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: Luca Bindi |
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.
Within a sliver of the Russian rock, the researchers found the signature
diffraction pattern of a quasicrystal, consisting of aluminum, copper, and
iron, embedded next to the khatyrkite and other minerals. “I was very
excited when I saw the diffraction pattern,” said Yao, who had come into work on New Year’s Day
to do the studies when the lab was quiet. The team—which included Yao, director
of the Imaging and Analysis Center at the Princeton Institute for the Science
and Technology of Materials, and Peter Lu at Harvard University—published the
evidence for the first natural quasicrystal, which today is known as
icosahedrite, in a 2009 article in Science.
The researchers studied a small sample of the mineral khatyrkite, which is mounted on a pyramid-shaped piece of clay next to a penny to illustrate the small size of the sample. Image: Paul Steinhardt |
Uncovering extraterrestrial origins
To uncover the origins of the natural quasicrystal sample, Steinhardt, Bindi,
and Yao teamed with John Eiler and Yunbin Guan
of the California Institute of Technology, Lincoln Hollister from Princeton, and Glenn MacPherson of the Smithsonian
Institution. The researchers examined numerous possibilities for the material’s
origin, including the chance that the sample was actually a byproduct of
industrial manufacturing that had somehow ended up in the museum’s collection.
Through a series of investigations, the team uncovered evidence that clearly
points to an otherworldly beginning.
One such clue was the presence of a mineral called stishovite, a type of
silica that forms only under extremely high pressures and temperatures far from
the conditions used in any human activity. Stishovite has been found in
meteorites. A key finding was that the quasicrystal was embedded in the
stishovite grain, indicating that the quasicrystal and the stishovite formed
together through some natural high-pressure process.
“We actually found physical contact between the quasicrystal and
meteoritic minerals, and that convinced us that we’d found something
important,” said Hollister, a professor of geosciences emeritus.
Next, the investigators probed the ratios of different versions, or
isotopes, of oxygen, which vary depending on whether the minerals formed on
Earth or in space. The researchers found that the ratio of oxygen isotopes in
pyroxene and olivine, two minerals intergrown among the slivers of
quasicrystal, were similar to ones found for some of the oldest-known
extraterrestrial meteorites, known as the CV3 carbonaceous chondrites. Other
minerals detected in the sample were also consistent with meteoritic origin.
The results came as a surprise, said Hollister, who initially thought the
quasicrystal would turn out to be an industrial byproduct given its unusual
configuration of copper, iron, and aluminum. “In nature it is highly
unusual to have metallic aluminum,” said Hollister, referring to the fact
that in nature aluminum grabs onto oxygen atoms and is always found in the form
of aluminum oxide. “We were trying to figure out where on Earth from the
core to the surface could we have conditions that would lead to formation of
quasicrystals.”
Other researchers have been impressed with the results. “I was very
surprised when I read that the previously reported icosahedral phase was of
extraterrestrial origin,” said Robert Downs, professor of geosciences at
the University of
Arizona, who was not
associated with the research. “But a moment later, it was obvious. How
else could such an exotic array of elements be formed and preserved?”
Downs described the
work as “a great find that crosses all sorts of science boundaries—materials
sciences, physics, chemistry, geosciences, astrophysics—all at once.” He
added, “And for kicks, it provides a snapshot of our solar system before
it formed.”
In the past year, Steinhardt and Bindi have launched an ambitious quest to
trace the origins of the Russian sample, with the goal of confirming its origin
and obtaining more quasicrystals. The researchers tracked down the widow of the
Amsterdam
collector who first sold the mineral to the Italian museum. She showed them a
long-hidden diary describing the acquisition of the rock from a government
laboratory during the Soviet era. Piecing together this information with a name
mentioned in a Russian scientific publication, Steinhardt and Bindi eventually
located the Russian mineralogist who in 1979 dug the rock from a thick
blue-green layer of clay in a streambed in the Koryak
Mountains of Chukotka in far eastern Russia. The
adventure culminated in an expedition last summer to that streambed, and the
samples gathered during the trip are in the process of being analyzed.
