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A tiny bit of rock trapped inside a diamond is now opening a brand-new window into what the planet’s lower mantle looks like. Inside the diamond is a newly identified silicate mineral dubbed davemaoite that can only have formed in Earth’s lower mantle, researchers report November 12 in Science. It’s the first time that scientists have managed to definitively prove that this type of lower mantle mineral — previously just predicted from laboratory experiments — actually exists in nature. The team named the mineral for well-known experimental high-pressure geophysicist Dave Mao.
The diamond bearing the telltale mineral inclusion came from a Botswana mine and formed at depths greater than 660 kilometers, the upper boundary of Earth’s lower mantle. Using analytical techniques including X-ray diffraction, X-ray fluorescence imaging and infrared spectroscopy, mineralogist Oliver Tschauner of the University of Nevada, Las Vegas, and colleagues identified the chemical makeup and structure of the new mineral, pegging it as a type of calcium silicate perovskite.
Scientists had previously estimated that about 5 percent to 7 percent of the lower mantle must be made up of this mineral, Tschauner says. But it’s fiendishly difficult to directly observe such deep-Earth minerals. That’s because minerals that are stable in the intense pressures of the lower mantle — which extends all the way to 2,700 kilometers below Earth’s surface — begin to rearrange their crystal structures as soon as the pressure lets up.
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