Crystal with "Forbidden Symmetry" Found in 4.5-Billion-Year-Old Meteorite

Crystal with "Forbidden Symmetry" Found in 4.5-Billion-Year-Old Meteorite

March 18, 2015 | by Janet Fang
photo credit: High-resolution transmission electron microscopy (HRTEM) image showing that the real space structure consists of a homogeneous, quasiperiodic and ten-fold symmetric pattern / L. Bindi et al., 2015 Scientific Reports
A crystal with an “unorthodox” arrangement of atoms has been discovered inside an ancient meteorite that crashed into a remote area of northeastern Russia thousands of years ago. This is only the second time a natural so-called quasicrystal has been found. The work is published in Scientific Reports this week.
To understand the difference between crystals and quasicrystals, imagine a tiled floor. Hexagon-shaped tiles (with 6 sides) fit neatly next to each other to cover the entire floor. But if you lay down pentagons (5 sides) or decagons (10 sides) next to each other, you’ll end up with gaps between the tiles. In ordinary crystals, the atoms are packed closely together in a repeated and orderly fashion. But with quasicrystals, “the structure is saying ‘I am not a crystal, but on the other hand, I am not random either,'” Princeton’s Paul Steinhardt says in a news release
Researchers used to think that these structures were too fragile and energetically unstable to be formed through natural processes. That is, until Steinhardt and colleagues stumbled on a crystal with these “forbidden symmetries” in 2009 in a rock collected years earlier in Chukotka, Russia. Called icosahedrite, that quasicrystal had the 5-fold symmetry of a soccer ball, and it originated in an extraterrestrial body formed around 4.57 billion years ago.
Based on experiments with X-rays, the newly discovered quasicrystal has a structure that resembles flat 10-sided disks stacked in a column (pictured to the right). This 10-fold symmetry is an impossible structure in ordinary crystals. "When we say decagonal, we mean that you can rotate the sample by one-10th the way around a circle around a certain direction and the atomic arrangement looks the same as before,” Steinhardt explains to Live Science
Still unnamed, this second quasicrystal was found in the same meteorite but in different grain (circled in red above). It’s made up of aluminum, nickel, and iron—three things that aren’t usually found together since aluminum binds so quickly to oxygen, blocking attachment to the other two. Finding a second naturally occurring quasicrystal confirms that these can, in fact, form in nature and remain stable over cosmic time scales.
A micro CT-scan of the whole Grain 126 is pictured to the right.
Quasicrystals are very hard, have low friction, and don’t conduct heat very well—making them perfect as protective coatings on airplanes and non-stick frying pans alike. The team is now trying to figure out how the mineral formed. “We know there was a meteor impact, and that the temperature was around 1000 to 1200 degrees Kelvin [726 to 926 degrees Celsius], and that the pressure was a hundred thousand times greater than atmospheric pressure, but that is not enough to tell us all the details,” Steinhardt says. “We’d like to know whether the formation of quasicrystals is rare or is fairly frequent, how it occurs, and whether it could happen in other solar systems. What we find out could answer basic questions about the materials found in our universe.”
Images: L. Bindi et al., 2015 Scientific Reports (top, bottom), Princeton (middle)

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