Peridotite
Peridotite is a dense, coarse-grained ultramafic rock made mostly of olivine and pyroxene. It is the main rock of Earth's upper mantle, and pieces of it reach the surface as fragments carried in lavas and as slabs of oceanic mantle in ophiolites.
Peridotite is a dense, coarse-grained ultramafic rock made mostly of olivine and pyroxene. It is the main rock of Earth's upper mantle, and pieces of it reach the surface as fragments carried in lavas and as slabs of oceanic mantle in ophiolites.
🪨 Geology Guide Specimen: Open Peridotite (Lherzolite) in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.
Upper Mantle Rock: What Is Peridotite Made Of?
Peridotite contains more than 40 percent olivine, plus orthopyroxene, bright green chromium-rich clinopyroxene and a small amount of spinel or garnet, depending on depth. Names depend on the mix: lherzolite has both pyroxenes, harzburgite is rich in olivine and orthopyroxene, and dunite is nearly pure olivine.
Melting and Basalt: Why Does Peridotite Matter?
Partial melting of peridotite produces basalt, the crust of the ocean floor, and leaves behind depleted peridotite. This cycle of melting and extraction has shaped the mantle and the crust over billions of years, and exposed peridotite records what the mantle was like.
Serpentinization and Carbon: What Happens at the Surface?
At the surface, peridotite is chemically out of balance with water and air. It reacts to form serpentine minerals and hydrogen gas, and it reacts readily with carbon dioxide to form carbonate minerals. This is why the peridotite of Oman is studied as a place where carbon dioxide may be stored as rock.
Peridotite at a Glance
| Property | Value |
|---|---|
| Type / variety | Olivine–Pyroxene Mantle Peridotite |
| Composition | >40% Olivine (Forsterite) + Orthopyroxene + Emerald-Green Cr-Diopside |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Ultramafic & Mantle |
| Mohs hardness | Mohs 6.5–7 |
| Texture / habit | Coarse granular interlocking crystals |
| Density | 3.25–3.40 g/cm³ |
| Lustre and cleavage | Sugary olive-green olivine & bright green clinopyroxene |
| Geological setting | Earth's upper mantle (40–400 km depth) & ophiolite complexes |
| Diagnostic feature (Geology Guide) | Dense, heavy granular rock glittering with apple- to olive-green forsterite crystals, bronzy orthopyroxene, and bright emerald-green chromium diopside brought up as nodules in alkali lava. |
| Olivine content | More than 40% |
| Varieties | Lherzolite, harzburgite, dunite |
See Also
- Dunite — the nearly pure olivine end member
- Kimberlite — a magma that carries peridotite fragments from depth
- Basalt — the melt peridotite produces