The rock sample from the mineral collection of the Museo di Storia Naturale in Florence was unearthed in the Koryak Mountains in Russia and found to include grains of icosahedrite, the first quasicrystalline mineral to be discovered in nature. Analysis of the oxygen isotope abundances in the rock indicate it is a fragment of a meteorite formed at the formation of the solar system over 4.5 billion years ago. (Image courtesy of Luca Bindi)
by Catherine Zandonella
A rare and exotic mineral, so unusual that it was thought impossible to exist, came to Earth on a meteorite, according to an international team of researchers led by Princeton University scientists. The discovery provides evidence for the extraterrestrial origins of the world’s only known sample of a naturally occurring quasicrystal.
Found in a rock collected in a remote corner of far eastern Russia, the natural quasicrystal was most likely formed during the early days of the solar system, roughly 4.5 billion years ago, making the mineral perhaps older than the Earth itself, according to the research team. The results, which come three years after the team identified the mineral as the first natural quasicrystal, recently were published in the Proceedings of the National Academy of Sciences.
"The finding is important evidence that quasicrystals can form in nature under astrophysical conditions, and provides evidence that this phase of matter can remain stable over billions of years," said physicist Paul Steinhardt , the Albert Einstein Professor in Science at Princeton and one of the leaders of the research.
Although quasicrystals are solid minerals that look quite normal on the outside, their inner structure makes them fascinating to scientists. Instead of the regularly repeating clusters of atoms seen in most crystals, quasicrystals contain a more subtle and intricate atomic arrangement involving two or more repeating clusters. As a result, a quasicrystal’s atoms can be arranged in ways that are not commonly found in crystals, such as the shape of a 20-sided icosahedron with the symmetry of a soccer ball.
The concept of quasicrystals — along with the term — was first introduced in 1984 by Steinhardt and Dov Levine, both then at the University of Pennsylvania. The first synthetic quasicrystal, a combination of aluminum and manganese, was reported in 1984 by Israeli materials scientist Dan Shechtman and colleagues at the U.S. National Institute of Standards and Technology, a finding for which Shechtman won the 2011 Nobel Prize.
Since Shechtman’s work was published, scientists have created about 100 types of synthetic quasicrystals, some of which are now used in durable coatings and surgical blades. Scientists are also exploring them for use in frying-pan coatings and heat insulation for engines
The search for natural quasicrystals
For years, many experts believed that quasicrystals, while interesting, could be made only under the carefully controlled conditions available in a laboratory. Many also thought that the materials were unstable and must, after an extended period of time, revert to ordinary crystals.
Steinhardt, who was skeptical of this view, decided to launch a search to see if perhaps nature had beaten scientists to the punch, and had already produced quasicrystals. In 1999, he and his collaborators began an intensive search for natural quasicrystals. The team scanned a database of experimental results from more than 80,000 known materials looking for signs of quasicrystalline structure. Next, the researchers started combing museums and private collections for samples containing certain combinations of metals including aluminum, often found in synthetic quasicrystals.
In 2008, the researchers finally uncovered a lead when they were contacted by Luca Bindi, a mineralogist at the Museum of Natural History in Florence, Italy. Bindi suggested that Steinhardt test some of his specimens, including a rare mineral called khatyrkite, which was composed of copper and aluminum. The sample had been stored in a box as part of 10,000 minerals acquired by the museum from a private collector in Amsterdam. The marking on the box indicated that the sample came from the Koryak Mountains, in the northeastern part of Russia’s Kamchatka Peninsula.
When the sample arrived from Italy, however, it had been cut away from the surrounding rock, leaving Steinhardt with microscopic grains to work with, and no room for error. "If we had dropped the sample, it would have been lost forever," said Nan Yao , Steinhardt’s Princeton colleague. Yao painstakingly ground the tiny sample, which measured the width of a human hair, into the even smaller slivers required for probing the structure to see if it was a quasicrystal. The technique they used, transmission electron microscopy, involves firing a beam of electrons at a sample and observing how the electrons bend, or diffract, when they hit the sample.
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