Jack-straw-textured olivines, which are elongate olivines of metamorphic origin, occur in complexly metamorphosed ultramafites within the Norwegian Caledonides. The ultramafic bodies are lenses enclosed within high-grade mica-shists or gneisses. In general they consist of a dunitic core surrounded by massive olivine orthopyroxenite with a mantle of jack-straw-textured olivines in contact with the country rocks.
The jack-straw olivines are elongate, randomly orientated platy crystals from 3 cm to 1 m long. They lie in a matrix of orthopyroxene, carbonate and minor tremolite. Field relations between various rock-types, including jact-straw textured olivine + carbonate + tale+/- bronzite veins, are well exposed at the Nordvernes ultramafic body, North Helgeland.