Komatiite Identification Guide
Identify Komatiite, a rare ultramafic Archean lava, by its spinifex texture, high-magnesium olivine, and dark color.
Read the full Komatiite encyclopedia entry →
What Komatiite Looks Like
Komatiite is a rare ultramafic volcanic rock—an extremely high-magnesium lava (MgO often >18 wt%) that erupted mostly during the Archean, when the mantle was hotter. It is dark green to greenish-black to dark gray, fine-grained, and most are now altered (serpentinized/chloritized). Its signature is the spinifex texture: large, bladed or skeletal crystals of olivine (and pyroxene) arranged in fans and sprays, resembling spinifex grass. Cumulate zones at flow bases are more equigranular olivine-rich rock.
Quick visual cues
- Dark green-black ultramafic lava
- Distinctive bladed/feathery "spinifex" crystal sprays
- Often serpentinized (soapy/greenish, soft alteration)
- Flow tops with spinifex grading down to olivine cumulate
Step-by-Step Field ID Checklist
- Look for the spinifex texture—elongate, randomly oriented olivine/pyroxene blades are diagnostic and almost unique to komatiite.
- Confirm an ultramafic, very dark, magnesium-rich character (few or no feldspars).
- Note alteration: most komatiites are serpentinized—greenish, locally soft and greasy.
- Check the flow structure: spinifex-textured upper zones over cumulate lower zones.
- Date the setting: overwhelmingly Archean greenstone belts.
- Test hardness/heft: fresh olivine ~6.5-7; altered serpentine softer (~3-4); rock is dense (~2.8-3.3).
Key Diagnostic Tests
- Texture is the key diagnostic—spinifex is the field fingerprint.
- Mineralogy: olivine + pyroxene (+ chromite), minimal feldspar.
- Acid: no fizz unless carbonate-altered.
- Magnetism: can be weakly magnetic from magnetite produced during serpentinization.
- Density: high (~2.8-3.3 g/cm^3).
Common Look-Alikes and How to Tell Them Apart
- Basalt: also dark and volcanic but lower MgO, plagioclase-bearing, and lacks spinifex blades.
- Peridotite/dunite: ultramafic but plutonic (coarse, equigranular cumulate), not a lava with spinifex texture.
- Serpentinite: the altered product—if the original spinifex/flow features survive, it is altered komatiite; structureless serpentinite cannot be called komatiite.
- Picrite: olivine-rich basalt, intermediate MgO, but without true spinifex and with more feldspar.
Where It Is Found
Komatiite is concentrated in Archean greenstone belts: the type area is the Komati River, Barberton, South Africa; major occurrences include Australia (Yilgarn—e.g., Kambalda Ni deposits), Canada (Abitibi), and Zimbabwe. Komatiite flows host important magmatic nickel-sulfide ores.
Frequently asked questions
How do you identify Komatiite?
Look for a dark green-black ultramafic lava with the distinctive bladed, feathery spinifex texture of olivine and pyroxene crystals, very high magnesium content, minimal feldspar, and usually some serpentine alteration, typically in Archean greenstone belts.
What is spinifex texture in Komatiite?
Spinifex is a texture of large, skeletal, randomly oriented blades of olivine and pyroxene that crystallized rapidly in fast-cooling komatiite lava, resembling spinifex grass; it is the rock's diagnostic feature.
Why are most komatiites Archean?
Komatiites require extremely high-temperature, high-magnesium magmas that formed when Earth's mantle was much hotter in the Archean; such conditions are rare today, so younger komatiites are very uncommon.
Komatiite vs basalt—what's the difference?
Komatiite is ultramafic with very high MgO, abundant olivine, spinifex texture, and little feldspar, while basalt is mafic, plagioclase-bearing, lower in magnesium, and lacks spinifex blades.
Komatiite identified by the community
Recent Komatiite specimens identified with Rock Identifier.
