Jun 15 2016
Quasicrystals are naturally formed crystal-like solids with apparently impossible symmetries. These structures are very rare on Earth, and so far only two naturally occurring quasicrystals have been found.
The electron backscatter diffraction pattern of the quasicrystal reveals its unusual structure. Credit: Asimow Laboratory/Caltech
A paper published on the June 13 issue of the Proceedings of the National Academy of Sciences, a team headed by Paul Asimow, professor of geology and geochemistry at Caltech, reveals one of the reasons for the scarcity of quasicrystals and describes how they may arise from rocky bodies as they collide in the asteroid belt with rare chemical compositions.
Crystals are structured and periodic at an atomic level and have a defined geometric structure which repeats itself again and again. They can only display one of four rotational symmetry types – two-fold, three-fold, four-fold, or six-fold – in order to produce a repeating structure without breaking down the original organization.
The number indicates the number of times an object will appear precisely the same in a complete 360-degree rotation around an axis. For instance, a two-fold symmetry object appears exactly the same twice, or each 180 degrees; a three-fold symmetry object appears exactly the same three times, or each 120 degrees; and a four-fold symmetry object appears exactly the same four times, or each 90 degrees.
Before 1984, it was assumed that it would not be possible for a crystal to develop with different type of symmetry, and as such there were no examples of crystals grown with other symmetries in a lab or discovered in nature. However, in that same year, Princeton’s Paul Steinhardt hypothesized a series of conditions in which other symmetry types could possibly exist, and Israel Institute of Technology’s Dan Shechtman published a paper stating the formation of a crystal-like structure having a five-fold rotational symmetry.
The crystal-like structures were adequately ordered in order to create identifiable diffraction patterns when hit with high-energy beams of X-rays and electrons, not like disordered structures that do not create any patterns. Nevertheless, these were not periodic structures, in other words their organization changed and moved as they grew, and the materials came to be known as "quasiperiodic crystals," or "quasicrystals".
In the next few decades, scientists discover ways to develop over 100 different types of quasicrystals by homogenizing, melting, and cooling specific elements in the lab at very specific rates. However, no naturally occurring quasicrystals were identified. In fact, scientists assumed that the formation of quasicrystals would not be possible, because nearly all quasicrystals grown in the lab were metastable, that is, the same grouping of elements could organize themselves into a crystalline structure with less amount of energy.
However, in the late 2000s everything changed. Steinhardt and Luca Bindi from the Museum of Natural History at the University of Florence (presently in the Faculty of the Department of Earth Sciences of the same University) identified a small particle of an iron, aluminum and copper mineral that displayed five-fold symmetry. The particle was derived from a small section of the Khatyrka meteorite, a celestial object found in the Koryak Mountains in Russia. In 2015, Steinhardt and his partners identified a second natural quasicrystal from same Khatyrka meteorite and confirmed that quasicrystals can exist naturally, but they are extremely rare.
A microscopic testing of the meteorite showed that it had experienced a significant shock at certain point in its life span before colliding into Earth, probably from a collision with a different rocky body in space. These types of collisions are frequent in the asteroid belt and emit maximum amounts of energy.
Asimow and his team theorized that the energy emitted during the shock event could have caused the formation of quasicrystal by activating a quick cycle of compression, decompression, heating and cooling. In order to test the theory, Asimow reproduced the crash between two asteroids in his lab. To execute this, he selected thin slices of minerals discovered in the Khatyrka meteorite and packed them together in a test case similar to a steel hockey puck, and then fastened the puck to the muzzle of a 4 m long, 20 mm bore single-stage propellant gun and blasted it with a projectile at almost 1 km per second, that is, equal to the high-speed rifle-fired bullets.
It should be noted that these minerals contained a metallic copper-aluminum alloy sample that can be found only in nature and that too in the Khatyrka meteorite. The sample was put into the chamber and shocked with the propellant gun and later it was opened, polished and investigated. The collision smashed the packed elements together and, in a number of spots, formed microscopic quasicrystals.
Equipped with this experimental proof, Asimow is certain that shocks are the basis of naturally produced quasicrystals.
We know that the Khatyrka meteorite was shocked. And now we know that when you shock the starting materials that were available in that meteorite, you get a quasicrystal.
Paul Asimow, Professor of Geology and Geochemistry, Caltech
Sarah Stewart, a planetary collision specialist from the University of California, Davis, and reviewer of the PNAS paper, acknowledges that she was amazed by the findings. "If you had called me before the study and asked if this would work I would have said 'no way.' The astounding thing is that they did it so easily," she says. "Nature is crazy."
Asimow admits that there are many questions that are still unanswered with these experiments. For instance, it is not uncertain at which moment the quasicrystal was created during the pressure and temperature cycle of the shock. According to Asimow, a bigger mystery is the basis of copper-aluminum alloy in the Khatyrka meteorite that has never been seen in nature before.
Asimow is planning to shock several combinations of minerals to understand the vital ingredients required for the formation of natural quasicrystal.
The research findings are published in a paper titled "Shock synthesis of quasicrystals with implications for their origin in asteroid collisions." Besides Asimow, Bindi and Steinhardt, other coauthors involved in the study are Chi Ma, director of analytical facilities in the Geological and Planetary Sciences division at Caltech; Oliver Tschauner from the University of Nevada, Las Vegas; and Lincoln Hollister and Chaney Lin from Princeton University.
The National Science Foundation (NSF), the NSF-Materials Research Science & Engineering Centers Program and the University of Florence supported the research through the Princeton Center for Complex Materials and New York University.