2021 11 15
Scientists have recovered a new mineral, long hypothesized but never seen, from some 900 kilometers below the Earth’s surface.
It’s called Davemaoite: made of calcium, silicon and oxygen (calcium silicate, or CaSiO3), it takes a crystal lattice-shape (known as a perovskite) and is the first compound of its kind to be identified in the lower mantle, the deepest layer of the earth’s geology outside of its core.
It was spotted by a team of researchers who came together from the University of Nevada Las Vegas, Florida State University, California Institute of Technology, Argonne National Laboratory, and Lawrence Berkeley National Laboratory, and detailed their findings in a peer-reviewed report in the journal Science on Thursday.
They located the compound in a mined diamond that originated in the Earth’s lower mantle using a powerful x-ray with precision down to the micrometer that can send signals through rocks, said Oliver Tschauner, first author on the study and research professor in mineralogy and crystallography at the University of Nevada Las Vegas. Soon after spotting it, Tschauner and his team reported the finding to the International Mineralogical Association, which has officially recognized it as a new mineral.
The findings offer new evidence of the existence of a mineral that’s been speculated about for years, but never sampled.
“This process took more than a year,” Tschauner told Motherboard in an email. “The deep Earth is not directly accessible. We have seismic data, geochemical data from rocks that may have originated there, we can perform experiments at the pressures and temperatures of the deep Earth but we had no actual samples of real minerals from that deep. The inclusions in diamond provide this missing information.”
The compound was found trapped within a diamond located in Orapa, Botswana—which was excavated and deposited in the Natural History Museum of Los Angeles. Its retrieval alone is worthy of scientific admiration, per an independent commentary on the paper’s findings, also published in Science on Thursday. Calcium Silicate perovskite is “unquenchable,” meaning it can only maintain its structural characteristics under the high pressure conditions of the lower mantle. The diamond it was found within kept it from changing structure in its travel hundreds of kilometers up to the earth’s outer layers, but it isn’t likely to be retrieved in other conditions in the future.
Tschauner notes that the lower mantle maintains a pressure that is 24,000 times that of the earth’s atmosphere, and is thus home to elements that could quite literally never exist in ambient conditions, outside the crust. Davemaoite, for instance, would turn into glass instantly in the atmosphere, he said.
“We reconstructed the pressure-temperature path that the diamond and its inclusions took and we concluded that the diamond grew somewhere between 660 and 900 kilometers deep,” Tschauner wrote.
The diamond that the researchers pulled out of the earth brought a number of trace minerals up with it—this type of structure is referred to as a ‘garbage can,’ by crystal chemists, because it’s capable of holding a slew of other minerals within it.
Tschauner believes Davemaoite constitutes five to seven per cent of the earth’s lower mantle—“not much,” he says, but he notes that it plays an important role in the overall heat levels of the earth’s interior because it decays radioactively (radioactive decay is responsible for one-third of the inner earth’s heat, Tschauner said).
After identifying the mineral, Tschauner swiftly named it after his once-postdoctoral supervisor, Dave (Ho-kwang) Mao, a prominent geophysicist. He told Motherboard he hopes to continue studying the complex processes occuring far below our feet.
“An element like calcium behaves differently at 200,000 times the atmosphere pressure,” he said. “How does this different chemistry control processes inside the Earth?”