An ancient meteorite that crashed on Earth thousands of years ago, in a region located in Russia, has revealed a precious surprise: a group of scientists have discovered a crystal with a puzzling arrangement of atoms in its midst. The crystal, called a quasicrystal, is the second of its kind to have been found. The findings of the study entitled “Natural quasicrystal with decagonal symmetry” have been published in Scientific Reports.
“High-resolution transmission electron microscopy (HRTEM) image showing that the real space structure consists of a homogeneous, quasiperiodic and ten-fold symmetric pattern.” Photo credits: L. Bindi et al., 2015 Scientific Reports
Quasicrystals are unlike normal crystals that consist of atoms closely packed together in a repeated pattern; rather, they have a weird symmetry of atoms.
It was initially believed that the structure of quasicrystals cannot be formed naturally because they would be too energetically unstable. However, two new discoveries have shown otherwise. It seems that they can indeed form in nature and maintain stability over cosmic time scales.
Formation of the second quasicrystal
“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,” explained Paul Steinhardt, one of the authors.
2 forbidden-symmetry crystals discovered
The first crystal with the “forbidden symmetry”, called icosahedrite, was found in a rock taken from Chukotka, Russia. The icosahedrite displayed the 5-fold symmetry of a soccer ball. It is said to come from a heavenly body that formed about 4.57 billion years ago.
The second quasicrystal (with no name yet) also originated from the same meteorite. Its X-ray analysis showed a structure resembling like flat 10-sided disks stacked in a column. The 10-fold symmetry cannot exist in ordinary crystals.
Image showing the two types of quasicrystal symmetry
“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,” Paul Steinhardt explained.
Rare composition of the second quasicrystal
Even its composition is unusual. It consists of the metals aluminium, nickel and iron – these three are generally not found together because aluminium has the tendency to bind to oxygen so fast that the other two would not have the time to stick to it.
“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,” said Steinhardt.