Scientists believe that davemaoite makes up about five percent of the Earth's lower mantle and may contain radioactive elements which have a half-life longer than the planet's geological history.
American geologists have discovered a new mineral called davemaoite inside a diamond. In an article published in the journal Science, the authors note that this mineral could make up approximately five percent of the Earth’s lower mantle. The existence of this mineral has been theorized since the 70s but it has never been found before in nature.
Davemaoite: a mineral believed to be impossible to find
Earth’s lower mantle
Currently, the exact composition of the lower mantle of the Earth, the depth of which is approximately from 660 to 2600 kilometers, is not fully known. Most researchers assume that the deep mantle is mainly composed of magnesium silicate.
It accounts for about 70 percent of its volume, another 20 percent falls on magnesiowüstite [(Mg,Fe)O], and the remaining 10 percent are dense tetragonal forms of silicon dioxide and oxide phases containing calcium, sodium, potassium, aluminum, and iron.
The study of diamonds
To clarify the composition of the lower mantle, scientists are studying the minerals that formed there. Typically, these minerals are enclosed in deep-earth diamonds, which retain their original composition as they rise to the surface.
Most diamonds form at a distance of 120 to 250 kilometers from the earth’s surface, but deep-earth diamonds appear in the lower mantle at a depth of 200 to about 1000 kilometers. Scientists have already found minerals from deep layers in them, but there are no easy ways to determine the exact depth of formation of each such diamond.
There is no way to know where the next sample will be found. Therefore, the process of finding them is rather complicated and long.
Scientists found a new mineral inside a diamond
Oliver Tschauner, a professor at the University of Nevada at Las Vegas, along with his colleagues decided to study the diamond, which was kept at the Los Angeles Museum of Natural History. This diamond, about 4 millimeters wide and weighing 82 milligrams, found in the Orapa mine in southern Africa, was acquired by an American scientist at the end of the 20th century, who transferred it to the mineral collection of the California Institute of Technology. The diamond later ended up in the Los Angeles Museum.
Using X-ray analysis, scientists have determined that there are inclusions of another mineral inside. The geologists then removed the crystals using a laser beam and analyzed the chemical composition of the inclusions using mass spectrometry.
Davemaoite
It turned out that they had extracted calcium silicate with a perovskite crystalline structure, which had not previously been found in nature, but, as was assumed, can form in the lower mantle. Scientists named the mineral davemaoite, after Dave Mao of the Carnegie Institute of Science for his outstanding contributions to deep-sea geophysics.
Why was it considered impossible to find?
It is practically impossible to find such a modification of calcium silicate in nature because it is stable at a pressure that is 198 thousand times higher than the pressure on the Earth’s surface. The perovskite would simply collapse when raised to the surface. However, this sample of davemaoite was enclosed in a hard diamond that was able to keep it in its original state.
On the surface, calcium silicate usually occurs in the form of wollastonite CaSiO3, a white mineral that differs from the davemaoite chemical formula and structure. Perovskite crystallizes in the orthorhombic system, while wollastonite crystallizes in the triclinic system.
Davemaoite in Earth’s lower mantle
Scientists believe that davemaoite makes up about five percent of the Earth’s lower mantle and may contain radioactive elements such as uranium, thorium, and the isotope potassium-40, which have a half-life longer than the planet’s geological history. When decaying, they release heat, so the distribution affects the thermal balance of the deep mantle, where the mineral is thermodynamically stable.
Importance for science
Geologists note that such natural samples of high-pressure minerals are very important for geology and geophysics, as they help to understand the complex process of formation and transformation of deep layers of the Earth. For example, this can help in calculating the risks of earthquakes or tsunamis, since the lower mantle contributes to the movement of lithospheric plates.
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Sources:
• Baker, H. (2021, November 11). Diamond hauled from deep inside Earth holds never-before-seen mineral. LiveScience.
• Carnegie Institution for Science. (2021, November 15). Introducing Davemaoite: A groundbreaking mineral discovery named after trailblazing Carnegie geophysicist.
• Klein, A. (2021, November 17). New mineral davemaoite discovered inside a diamond from Earth’s mantle. New Scientist.
• Pappas, S. (2021, November 11). New mineral discovered in deep-earth diamond. Scientific American.
• Tschauner, O., Huang, S., & Yang, S. (2021, November 11). Discovery of davemaoite, CASIO3-perovskite, as a mineral from the lower mantle. Science.
• Witze, A. (2021, November 11). Diamond delivers long-sought mineral from the Deep Earth. Nature News.
