When Oliver Tschauner and colleagues dusted off a sample of volcano-ejected diamond found in a South African mine, they had no idea that they were holding the first-ever natural sample of a new high-pressure mineral from deep within Earth.
While minuscule, researchers predict that this mineral is responsible for the movement of crucial components like rare earth metals and radioactive isotopes through the Earth’s mantle. To figure this out, the team turned to X-ray beams and lasers.
Tschauner is a research professor of mineralogy at the University of Nevada, Las Vegas, and the first author of a new paper published in Science earlier this month that describes the discovery. He tells Inverse that this discovery was decades in the making — and simultaneously stumbled into at the last minute.
“We were actually not really looking for [davemaoite],” Tschauner says. “We had no suspicion that it occurs in this diamond.”
Encapsulated in a light-green diamond and invisible to the human eye, the team discovered the first sample of a mineral later named davemaoite. Formed roughly 400 miles under the Earth’s crust, Tschauner says this mineral is one in about 50-100 new minerals classified each year by the International Mineralogical Association. Named after experimental high-pressure geologist Ho-kwang “Dave” Mao, this sample is only the second-ever high-pressure mineral to be uncovered.
These minerals typically lie deep in the Earth’s lower mantle and are held together by pressures 200,000 times greater than the kind of atmospheric pressure we typically experience. If scientists extracted these samples by themselves, they would liquify instantly, Tschauner explains, but in this case, the same sample was held together by the outside pressure of a diamond.
What’s new — Under the surface, davemaoite is known to its friends as calcium silicate perovskite. Geologists had theorized about the existence of this mineral in nature for decades, but until now, they had only ever observed it in its synthetic, laboratory-created form.
The discovery of the davemaoite sample gives scientists an opportunity to finally study how this mineral might form in nature, Tschauner says, and how it interacts with other deep earth elements.
Why it matters — While the serendipitous discovery of such a high-pressure mineral is significant on its own, Tschauner says that this sample will also help the team better understand the movement of elements and minerals through the Earth’s mantle as well. One significant finding is the role of davemaoite in “concentrating” or chemically reacting with rare Earth metals and radioactive isotopes.
Without these reactions, such minerals and isotopes may never make it to the surface intact for our extraction and use.
“When we talk about concentration, we mean it that relative to the surrounding other minerals,” explains Tschauner, meaning that “concentrating” is more an act of grouping or attracting than a reducing.
“We know from experiments that all these elements [e.g., uranium and thorium] like to go into the structure of davemaoite,” Tschauner says.
How they did it — One tricky aspect of handling high-pressure minerals, like the diamond-encased davemaoite sample, is that excavating the sample to handle hands-on would reduce it to a liquid mess. Instead, the team turned to less invasive techniques to analyze the structure, including:
- Synchrotron X-ray diffraction to determine the atomic structure
- X-ray fluorescence to analyze elements in the mineral
- Laser-ablation to excavate the (then liquid) sample for mass spectrometry analysis
Through these different analyses, the researchers were able to learn more about the natural composition of davemaoite and the lower mantle, including that it may not be as homogenous as scientists once believed.
Another significant finding of this research, Tschauner says, is the realization that carbon (aka the diamond housing) could be found so deep in the mantle as well. This finding will help future scientists better study the carbon cycle of the Earth’s mantle.
What’s next — The successful excavation and analysis of davemaoite is still only just the beginning of what scientists like Tschauner hope to learn about deep earth minerals.
One item on his wishlist is to find larger intact samples of davemaoite to study its trace mineral content. Tschauner says that this — like much of high-pressure geology — is easier said than done.
“Now the issue is if it’s too large, then the diamond would not be able to sustain the pressure, and it will crack,” Tschauner laughs.
Abstract: Calcium silicate perovskite, CaSiO3, is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO3-perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable.