Flecks of minerals captured in diamonds show hidden connections between Earth’s surface and its deep interior

By James Dinneen edited by Andrea Thompson

Nester Korolev swivels a knob on a microscope, and a tiny speck of a mineral that formed hundreds of kilometers beneath Earth’s surface comes into view. Until Korolev peered at it a few years ago, no one had ever seen the mineral in nature. Normally, such deeply formed material doesn’t survive its long journey up to the planet’s surface. But this minuscule bit was trapped within a diamond, keeping its crystal structure intact.

Powerful new lasers and x-rays are enabling geologists like Korolev, a researcher at the American Museum of Natural History (AMNH) in New York City, to probe increasingly small flecks of minerals in deep diamonds. These tools have led to what one researcher calls an “explosion” of new mineral discoveries from Earth’s mantle, the slowly creeping layer of rock that lies between the planet’s crust and core.

Discoveries in the past few years include breyite, grahampearsonite and goldschmidtite. (The minerals are generally named in honor of leading researchers in the field.) Each new mineral adds a new piece to the puzzle of how rocks transform under heat and pressure within the planet. This in turn shapes estimates of the volume of elements, such as carbon and hydrogen, that are stored in the Earth’s interior.

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The crystal seen in Korolev’s microscope is a case in point. Called bernwoodite, it’s one of two new minerals Korolev and his adviser at the AMNH, Kate Kiseeva, and their colleagues recently discovered trapped in deeply formed diamonds. Both were just designated as new minerals by the International Mineralogical Association, though it has not yet officially announced bernwoodite’s designation.

Subtle chemical differences in these flecks of material offer direct evidence that “there is a relatively efficient cycle of moving material from the surface into the depth and then back to the surface,” Kiseeva, a petrologist and curator, says. This cycling produces a greater variety of mineral structures in the mantle than there would be if Earth’s layers didn’t mix.

One is kopylovite, the other mineral that the researchers discovered. Found in a rare American diamond recovered from a now defunct mine in Wyoming, it forms in the upper mantle between a few dozen and 200 kilometers beneath the surface. Because the mineral contains titanium and potassium, and those elements are associated with the rocks in Earth’s crust, the researchers think kopylovite is produced when sediments sink into the mantle in subduction zones atop slabs of oceanic crust. Though subducted slabs have been detected by seismologists sinking all the way to the edge of the core, it’s been uncertain how far the sediments make it down. “You need a lot of sediments to produce kopylovite,” Korolev says, suggesting they do survive the trip—at least that far down.

(Kopylovite is also one of just 3 percent of the thousands of known minerals that are named after women. The researchers named it after Maya G. Kopylova, a geoscientist at the University of British Columbia, as well as her father, Vladimir Kopylov, a Russian physicist, poet and political dissident.)

Bernwoodite—named in honor of British geoscientist Bernie Wood—was found in a diamond from Brazil and comes from deeper in the mantle. The researchers think it is formed when yet another recently discovered mineral from the lower mantle, called davemaoite, breaks down as it ascends into the transition zone. This is a dynamic layer between about 410 and 660 kilometers depth where the physical properties of minerals see sudden changes as their atoms rearrange under immense pressure. The presence of aluminum in bernwoodite’s chemical structure also suggests subducted material from the crust is involved in its formation, even all the way down to the lower mantle.

These and the other recent discoveries show “there is more mineral variety in the mantle than previously believed; they just have been overlooked,” says Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas, who was not involved with the research.

The researchers are already at work to verify several more candidate minerals from the deep Earth in diamonds obtained by the museum. “We carry on discovering them,” Kiseeva says.