New mineral discovered in diamond found in deep Earth

12 Nov 2021

Davemaoite enclosed in a diamond. Image Aaron Celestian/Los Angeles County Natural History Museum

The calcium silicate perovskite found in the lower mantle had been theorised but never seen before.

Scientists have found a never-before-seen calcium silicate mineral deep within the Earth, which could potentially fill knowledge gaps in our understanding of the composition of the planet.

The discovery was made after scientists studied black spots found in a diamond from the planet’s lower mantle, which formed at high pressure and temperature more than 660km below the Earth’s surface.

Future Human

The new mineral, davemaoite, has been named after Chinese-American geophysicist Ho-Kwang ‘Dave’ Mao in a study published in the journal Science today (12 November).

Oliver Tschauner, a researcher at the University of Nevada Las Vegas who co-led the study that led to the discovery, said that calcium silicate is one of the most important minerals in the lower mantle because it concentrates elements that are incompatible in the upper mantle, including rare-earth elements and radioactive isotopes that contribute to the heat of the Earth’s mantle.

Although theorised for decades, a silicate of this kind, formed under high pressure, has never been successfully retrieved from the Earth’s lower mantle before.

Fills ‘knowledge gap’

Yingwei Fei, a geochemist at the Carnegie Institution for Science, said that Tschauner and his team “inspire hope” for the discovery of other difficult high-pressure phases in nature, “either through careful search in deep-origin diamonds or in highly shocked meteorites”.

“Such direct sampling of the inaccessible lower mantle would fill our knowledge gap in chemical composition and heterogeneity of the entire mantle of our planet,” he said.

Davemaoite and other similar silicates cannot retain their original mineralogical structure after being removed from their natural high-pressure and high-temperature environments. Researchers were able to observe the davemaoite as it was trapped within a diamond.

According to Nature, the greenish, octahedral diamond was found in Botswana’s Orapa mine, the world’s largest open-pit diamond mine, decades ago. A dealer then sold it to George Rossman, a mineralogist at the California Institute of Technology in Pasadena.

Tschauner and Rossman, along with their colleagues, began to study the diamond many years ago. The team used x-ray diffraction to identify and characterise the davemaoite in the diamond, showing its ability to host potassium, thorium and uranium – three major heat-producing elements.

“When we broke open the diamond, the davemaoite stayed intact for about a second, then we saw it expand and bulge under the microscope and basically turn into glass,” Tschauner told New Scientist.

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Vish Gain is a journalist with Silicon Republic

editorial@siliconrepublic.com