Komatiite

Komatiite is a rare, ultramafic volcanic rock with very high magnesium content, formed from some of the hottest lavas ever erupted on Earth, with temperatures around 1600°C. It is named after the Komati River in South Africa, where it was first described.

Most komatiites are extremely old, dating to the Archean Eon (over 2.5 billion years ago), when the Earth's mantle was hotter and could produce such magnesium-rich melts. Younger komatiites are exceedingly rare.

Its hallmark is the spinifex texture, a pattern of long, bladed, criss-crossing olivine or pyroxene crystals that grew rapidly as the hot, runny lava quenched.

Komatiite forms from extraordinarily hot, magnesium-rich, low-viscosity magma generated by high degrees of mantle melting. Such conditions were common in the hotter Archean mantle, which is why nearly all komatiites are ancient.

The runny lava flowed easily and cooled quickly at its surface, allowing distinctive skeletal, blade-shaped crystals (spinifex texture) to form. Komatiites occur in Archean greenstone belts such as the Barberton belt in South Africa, the Abitibi belt in Canada, and Western Australia, where they are also associated with nickel ore.

Look for a dark green to greenish-black, dense ultramafic rock, ideally showing the diagnostic spinifex texture of long, thin, criss-crossing crystal blades. Many komatiites are altered to serpentine and chlorite, giving a greenish, soft surface.

Its high magnesium content, antiquity, and spinifex texture are the strongest clues. Streak is pale to greenish.

Look-alikes include basalt (lower magnesium, no spinifex, younger), peridotite (intrusive, coarse-grained, no quenched texture), and serpentinite (alteration product, greasy feel). The spinifex texture is essentially unique to komatiite.

Komatiite itself has little direct commercial use, but it is enormously important economically and scientifically. Many komatiites host major nickel-copper sulfide deposits, making them key exploration targets, especially in Western Australia and Canada.

Scientifically, komatiites are prized because they record the high temperature of the early Earth's mantle and provide insight into Archean tectonics and the planet's thermal evolution. They are not used as gemstones, though spinifex-textured specimens are valued by collectors and researchers.