Scientists looking for quasi-crystals – so-called “impossible” materials with unusual, non-repeating structures – have identified one among the remains of the world’s first nuclear bomb test.
The previously unknown structure, made of iron, silicon, copper and calcium, likely formed from the fusion of cables of vaporized desert sand and copper. Similar materials have been synthesized in the laboratory and identified in meteorites, but this one, described in Proceedings of the National Academy of Sciences May 17, is the first example of a quasi-crystal with this combination of elements1.
Quasi-crystals contain building blocks of atoms which, unlike those of ordinary crystals, do not repeat in a regular pattern similar to that of masonry. While ordinary crystal structures look identical after being translated in certain directions, quasicrystals have symmetries that were once thought to be impossible: for example, some have pentagonal symmetry, and therefore look the same when rotated. one-fifth of a full twist.
Materials scientist Daniel Shechtman, now at the Technion Israel Institute of Technology in Haifa, first discovered such impossible symmetry in a synthetic alloy in 1982. It had pentagonal symmetry when rotated in each of the different possible directions, which would happen if its basic elements were icosahedral – that is, had a regular shape with 20 faces. Many researchers first questioned Shechtman’s findings, as it is mathematically impossible to fill space using only icosahedra. Shechtman ultimately won the 2011 Nobel Prize in Chemistry for this discovery.
Around the same time, Paul Steinhardt, a theoretical physicist now at Princeton University in New Jersey, and his collaborators had begun to theorize the possible existence of non-repeating 3D structures. These had the same symmetry as an icosahedron, but were assembled from building blocks of several different types, which never repeated in the same pattern – thus explaining why the mathematics of symmetrical crystals had missed them. Mathematical physicist Roger Penrose, now at the University of Oxford, UK, and other researchers had previously discovered similar two-dimensional models, called Penrose tilings.
Steinhardt remembers the time in 1982 when he first saw the experimental data of Shechtman’s discovery and compared it to his theoretical predictions. “I got up from my desk and went to look at our model, and you couldn’t tell the difference,” he says. “So it was an incredible moment.”
Over the following years, materials scientists synthesized several types of quasicrystals, expanding the range of possible forbidden symmetries. And Steinhardt and his colleagues later discovered the first natural “icosahedrite” in fragments of a meteorite recovered from the Kamchatka Peninsula in eastern Siberia. This quasi-crystal likely formed during a collision between two asteroids at the start of the solar system, says Steinhardt. Some of the lab-made quasicrystals were also produced by shattering materials together at high speed, so Steinhardt and his team wondered if shock waves from nuclear explosions could also form quasicrystals.
‘Slice and dice’
Following the Trinity test – the very first detonation of a nuclear bomb, which took place on July 16, 1945 at Alamogordo Bombing Range in New Mexico – researchers discovered a large field of greenish glassy material that had formed from the liquefaction of desert sand. They nicknamed this trinitite.
The plutonium bomb had exploded at the top of a tower 30 meters high, loaded with sensors and their cables. As a result, some of the trinitite that formed had reddish inclusions, says Steinhardt. “It was a fusion of natural material with the copper of the transmission lines.” Quasi-crystals often form from elements that would not normally combine, so Steinhardt and his colleagues thought that samples of red trinitite would be a good place to look for quasicrystals.
“For ten months, we sliced and diced, examining all kinds of minerals,” says Steinhardt. “Finally, we found a little grain.” The quasi-crystal has the same type of icosahedral symmetry as that of Shechtman’s original discovery.
“The dominance of silicon in its structure is quite distinct,” says Valeria Molinero, theoretical chemist at the University of Utah in Salt Lake City. “However, after a lot of quasicrystals have been synthesized in the lab,” she says, “what I find really fascinating is that they are so rare in nature.” Steinhardt says this could be because the formation of quasicrystals involves “unusual combinations of unusual elements and arrangements.”
Like most known quasi-crystals, the trinitite structure appears to be an alloy – a metal-like material made up of positive ions in a sea of electrons. This is unusual for silicon, which typically occurs in rock in an oxidized form: reversing the oxidation would require extreme conditions, such as the intense heat and pressure of a shock wave, says Lincoln Hollister, a geoscientist. at Princeton.
Steinhardt suggests that the quasicrystals could be used for some sort of nuclear forensic science, as they could reveal sites where a secret nuclear test took place. Quasi-crystals can also form in other materials generated under violent conditions, such as fulgurite, the material made when lightning strikes rocks, sand, or other sediment. “The saga of the quasi-crystals will continue!” Hollister said.