Ringwoodite: a unique mineral that can ‘store’ water and reveal secrets of the inner Earth

The minerals deep beneath our feet hold all kinds of secrets about our planet, such as the super-deep diamonds that tell us about the formation of continents. But there’s one particularly rare, buried mineral you may not have heard of, and it too has uncovered some mysteries. His name? Ringwoodite.

What is ringwoodite?

Ringwoodite was first discovered in 1969 in a fragment of the Tenham meteorite, a space rock that was favored in Australia. The deep blue mineral is named after Australian geophysicist and geochemist Ted Ringwood, who predicted its structure could exist.

Speaking of structure, ringwoodite is a member of the spinels, a group of minerals that crystallize in the shape of a cube. It’s also a high-pressure form of the mineral olivine – both are magnesium silicates, but only olivine has some iron for good measure.

Trapped in a diamond

It’s not just a pretty blue face. It turns out that ringwoodite has an excellent ability to store water; the oxygen and hydrogen atoms that make up the fluid of life can take the place of magnesium atoms and oxides in the mineral, bundled as hydroxyl.

“Ringwoodite is like a sponge that soaks up water. There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and retain water,” geophysicist Steve Jacobsen explained in a statement.

So when the very first terrestrial rock sample of ringwoodite was confirmed in a small ‘ultra-deep’ diamond in 2014, scientists got pretty excited. These super-deep diamonds often form within what is known as the transition zone, the region in the Earth that separates the upper and lower mantle and is located about 250 to 400 miles (410 to 660 kilometers) below the surface.

“These high-pressure diamonds give you a glimpse into the deep Earth,” said Graham Pearson, lead author of the diamond study, in conversation with Nature News. And not only did that window confirm the long-standing theory that ringwoodite existed as an important part of the transition zone, it also provided direct evidence that water was present deep beneath our feet.

Water world

However, there’s no giant ocean hidden beneath the ground (sorry, sorry, Hollow Earth lovers). As mentioned above, it’s a case of hydroxyl being stored within the mineral – it’s more like a crystal reservoir of potential water, which sounds like a rather delightful place to be if it weren’t deep underground.

But how much ‘water’ is in the transition zone? Another 2014 study suggests it could be quite a lot.

Using seismic wave data from more than 500 earthquakes, researchers from Northwestern University and the University of New Mexico examined the speed of the waves at different depths to find out what types of rocks the waves traveled through. From this they added further evidence that ringwoodite was indeed present in the transition zone.

According to their results, this would mean that even if it only made up 1 percent of the rock in the mantle, the total amount of ringwoodite in this area could contain three times the amount of water found in Earth’s oceans.

The authors also found evidence that Earth’s water cycle is more than just what happens on the surface. “If there is a significant amount of H2O in the transition zone, some melting should occur in areas where there is flow to the lower mantle,” study author and seismologist Brandon Schmandt explained in a statement, “and that is consistent with what we found.”

“I think we’re finally seeing evidence for a water cycle across the Earth, which could help explain the enormous amount of liquid water on the surface of our habitable planet,” Jacobsen added. “Scientists have been looking for this missing deep water for decades.”

Not a bad day for ringwoodite, it seems.

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