Dunite

Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine. “Ultramafic” means that mafic minerals form more than 90% on the rocks composition. Most common mafic minerals in ultramafic rocks are definitely pyroxenes and olivine (if hornblende is present it is added to pyroxenes). Rocks that contain more than 40% olivine are peridotites. Note that this 40% means 40% of olivine-pyroxene(hornblende) pair, all other minerals are excluded in current classification scheme. Peridotite that contains more than 90% olivine have a special name, they are called dunite (named in 1864 after Dun mountain in New Zealand).

Dunite xenolith in basaltic lava from Hawaii. The sample is 8 cm in width.

‘Plutonic’ means that the rock is not volcanic, it didn’t form at or near the surface. In the case of dunite the formation place was probably very deep in the mantle. That’s why it is so rare on the surface. This rock type is rare, but it is pretty. However, its beauty is not the reason to reserve a separate rock name for it. It is an important rock type because it is probably very common in the mantle.

Dunite is mostly composed of olivine which is a bright green mineral. Fresh rock is green as well. However, olivine readily alters and loses its bright green color pretty quickly. Chances are very high that on the way up in the crust olivine grains lost some of its brightness. Hence, many samples look dull yellow, not green anymore. This rock usually contains chromite (Mg-bearing spinel group mineral). However, if the most common spinel mineral is magnetite, dunite is named olivinite instead.

Magnesium is a very common chemical element in the mantle. So we should expect to see lots of unusual mineral varieties in dunite. I already mentioned chromite, but Mg-bearing garnet pyrope is quite common as well. If the sample contains significant amount of garnet, then it should be added to its name.

A sample from Norway which is mined because of its high olivine content. Olivine is used as a refractory material. Gusdal Quarry, Møre og Romsdal, Norway. Width of sample 9 cm.

Weathered sample with chlorite. Width of sample 14 cm. Helgehornsvatnet, Norway.

Outcrop of dunite in Norway. Dunite loses its green color rapidly when exposed to rain.

Grooves on the outcrop of dunite is a result of rain-induced weathering.

Outcrop of dunite with layers of pyroxenite (layered ultramafic intrusion). Helgehornsvatnet, Norway.

This greenish sand is composed of almost pure olivine and is a result of weathering of dunitic rocks. Gusdal quarry, Norway.