How does an impossible material with forbidden symmetries form in the scattered grains of a meteorite? There’s no simple answer.
The phrase "quasicrystal from outer space" sounds like the title to a science fiction thriller with a New Age twist. But a study published Thursday in Scientific Reports announced that a bonafide quasicrystal has indeed been discovered within the grainy remains of a meteorite found in the Khatyrka region of the Russian far east, five years ago. The paper marks the first time that a type of quasicrystal has been identified in nature before it was created artificially in a lab.
You might be asking: That's neat, but what the heck is a quasicrystal? An understandable question. First identified by material scientist Dan Shechtman in 1982—who won the 2011 Nobel Prize in Chemistry for his discovery—quasicrystals are crystal-like arrangements of atoms that defy expectations of crystalline behavior with their "forbidden" rotational symmetries.
Regular crystal structures are periodically arranged in neatly fitted shapes like cubes (four-fold symmetry) or triangles (three-fold symmetry), but quasicrystals are arranged in non-periodic patterns such as pentagonal, five-fold symmetries.
Patricia Thiel, a chemistry and materials science expert based at Iowa State University, used this helpful analogy of tiling a floor to explain the unique properties of quasicrystals in this NPR article: "If you want to cover your bathroom floor, your tiles can be rectangles or triangles or squares or hexagons," she said. "Any other simple shape won't work, because it will leave a gap. In a quasicrystal, imagine atoms are at the points of the objects you're using. What Danny [Shechtman] discovered is that pentagonal symmetry works."
READ MORE: Quasicrystals Are Nature's Impossible Matter
It should be impossible, but material science is full of surprises. While quasicrystals are most often studied via artificial synthesis in labs, the Khatyrka meteorite demonstrates that they can form naturally. Two previous quasicrystal forms have already been identified in the fragments of this outer space visitor, but both of those arrangements had been synthesized prior to their discovery in nature. However, the latest version features icosahedral symmetry, an exotic pattern featuring 60 points of rotational symmetry, somewhat like that of a soccer ball.
Paul Steinhardt, who serves as the Albert Einstein professor in science at Princeton University, was one of the authors of the new research. When I asked him why this one meteorite, which only survives as a few stray grains, seems to have such an abundance of quasicrystals, he told me there's no simple answer.
"There are perhaps one or two other groups [of researchers] at most searching," Steinhardt said. "Since we found an example, we have been focusing on understanding how this particular one formed since that will tell us something about the likelihood of finding others. But it is very very early times for these kinds of studies."
"What is encouraging is that we have already found three different types of quasicrystals in the same meteorite, and this new one has a chemical composition that has never been seen for a quasicrystal," he added. "That suggests there is more to be found, perhaps more quasicrystals that we did not know were possible before."
Scientists have come a long way towards a better understanding of quasicrystals since their discovery some three decades ago, and they have found many practical applications for them in everything from frying pans to LED lights. But as Steinhardt emphasized, much of the field is still in its infancy, and there is much left to discover. Stay tuned.
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