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THE EARTH’S MOST ABUNDANT MINERAL

THE MINERAL bridgmanite got its name only recently. Despite this, specialists in Earth science have long known that the stuff is Earth’s most abundant mineral.

I gleaned this, and other insights into the processes of science, from “Bridgmanite—Named At Last,” by Thomas Sharp, in the November 28, 2014, issue of Science magazine published by the American Association for the Advancement of Science.

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Scientists learn about the Earth’s interior through studies of seismic activity. Seismic vibrations exhibit velocity changes at particular depths. These discontinuities show that Earth is made of an upper and lower mantle, intermediate layers of transitional materials, an outer core and an inner core.

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Mineralogical model of Earth. Its four principal layers and transitional zones. Image from Science, November 28, 2014.

Because access is impossible, knowledge of these materials is gained indirectly. Thermodynamic principles of temperatures and pressures predict the existence of materials in each layer. High-pressure experimentation and simulation studies add to the knowledge. For instance, it’s known that the lower mantle makes up the largest single volume, 38 percent of our planet, and consists of (Mg, Fe)SiO3, magnesium iron silicate.

Natural samples don’t come readily; the upper mantle continues to a depth of 410 km (255 miles). Only rarely do materials end up at the surface, as diamonds, for instance.

Last year, though, a Brazilian diamond was found that contained an inclusion of ringwoodite, named for Australian geologist Alfred Ringwood. He had hypothesized the existence of such a mineral from analyses of seismic discontinuities. Later, other specialists grew artificial crystals of the material.

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Photomicrograph of a ringwood crystal, about 150 micrometers (0.006 in.) across. Image by (and crystal grown by) Jasperbox..

Studying the natural inclusion of ringwoodite, scientists confirmed that substantial amounts of water exist in the Earth’s Transition Zone. In fact, it’s estimated that an equivalent of from one to three times of the world oceans resides in this zone.

Extraterrestrial evidence is also beneficial to Earth science, derived from studying meteorites. (See “Catch a Falling Star,” http://wp.me/p2ETap-2xe, for details on these.) The meteoroid’s hypervelocity entry into the atmosphere simulates temperature and pressure conditions of deep Earth. Meteorite materials similar to those of the Earth’s upper mantle are “shocked” into lower-mantle-like minerals.

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Deep-mantle minerals in a meteorite. The deep blue portions are ringwoodite. Image from Science, November 28, 2014.

In particular, researchers at University of Nevada, Cal Tech and University of Chicago published “Discovery of Bridgmanite, the Most Abundant Mineral in Earth, in a Shocked Meteorite,” in Science, November 28, 2014. Their Abstract notes, “The discovery concludes a half century of efforts to find, identify, and characterize a natural specimen of this important mineral.”

This phase of magnesium silicate, MgSiO3, was officially assigned the name bridgmanite by the International Mineralogicial Association.

Researchers chose this name in honor of Harvard physicist Percy W. Bridgman, regarded as the father of high-pressure experimentation.

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Percy Williams Bridgman, 1882 – 1961, American physicist, Harvard University, Nobel Laureate, 1946 Physics.

It’s fascinating that science derives knowledge of the Earth’s inner structure from extraterrestrial sources. ds

© Dennis Simanaitis, SimanaitisSays.com, 2015

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