X-ray studies, optical studies and chemical composition of pyrrhotite indicate that in the Ingaladhal copper mine it occurs in both hexagonal and monoclinic modifications as well as in transitional forms. The monoclinic phase might have formed by inversion from the hexagonal phase. The hexagonal form has a composition that appropriates to solvus temperatures of < 300°–360°C. Heated monoclinic and hexagonal monoclinic mixtures of pyrrhotite yielded temperatures ranging from 425°–460°C. From the questionable validity of the pyrrhotite geothermometer it is concluded that the temperature estimates based on the pyrrhotite solvus cannot be used to know the temperature of formation of pyritepyrrhotite assemblage in the Ingaladhal sulphide lode.