Geologists describe silicate rocks that form out of magma as igneous, from the Latin word for “fiery.” That’s a dramatic origin story in and of itself, but one such rock, peridotite—named after peridot, a semiprecious gem and the August birthstone—really pushes the limits.
The extreme
Most igneous rocks range from felsic (a portmanteau geochemistry word that combines “feldspar” and “silica”) to mafic (another portmanteau, derived from “magnesium” and “ferric”). Because of the minerals each contains, felsic rocks tend to be light in color, like granite, while mafic rocks are dark, like the basalt lavas of Hawaii’s volcanoes. Peridotite, on the other hand, is ultramafic, that is, “extremely enriched in iron and magnesium.”
It’s extremely old, too. This coarse-grained, dark green to bluish gray rock makes up Earth’s upper mantle and so dates back billions of years to when the planet first developed an inner structure.
Peridotite usually exists in the mantle as molten, solid or “squishy” matter, but plate tectonic processes can bring it up to the surface. When freed from extreme heat and pressure this way, part of the rock actually becomes unstable and changes into a different rock when exposed to water. Geoscientists call this process metamorphism.
Turning carbon dioxide into carbon
Peridotite’s uses range from industrial manufacturing, decoration and jewelry making to scrubbing greenhouse gases from the atmosphere.
The rock is composed of the minerals olivine and pyroxene and is classified as as an intrusive. Chromite, platinum and nickel are often concentrated around peridotite, where they can be mined when the rock has reached the surface. Once here, peridotite’s instability turns it into such useful materials as serpentine, soapstone, asbestos, talc and limestone.
If there are volatiles like carbon dioxide or water in molten peridotite down in the mantle, where the pressure is very great, it might explode to the surface as a kimberlite pipe. collecting diamonds along the way that have formed underneath very stable areas of the continents. Fortunately, this isn’t happening now—the youngest of these structures is some 50 million years old, according to the American Museum of Natural History. However, the gemstone peridot is still being formed hundreds of miles below our feet.
Peridotite’s high melting temperature means that it crystallizes fairly quickly out of molten rock and then just sit there for a long time under extremely high pressures and temperatures until brought to the surface, usually as a result of continental collision. This slow, stable cooling sometimes turns its iron-rich olivine into clear green to yellow-green crystals of peridot. This, “the other green gem,” has been mined since at least the days of Old Egypt and today is found mostly in the United States, China and Pakistan.
The ferric part of ultramafic peridotite provides jewels, but its magnesium content helps deal with global warming. In 2008, Columbia University geologists discovered that a huge deposit of peridotite in Oman was actively converting the carbon dioxide in ground water into carbonate rocks, absorbing somewhere between 10,000 and 100,000 tons of carbon dioxide a year. Some researchers have proposed to take advantage of this by grinding up peridotite into filters and then attaching these to industrial chimneys. Per the abstract of their 2011 scientific paper, however, the Columbia University team believes it would be better to put the carbon dioxide into water that can be injected directly into the rock itself.
Peridotite may thus become a valuable tool in the fight against global warming. It also provides diamonds, forms the beautiful green to yellow-green peridot, and has many practical uses. It is the oldest of the igneous rocks, existing under a wide range of temperatures and pressures, and it is the main rock of the upper mantle, a key sector of our planet where massive heat and pressure from deep within transition to the much more benign conditions at the surface. That’s a lot to ask of just one kind of rock, but peridotite is extreme enough to handle it all very well.
