Natural quasicrystal found in a Khatyrka meteorite. Credit: Paul J. Steinhardt et al.

Scientists might have finally figured out where the rarest crystals on Earth formed

This is crazy.

BEC CREW
14 JUN 2016
 

Scientists have just come up with an explanation for how one of the rarest structures on Earth came to be, and it’s got heads spinning. 

Natural quasicrystals are exceptionally scarce, and have only been found in one place since their existence was first proposed back in 1982. Found inside a Russian meteorite with an 'impossible symmetry' that no one could explain, they cost the scientist who discovered them his job.

 

Now scientists have an explanation for why these things are so rare: quasicrystals appear to have come from outer space, where conditions are as strange as their atomic structure. 

"If you had called me before the study and asked if this would work I would have said 'no way,'" Sarah Stewart, a planetary collision expert from the University of California, Davis, who reviewed the paper, told Robert Perkins at Phys.org. "The astounding thing is that they did it so easily. Nature is crazy."

Crystals are one of nature’s most stunning formations, and they’re pretty simple to wrap your head around - they're made up of atoms that are arranged in near-perfect symmetry to form tiny symmetrical wonders like snowflakes, diamonds, and table salt.

Other types of structures include polycrystals, such as most metals, rocks, and ice, and amorphous solids, including glass, wax, and many plastics.

Unlike crystals, which are both ordered and periodic, and have a perfectly defined geometric structure, polycrystalline and amorphous structures are disordered and random, and this gives them unique physical properties related to how to respond to things such as heat and pressure.

Back in 1982, Israeli chemist Daniel Shechtman proposed that another type of atomic structure exists, which he found in a sample of synthetic material he created in the lab. 

 

Known as quasicrystals, these structures consist of a strange, semi-ordered form of matter, with an atomic structure that displays no repeating patterns anywhere you look. What he’d discovered was so strange, he reportedly told himself, "Eyn chaya kao," which translates to "There can be no such creature," in Hebrew.

Shechtman was awarded the 2011 Nobel Prize in Chemistry for his trouble, but not before being literally laughed out of his lab and ridiculed by his peers for decades for daring to suggest something so preposterous as a semi-ordered structure. 

Science can be savage.

"The first sample, made in 1982, was so improbable that eventual Nobel prize winner Shechtman was ridiculed and ultimately asked to leave his lab," Nadia Drake reports for Wired. 

"Then, for years, no one believed that quasicrystals could exist anywhere but the lab - assembling the strange, quasi-periodic structures was simply too tricky, requiring precise temperatures and strange conditions including vacuums and an argon atmosphere."

Fastforward to 2007, and the story of quasicrystals get even weirder.

Physicist Paul Steinhardt of Princeton University and geologist Luca Bindi from the University of Florence, Italy cracked open a meteorite found in the Koryak mountains of east Russia in the late 1970s, and found the first example of naturally formed quasicrystals. 

"Bindi and Steinhardt eventually proved, in 2012, that the quasicrystals inside the rock had been forged in space, and were the natural result of an astrophysical process, and not the product of terrestrial furnaces or a consequence of the rock’s collision with Earth," says Drake.

Another quasicrystal was discovered in this same meteorite in 2015, but this is still the only known natural source. 

Around 100 different types of quasicrystals have been created in the lab, and they’ve been used in everything from non-stick cookware and LED lights to surgical instruments, but scientists have been trying to narrow down the origin of naturally occurring quasicrystals. Now we’re finally getting close.

A new paper builds on Bindi and Steinhardt's discovery by pinpointing exactly where in space these quasicrystals likely originated. 

Led by geochemist Paul Asimow from Caltech, the team proposes that the only natural quasicrystals we know about formed out of collisions in the asteroid belt - a floating disc of irregularly shaped asteroids or minor planets located between the Mars and Jupiter orbits - before falling to Earth as meteorites.

The reason quasicrystals are so unlikely is because perfect symmetry follows a very strict set of rules (or so we thought). Before their existence was confirmed, scientists assumed that for a structure to grow with a repeating, symmetrical structure, it could exhibit one of four types of rotational symmetry: two-fold, three-fold, four-fold, or six-fold.

Perkins explains for Phys.org:

"The number refers to how many times an object will look exactly the same within a full 360-degree rotation about an axis. For example, an object with two-fold symmetry appears the same twice, or every 180 degrees; an object with three-fold symmetry appears the same three times, or every 120 degrees; and an object with four-fold symmetry appears the same four times, or every 90 degrees."

Quasicrystals broke this rule, because they have crystal-like structure with a five-fold rotational symmetry. "The rules of crystallography had been around since 1820," Asimow told Jennifer Ouellette at Gizmodo. "So they were completely unexpected when they were discovered."

Asimow hypothesised that the strange structure was the result of massive cosmic collisions, because he noticed that textures of iron metallic beads inside the meteorite quasicrystals were similar to what he’d seen in previous shock compression experiments, which involves firing projectiles at various materials to see how they respond. 

An analysis of the microscopic structure of the meteorite suggested that this collision happened before it slammed into Earth, and its outer space origin was made more likely by the fact that the Khatyrka meteorite contained a metallic copper-aluminium alloy that's not be found anywhere else on Earth.

Asimow's team performed new shock compression experiments on slivers of meteorite minerals, including a sample of a metallic copper-aluminium alloy, by blasted them with projectiles at nearly 1 kilometre per second. 

"The impact smashed the sandwiched elements together and, in several spots, created microscopic quasicrystals," Perkins reports

"We know that the Khatyrka meteorite was shocked," Asimow told him. "And now we know that when you shock the starting materials that were available in that meteorite, you get a quasicrystal."

The results, published in Proceedings of the National Academy of Sciences, strengthen Asimow's hypothesis that a collision between asteroids - most likely in the bustling asteroid belt - caused the quasicrystals inside to form inside the meteorite. Now he plans to collide different types of minerals together to see if something other than copper-aluminium alloy can produce natural quasicrystals. 

"It explains the mechanism for making natural quasicrystals, and why we haven’t found any others," Asimow told Gizmodo. "We have a unique starting material, and we have a unique environment. Now the biggest mystery is why there were copper aluminium alloys in that meteorite in the first place."

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