Ferrous thio-orthophosphate, Fe₃(PS₄)₂

Ferrous thio-orthophosphate, Fe₃(PS₄)₂, results on heating together ferrous sulphide and phosphorus pentasulphide, the latter being present in excess of that required by the equation

3FeS + P₂S₅ = Fe₃(PS₄)₂.

The excess of pentasulphide distils away, leaving a black residue contaminated with ferrous sulphide, from which it is readily freed by treatment with dilute hydrochloric acid.

Ferrous thio-orthophosphate is a black crystalline substance which leaves a mark like graphite. When heated it does not fuse but burns, leaving a brown residue. It is insoluble in water and dilute acids.