A small outcrop of massive Sudbury-type mineralization has recently been exposed by roadworkers at Notträsk in northeastern Sweden. Two pentlandite types occur:type A, as large irregular patches between pyrrhotite grains with well-developed octahedral partings andtype B, as flames in pyrrhotite. The type A pentlandite has a composition approximating Ni4.2Fe3.9Co0.9S8and the type B contains less cobalt, Ni4.2Fe4.1Co0.7S8. Type A pentlandite is altered to violarite with a composition Ni1.5Fe1.2Co0.3S4to Ni1.5Fe1.1Co0.4S4. Violarite formation occurs through the reaction Ni4.3Fe3.9Co0.8S8→. 2Ni1.5Fe1.1Co0.4S4+1.3Ni2++2.7Fe2+. This results in a decrease in the Fe:Ni ratio of the co-existing violarite and indicates that cobalt behaves conservatively. Pentlandite-A formed below 300°C from the monosulfide solid solution, whereas pentlandite-B formed at even lower temperatures by late-stage readjustments in the Notträsk mineral assemblage.