Ilvaite from the Oslo Graben, Norway

Ilvaite occurs in Permian veins transecting Proterozoic granitic gneisses on the southwestern flank of the Late Paleozoic Oslo Graben, Norway. The mineral occurs in hydrothermal quartz-magnetite veins in association with hedenbergite, andradite, fluorite and calcite. Ilvaite as an accessory mineral in hydrothermal veins is a very unusual mode of occurrence. Ilvaite also occurs in some typical contact deposits within the region. These ilvaites are generally much richer in Mn than the ilvaites from the hydrothermal veins. The present study shows that increased Mn substitution leads to an expansion of the b axis and a decrease in the monoclinic β angle of the ilvaite unit cell. This is in accordance with what has been previously reported. Ilvaite from Konnerudkollen, varying from 8.60 % MnO to 15.03 % MnO, has the highest Mn content ever reported for naturally occurring ilvaites, and a new mineral species, a manganese dominant ilvaite, probably occurs at the locality. Ilvaites from the Oslo Graben display rather simple crystal morphology.