The iron sulfide having a mackinawite structure which contains {HEAD ~ FeMxNySz} wherein {TAIL ~ M} represents an alkaline earth metal, N represents an alkali metal, and x, y and z, indicating the molar proportions of the respective elements, represent numbers satisfying 0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component 
wherein
The iron sulfide having a mackinawite structure which contains { HEAD ~ FeMxNySz } wherein { TAIL ~ M } represents an alkaline earth metal , N represents an alkali metal , and x , y and z , indicating the molar proportions of the respective elements , represent numbers satisfying 0.01 < x≦0.5 , y≦0.2 and 0.7≦z≦1.4 , as an essential component
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH