Fe2IIIS3-II + 4 O20 2 FeIIO-II + 3 SIVO2-II

This is an oxidation-reduction (redox) reaction:

3 S-II - 18 e- 3 SIV (oxidation)

2 FeIII + 2 e- 2 FeII (reduction)

8 O0 + 16 e- 8 O-II (reduction)

Change the oxidation states