Dunite Identification Guide

What It Looks Like

Dunite is an ultramafic igneous rock made of more than 90% olivine, giving it a distinctive yellowish-green to olive-green color when fresh, weathering to brown or reddish on exposed surfaces. It is coarse- to medium-grained, granular (sugary), and equigranular, with a dull to slightly greasy luster. Fresh surfaces sparkle faintly from olivine grains; weathered crusts are often rusty due to iron oxidation.

Telltale Visual Cues

• Uniform green to yellow-green sugary texture, often with a brown weathering rind.
• Heavy feel — dunite is dense.
• Few or no dark mica or feldspar grains; it is nearly monomineralic olivine.

Step-by-Step Field-ID Checklist

1. Assess color and grain: even, granular green to yellow-green crystals point to olivine.
2. Heft it: dunite is noticeably heavy (density ~3.3 g/cm³).
3. Check hardness: olivine grains are Mohs 6.5–7 and scratch glass.
4. Look for weathering: a rusty-brown crust over green interior is characteristic.
5. Look for serpentinization: greenish, soapy, slippery patches indicate alteration toward serpentinite.
6. Examine grain uniformity: lack of feldspar and pyroxene-dominant zones distinguishes it from peridotite.

Key Diagnostic Tests

• Mohs hardness: ~6.5–7 (olivine); scratches glass.
• Density: ~3.2–3.4 g/cm³ — heavy for its size.
• Streak: white to pale (grains too hard to streak easily).
• Magnetism: weak; may contain accessory chromite or magnetite that responds to a magnet.
• Acid: inert to dilute HCl (silicate, not carbonate).
• Fracture: granular/uneven on the rock; olivine grains show conchoidal fracture.

Common Look-Alikes

• Peridotite (lherzolite/harzburgite): also olivine-rich but contains significant pyroxene, giving darker grains and more textural variety; dunite is nearly pure olivine.
• Serpentinite: the altered product — soft (2.5–4), soapy, slippery, often darker green with a waxy luster.
• Basalt: fine-grained and dark, not coarse granular green.
• Olivine-rich gabbro: contains plagioclase feldspar, absent in dunite.

Where It Is Found

Dunite forms in the Earth's upper mantle and appears at the surface in ophiolite complexes (slices of oceanic mantle), layered mafic intrusions, and as mantle xenoliths in basalt. Classic occurrences include Dun Mountain in New Zealand (the namesake), the Twin Sisters in Washington (USA), North Carolina, Oman's ophiolite, Norway, and the Ural Mountains of Russia. It is often associated with chromite and platinum-group ores.