Both intermediate and monoclinic pyrrhotite occur in the Notträsk massive sulfide deposit. The monoclinic form makes up 5–10 wt.% of the total and occurs as lamellae oriented parallel to the (0001) crystallographic plane. The intermediate (hexagonal) pyrrhotite has been identified by X-ray powder diffraction methods. It consists of 11C type with a composition of 47.7 at.% Fe. The dimensions of the intermediate pyrrhotite supercell, based on a hexagonal unit cell, are ao= 6.8959±0.0014 Å and co=31.6109±0.0186 Å. The phase relations indicate that cobaltian pentlandite and monoclinic pyrrhotite exsolved at Notträsk when intermediate 1C pyrrhotite inverted to 11C pyrrhotite at a temperature of less than 250°C.