4 FeIIS-II + 7 O20 2 Fe2IIIO3-II + 4 SIVO2-II

This is an oxidation-reduction (redox) reaction:

4 S-II - 24 e- 4 SIV (oxidation)

4 FeII - 4 e- 4 FeIII (oxidation)

14 O0 + 28 e- 14 O-II (reduction)

FeS is a reducing agent, FeS is a reducing agent, O2 is an oxidizing agent.