Peridotite
Ultramafic intrusive rock (olivine + pyroxene)
A dense, coarse-grained ultramafic rock rich in olivine that makes up most of the Earth's upper mantle.
- Mohs hardness
- 6-7
- Color
- dark green to greenish-black, brownish when weathered
- Type
- igneous
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Overview
Peridotite is a dense, coarse-grained ultramafic igneous rock composed largely of the mineral olivine, together with pyroxene and minor amounts of other minerals. Its high olivine content gives it a characteristic dark green to greenish-black color.
Peridotite is the dominant rock of the Earth's upper mantle, making it one of the most important rocks for understanding the planet's interior. Varieties are named by mineral content, including dunite (almost pure olivine), lherzolite, and harzburgite.
When olivine within it is gem-quality, it is the source of the gemstone peridot.
Formation & geology
Peridotite forms in the Earth's upper mantle and crystallizes from ultramafic magma or accumulates as olivine and pyroxene settle in layered intrusions. It reaches the surface in several ways: as mantle xenoliths carried up by basalt and kimberlite, as slices of oceanic mantle thrust onto continents in ophiolites, and within large layered igneous complexes.
When exposed to water near the surface, olivine in peridotite alters readily into serpentine, forming serpentinite. Major occurrences include ophiolite belts worldwide and mantle-derived xenoliths in volcanic regions.
How to identify it
Identify peridotite by its dense, heavy feel, coarse granular texture, and dark green to greenish-black color dominated by sugary olivine grains with darker pyroxene. Hardness is about 6-7.
Fresh surfaces show glassy green olivine; weathered surfaces are brownish or reddish from oxidation, and altered zones become soft, slippery serpentine.
Distinguish it from gabbro, which contains abundant plagioclase feldspar (lighter grains), and from basalt, which is fine-grained. The dominance of green olivine and its heavy, quartz-free character are diagnostic.
Uses & significance
Peridotite is of major scientific value as a direct sample of the Earth's mantle, used to study the planet's composition and dynamics. Gem-quality olivine within it provides peridot for jewelry.
Economically, peridotite-hosted deposits supply chromium (chromite), nickel, platinum-group metals, and, through alteration, serpentinite and talc. It is also studied for carbon capture, as olivine reacts with CO2 to form stable carbonate minerals.
Crushed peridotite serves as aggregate, and it is the parent rock of asbestos-bearing serpentinites.
Frequently asked questions
What is peridotite made of?
It is an ultramafic rock made mostly of olivine with pyroxene, giving it a dense, dark green character.
Why is peridotite important to geologists?
It is the main rock of the Earth's upper mantle, so it provides direct samples for studying the planet's interior.
Is peridotite related to peridot?
Yes. Gem-quality olivine within peridotite is the gemstone peridot.
What does peridotite turn into when altered?
When exposed to water, its olivine alters into serpentine, forming the rock serpentinite.
Peridotite guides
In-depth guides for identifying, valuing, and understanding Peridotite.
Peridotite identified by the community
Real specimens identified with Rock Identifier.
