Dunite
Dunite is a coarse-grained ultramafic igneous rock composed almost entirely of olivine, typically more than 90 percent, with minor chromite, pyroxene, or spinel. It represents mantle material or cumulate layers formed by the settling of olivine crystals in a magma chamber, making it one of the most direct samples of Earth's upper mantle available at the surface. Its name comes from Dun Mountain in New Zealand, where it was first described.
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Dunite properties at a glance
| Type | Igneous Rocks |
|---|---|
| Mohs hardness | 6.5–7 |
| Luster | Vitreous (olivine grains) |
| Fracture | Conchoidal to uneven |
| Specific gravity | 3.2–3.3 |
| Transparency | Opaque (translucent in thin section) |
| Common colors | Olive green, Yellowish green, Dark green, Brown (weathered) |
| Rarity | common |
Geology & background
Dunite forms either as a cumulate rock, accumulating from crystal settling in mafic and ultramafic magma chambers, or as fragments of Earth's mantle brought to the surface through tectonic processes such as ophiolite obduction, where oceanic lithosphere gets thrust onto continental crust. Because it derives so directly from mantle material, dunite hands geologists important information about mantle composition and melting processes. Notable exposures occur in ophiolite complexes such as Oman's Samail ophiolite, the Twin Sisters massif in Washington State, and the Dun Mountain type locality in New Zealand.
Rockhounds should know that dunite weathers readily, since its olivine content is chemically unstable at Earth's surface and alters into serpentine minerals, often giving outcrops a greenish, waxy weathered rind. Fresh dunite is olive-green to yellow-green and notably heavy for its grain size, reflecting a high iron and magnesium content. Because chromite deposits often occur within dunite bodies, some dunite outcrops carry real economic importance for chromium mining, on top of their scientific value as mantle samples.
How to identify Dunite
Dunite is a granular, typically olive-green to yellowish-green rock composed almost entirely of olivine, with a vitreous luster on fresh grain faces and a white to pale streak. It has a hardness around 6.5 to 7, reflecting olivine's own hardness, and breaks with a conchoidal fracture since olivine lacks distinct cleavage. Weathered surfaces often show a reddish-brown or serpentinized greenish rind that contrasts with the fresher green interior. It can be mistaken for other ultramafic rocks like peridotite, which carries more pyroxene.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Dunite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Dunite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Construction or aggregate when suitable
- Landscape interpretation
- Education
Interesting facts
- Dunite is named for Dun Mountain in New Zealand, one of the first places geologists formally described the rock.
- Because dunite is essentially solid olivine, researchers often study it as an analog for the composition of Earth's upper mantle.
- Olivine within dunite readily alters to serpentine at the surface, which is why many dunite outcrops develop a slick, greenish weathered rind.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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