What's in a name? Earth's most abundant mineral finally gets one

The minute particles seen in this microscope image actually make up 38 percent of the volume of our planet (Image: Caltech/Argonne National Laboratory)

When a mineral is the most abundant on the planet, (making up an estimated 38 percent of the Earth's entire volume, in fact), you would think that someone would have given it a name by now. But things are never as simple as they seem. Despite being so prevalent, the substance in question has only ever existed in synthetic form until recently, and the first naturally-occurring example of it didn't even come from beneath the ground; it arrived from outer space.

The inner layers of the Earth are strange and forbidding places where extreme pressures and temperatures do weird things to minerals, often rendering them in mixtures and properties only possible in the crucible of the planet, at great distances below the surface. For many years now, scientists have believed that a type of perovskite-structure material (a substance with a unique crystalline makeup) – in this case, a high-density form of magnesium iron silicate – has existed at these depths and has been a major, but unseen, influence on the flow of mineral elements and heat within the Earth's mantle.

However, the structural characteristics of the unseen substance have never truly been determined before because natural versions of it only remain stable at depths beneath the Earth's surface of some 660 kilometers (410 miles) and at pressures of above 230 kbar (23 GPa). Some scientists even believe that a number of inclusions found on diamonds are the telltale remnants of an encounter with the elusive subterranean substance on their long journey to the surface. Even so, when it is occasionally brought up from the deep Earth caught up in a conglomerate of other minerals, it quickly decomposes into far less dense minerals as heat and pressure decreases.

Due to this instability above ground, no intact example of the mineral has ever managed to make its way to the surface. As a result, and despite many years of research and examination pointing strongly to its existence, without a physical sample, naming or registering by the International Mineralogical Association was not possible.

Determined to prove the existence of the mineral, but unable to obtain a specimen from deep within the Earth's crust, a team of scientists led by Oliver Tschauner, a mineralogist at the University of Las Vegas, turned its attention to meteorites.

Because the shock-compression encountered in collisions of asteroids in our solar system replicates the same extreme conditions found deep within our planet (approximately 2,100 °C/3,800 °F, and pressures more than 240,000 times greater than the air pressure at sea-level on Earth), the material forms rapidly within pieces of space debris subject to such forces.

And due to the fact that the shock occurs at the enormous speeds involved in interplanetary encounters, it "freezes" the reaction deep in a mineral vein within an encapsulating lump of rock, protecting it from breakdown when (and if) it is ever exposed to lower pressures. Thus preserved, the encapsulated material, along with other asteroid debris, often eventually falls to Earth as meteorites.

Unfortunately, even when presented with such a conveniently cocooned sample, past attempts by other researchers to uncover the perovskite material from other meteorites using less-than-delicate techniques, such as transmission electron microscopy, has resulted in radiation damage to the samples and incomplete or inconclusive results.

In an attempt to avoid such sample degradation, the researchers at Las Vegas University bet on a different method of examination and hit the jackpot with the use of non-destructive micro-focused X-rays for diffraction analysis and fast-readout area-detector techniques, using the X-ray beamline at the US Department of Energy's Argonne National Laboratory in Chicago.

Specifically, the University of Las Vegas team focused its attention on an ancient meteorite that had plummeted to the ground in outback Australia back in 1879. A highly-shocked L-chondrite Tenham meteorite, it is a nugget of space rock deemed to be a prime candidate to contain microscopic, naturally-occurring, particles of the perovskite material.

To conduct their investigations,Tschauner and his colleagues from the GeoSoilEnviroCARS (GSECARS) and Caltech used the University of Chicago-operated, beamline instrument to illuminate the interior of the meteorite. The incredibly intense beams of the GSECARS beamline leave little radiation behind to cause damage and is a machine at the forefront of delicate high-pressure research.

Even with such high-capability equipment, finding intact samples of the elusive material in the space debris was difficult; not only because they are rare in the Tenham meteorite, but because they are also less than 1 micrometer in diameter. The team persevered, however, and with the invaluable help of the dazzlingly bright and strongly-focused beam of the GSECARS apparatus, conducted diffraction mapping analysis until an aggregate of the material was finally identified and its structural and compositional analysis confirmed.

At last, with proof of its material existence firmly in hand, the substance has been named: Bridgmanite. So dubbed to pay homage to Percy Bridgman, 1946 Nobel laureate and pioneer of high-pressure research, the newly-recognized substance is surprisingly different than first surmised. Analysis of the first naturally-occurring specimen of Bridgmanite shows that it contains much higher amounts of ferric iron than even guessed at in synthetic versions of the material, along with a lot more sodium than the researchers expected.

Already the crystal structure of natural Bridgmanite is providing invaluable insights into deep mantle rocks and research into its makeup will help scientists in their understanding of the unseen forces and materials that occur very deeply below the surface of our planet.

And, as a result of this research, we can also now put a name to the huge amount of hidden mineral material that lays deep beneath our feet. A surprisingly long time in coming, it is nice to know that we are still discovering much about the inner realm of our planet and constantly adding to our knowledge of the unseen mechanisms at work under the surface.

The results of the research team's findings were published in the journal Science.

The research was also funded by the US Department of Energy, NASA, and the National Science Foundation.

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