2 Ca3(PO4)2 + 6 SiO3 + 16 C → P4 + 6 CaSiO3 + 16 CO
This is an oxidation-reduction (redox) reaction:
4 PV + 20 e- → 4 P0 (reduction)
16 C0 - 32 e- → 16 CII (oxidation)
6 SiVI + 12 e- → 6 SiIV (reduction)
Ca3(PO4)2 is an oxidizing agent, C is a reducing agent, SiO3 is an oxidizing agent.