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SYFY WIRE Bad Astronomy

Scientists Discover 38 Percent of the Earth

By Phil Plait

OK, so the title is a little tongue-in-cheek, but it's sorta true: Mineralogists have finally found naturally occurring samples of what may be the most common mineral on Earth: what’s called silicate perovskite, or (Mg,Fe)SiO­3.

They’ve also officially given it a name now too: bridgmanite. Percy Williams Bridgman won the Nobel in 1946 for studying high-pressure minerals … and that’s a clue to why this mineral was so hard to identify.*

Bridgmanite can only exist under conditions of high temperatures (at least 2,100 C) and pressure (240,000 times the sea level atmospheric pressure—a crushing 240 metric tons per square centimeter!). It’s thought to be abundant in the Earth’s lower mantle—a region 660 to 2,900 kilometers beneath Earth’s surface. The molten rock in the mantle is fluid, moving incredibly slowly inside our planet. Any bridgmanite in the mantle brought up toward the surface slowly breaks down under the cooler and lower pressure conditions, which is why it’s remained elusive, even though the mineral may make up as much as 90 percent of that part of the mantle (and therefore more than a third of the entire planet).

The scientific break came in the form of a meteorite, called Tenham. Long ago, two asteroids collided, and the impact created high temperatures and pressures. Bridgmanite formed, and the piece cooled too rapidly for the mineral to decompose. In 1879 the rock fell to Earth in Australia, where it was found and eventually determined to have different kinds of high-pressure minerals in it. Bridgmanite exists in it in very small grains, typically only about 1 micron wide (a human hair is typically 100 microns in width), but it’s there. It was announced earlier this year, but the scientists just published their paper about it in November.

This is quite a boon! It’s difficult to reproduce the conditions in the deep Earth, and even if you can it’s even harder to study what you get. In this case, it’s like we got a sample of the Earth’s lower mantle for free. It’s also a nifty crossover between different disciplines: meteoritics, high-pressure physics, mineralogy, just to name some.

And also, it’s just amazing. We live on a ball of rock and metal 12,740 km across, with a staggering 1 trillion cubic kilometers of material in it, the vast vast majority of which we can never directly see. I wasn’t even aware that we didn’t actually know for sure what made up over a third of our own planet.

Science! Astronomy may be my passion and my love, but sometimes it’s good to remember that science also tells you, literally, what’s going on right underneath your feet.

Correction, Dec. 18, 2014: This post originally misstated when Bridgman won the Nobel. It was in 1946, not 1964.

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