sexta-feira, 10 de agosto de 2012

Researchers Hunt For Naturally Occurring Quasicrystals

Image Caption: Field operations at the Listvenitovyi stream (clockwise): mapping the structural geology, examining mineral samples, extracting clay from areas along the stream, and panning the clay down to mineral separates. Credit: Paul J Steinhardt, Luca Bindi

An expedition to far eastern Russia, a 15,000 year old meteorite, secret diaries, smugglers, gold prospectors and bears; sounds like the plot to an Indiana Jones movie, right?

This is the story of two researchers looking for the origin of naturally occurring quasicrystals. Who knew science could be so exciting?

Writing in Reports on Progress in Physics, Paul J. Steinhardt and Luca Bindi reveal that new, naturally occurring quasicrystal samples have been found in an environment that does not have the extreme terrestrial conditions needed to produce them. This strengthens the case that they were brought to Earth by a meteorite that landed during the last glacial period, about 15,000 years ago.

“The fact that the expedition found more material in the same location that we had spent years to track down is a tremendous confirmation of the whole story, which is significant since the meteorite is of great interest because of its extraordinary age and contents,” said Steinhardt.

To add to the adventure movie plot of this expedition, Steinhardt and Bindi describe the journey that took ten scientists, two drivers, and a cook 230 kilometers into the Koryak Mountains of far eastern Russia to pan one and a half tons of sediment by hand, and survey local streams and mountains in the paper, “In Search of Natural Quasicrystals.”

Quasicrystals are a unique class of solids that were first synthesized in the laboratory by Israeli scientist Dan Shechtman in 1982. Shechtman was awarded the Nobel Prize for Chemistry in 2011 for this discovery.

The concept of quasicrystals was first introduced by Steinhardt and his student Dov Levine. Until their work, it had been believed that all solids, synthetic or natural, form ordinary crystals — materials whose entire structure is made of a single-type cluster of atoms that repeat at regular intervals, joining together in much the same way as identical tiles in bathroom tiling.

It was also thought that crystals could only have two-, three-, four- and six-fold symmetries; however, Steinhardt and Levine found a new theoretical possibility, which they dubbed quasicrystals. A quasicrystal has two or more types of clusters that repeat at different intervals with an irrational ratio, which allows all the symmetries that were thought to be forbidden, such as five-fold symmetry, to be possible.

Since their discovery in the laboratory, researchers have created over one hundred artificial quasicrystals that have been used in a variety of applications, from non-stick frying pans and cutlery to ball bearings and razor blades.

This is where the story gets interesting. Only one naturally occurring quasicrystal has been documented before this expedition, a sample in the Museum of Natural History in Florence, Italy. The sample was located and identified by Steinhardt, Bindi, and their collaborators in 2009. They found the sample to have the symmetry of a soccer ball, with six axes of five-fold symmetry forbidden to ordinary crystals. This discovery is what set off the incredible investigation to find the origin of the crystal.

Eventually, the research team found Valery Kryachko, who had removed the sample from a remote area of Chukotka in the Russian mountains in 1979.

Experiments in the summer of 2010 revealed that the sample was meteoric and had come from not just any type of meteorite, but a CV3 carbonaceous chondrite – a 4.5 billion year old meteorite formed at the beginning of the solar system.

“Now there was real motivation to turn this fantasy trip into a reality. It was a long shot, but if we could find even one sample there, it would prove the bizarre story we had put together beyond any shadow of doubt and provide new sources of material for studying this very strange meteorite that formed at the beginning of the solar system,” Steinhardt said.

Now that the expedition team has collected even more samples from the original site in Chukotka, there are a number of questions that can now be answered with further investigation.

“What does nature know that we don’t? How did the quasicrystal form so perfectly inside a complex meteorite when we normally have to work hard in the laboratory to get anything as perfect? What other new phases can we find in this meteorite and what can they tell us about the early solar system?

“At the moment, we are at the tip of the iceberg,” said Steinhardt.


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