OH)3 + 4 S
This is an oxidation-reduction (redox) reaction:
4 S-II - 8 e- → 4 S0 (oxidation)
4 FeII - 4 e- → 4 FeIII (oxidation)
6 O0 + 12 e- → 6 O-II (reduction)
FeS is a reducing agent, FeS is a reducing agent, O2 is an oxidizing agent.