6 June 2014
By JoAnna Wendel
The mineral said to be the most abundant of our planet, but found so deep within Earth’s interior that scientists usually cannot observe it directly, now has a name.
On June 2, bridgmanite was approved as the formal name for one of the Earth’s most plentiful yet elusive minerals known to exist in the Earth’s lower mantle. Bridgmanite, which was formerly known simply as silicate-perovskite, is named after the 1946 Nobel Prize winning physicist Percy Bridgman.
Scientists have known for decades that bridgmanite existed in the Earth’s interior, but had been unable to successfully characterize a naturally occurring sample until this year.
“This [find] fills a vexing gap in the taxonomy of minerals,” Oliver Tschauner, an associate research professor at the University of Nevada-Las Vegas who characterized the mineral, said in an email.
Tschauner, along with Chi Ma, a senior scientist and mineralogist at the California Institute of Technology in Pasadena, Calif., have been working to chemically and structurally characterize natural silicate-perovskite (MgSiO3) since 2009.
Scientists think the mineral, suspected to be the most plentiful of the planet, exists in an interior region that extends from the bottom of the transition zone of the Earth’s mantle down to the planet’s core-mantle boundary, at depths between 670 and 2,900 kilometers (416 and1,802 miles). The transition zone, which scientists discovered from studying the way earthquake waves travel through the Earth, lies between the upper and lower mantle and marks the point where mineral structures undergo change at an atomic level due to increasing temperature and pressure.
The lower mantle is a place scientists can only dream of observing directly. Instead, Tschauner and Ma found submicrometer-sized crystals of the then yet-to-be-named bridgmanite in the Tenham meteorite, a space rock that fell in Queensland, Australia in 1879.
The meteorite formed 4.5 billion years ago, and has been “highly shocked,” meaning it survived high-energy impacts in space. These impacts submitted the meteor to intense pressure and temperature—much like what rocks in the Earth’s mantle experience, making it a likely source of bridgmanite.
“Shocked meteorites are the only accessible source of natural specimens of minerals that we know to be rock-forming in the transition zone of the Earth,” said Tschauner.
To begin the hunt for bridgmanite, Ma analyzed shock-induced melt veins in the Tenham meteorite. The melt veins are structures that formed as a result of the meteor’s impacts in space and often contain high pressure minerals.
Ma was able to identify a bridgmanite-like mineral under a high-resolution analytical scanning electron microscope. However, when he tried to analyze its crystal structure using electron diffraction, the structure disintegrated.
“This material is very sensitive to electron beams,” Ma said, and noted that other scientists who had tried to characterize the mineral in the past had faced the same setback.
Ma then sent the samples off to Tschauner who used a different method—synchrotron X-ray diffraction—to characterize the mineral. After five years and multiple experiments using synchrotrons in Chicago and Berkeley, California, Tschauner and Ma were finally able to gather enough data to submit the mineral to the International Mineralogical Association (IMA) for official review.
In March 2014, Tschauner and Ma sent the proposal, which included information such as bridgmanite’s chemical composition, crystal structure, and other physical properties, to the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC), where it underwent a rigorous review.
The CNMNC has strict guidelines for naming new minerals, one of which is the requirement that the crystal structure be defined. Because Tschauner and Ma had obtained the mineral’s crystal structure, it could now be approved as a new mineral.
On June 2, the CNMNC accepted the mineral and its name. They chose Bridgman as the mineral’s namesake because of his contributions to the study of how materials react when they are submitted to extremely high pressures.
“We are glad no one used [Bridgman] for other minerals,” said Ma, “this one is so important.”
— JoAnna Wendel is a staff writer for the AGU’s weekly newspaper, Eos. She primarily writes Research Spotlights, short summaries highlighting exciting research from AGU’s journals