Pyrrhotite (Fe(1−x)S) is a non-stoichiometric structure where x varies from 0 to 0.125 and the non-stoichiometry is due to a system of ordered vacancies within the Fe lattice [13]. The grade of the pyrrhotite particles used in this study was very high (Fe + S > 95.15%), so the surface of the pyrrhotite particles could be considered as the surface of pure Fe(1−x)S.