A prime example of this can be seen in the small section of the dog’s breakfast magnified below, which I call the “turkey.” The bird’s head and beak are in the upper left quadrant and the plump round turkey body is in the middle.
The turkey represents a region where metal and silicate melted and reacted with each other due to an impact, likely the major collision the Zurich isotope test had identified as happening hundreds of millions of years ago. All along the boundary between the metal and the silicate was a thin layer full of mysterious round beads which were nearly pure iron. There was also a delicate arrangement of nonmetallic spinel crystals, which were oxides containing aluminum and magnesium.
It was the first example of this configuration of minerals we had seen among the Khatyrka samples. The spinel and iron beads were the product of a rapid heat-producing chemical reaction that took place when the aluminum in the surrounding metal came in contact with the silicate’s oxygen, magnesium, and iron. The aluminum atoms joined with the magnesium and oxygen in the silicate to form the spinel; the iron liberated from the silicate condensed to form the beads.
But what caused that chemical reaction? And how could we be certain?
CANNONS AND TURKEYS
I decided we needed to try to find the answers experimentally. Iron beads have been observed in lunar samples that experienced impacts on the moon’s surface. So I wondered if impacts in outer space might account for the iron beads in Khatyrka, even though they have a completely different chemistry from lunar surface material.
Hoping to find a way to test my idea, I spent the next few months holding conversations with various experts who had written about the iron beads found in lunar samples. They, in turn, pointed me to other experts, who pointed me to other experts—the usual sort of laborious, time-intensive search for information that had characterized the entire quest.
While I was speaking with an engineering professor at Caltech, a familiar name surfaced. He told me that a colleague, a geophysicist named Paul Asimow, had once studied the formation of iron beads during high-velocity impacts. Bingo.
My youngest son Will had been a geophysics major at Caltech. During his undergraduate years he had introduced me to Paul, a professor he greatly admired. Paul was thin and wiry and a bundle of energy. He was also brilliant, creative, and intensely curious. Once he had an idea for an experiment, he was lightning quick to act.
Paul, at left, had access to the Caltech research laboratory that contained a rare piece of equipment called a “propellant gun,” which is basically equivalent to a specialized cannon. The cannon itself is about five meters long and works just like a traditional cannon. The front end of the 20-millimeter-bore cannon, known as the breech, is loaded with gunpowder along with a two-millimeter-thick projectile composed of a hard, rare metal called tantalum. At the other end of the cannon is a custom-designed target consisting of a stack of synthetic or natural materials embedded in a stainless steel chamber, which is about three inches wide and equally thick. The particular materials used for the stack vary depending on the experiment being performed. The chamber containing the stack of materials is attached with nylon screws at the far end of the cannon, where the target assembly is encased in a large rectangular “catch” box.
When the cannon is fired, the projectile travels at about 3 times the speed of sound and generates a shock wave that passes through the target stack and lasts for less than a millionth of a second. At its peak, the shock pressure duplicates the pressure that Khatyrka experienced in space. The force of the impact shears the nylon screws, sending the steel chamber flying into the rectangular catch box at the rear of the assembly, where it is later recovered and taken apart for study.
My first email to Paul included an image showing just a small section of our iron beads in the turkey, asking if he had ever seen anything like it. He responded right away, excited because he had previously used the gas gun to study the formation of iron beads in stacks of various synthetic metals. Here was a natural example of the same phenomenon he had studied. It was immediately fascinating to him.
We soon began to discuss how to perform a test by putting together a stack of materials that may have been part of Khatyrka prior to its high-velocity impact, as established by the Zurich isotope test. We were hoping we could re-create the formation of iron beads by smashing the stack with the tantalum projectile from his cannon.
I had thought about trying a cannon experiment several years earlier. But at that time, it had not been clear what kind of materials should be included in the stack. By the time we discovered the iron beads in Grain #126A, we knew much more about the composition of Khatyrka. Using that information, Paul designed a test using a target consisting of various layers, as shown on the previous page. The first layer would be olivine, a typical meteoritic silicate, followed by a synthetic copper-aluminum alloy, then a natural iron-nickel alloy from the Canyon Diablo meteorite topped off with a synthetic aluminum bronze. Everything would be held together tightly in the stainless steel container.
Once the shot was fired, the impact part of the experiment was over in an instant. But after removing the sample from the catch box, we spent many additional months carefully dissecting its contents to determine what happened. We were hoping to prove that the impact could produce the iron beads that we had found in Grain #126A. But that would turn out to be the least of our discoveries.
A series of reactions had occurred at the side of the container when the impact created a shock wave that propagated through the stack. Incredibly enough, there was a tiny region of silicate surrounded by metal with iron beads along the boundary, remarkably similar to the turkey in Grain #126A. It was proof that the iron beads observed in Khatyrka could have been created by an impact. Mission accomplished.
But the test revealed something even more incredible. The impact had created grains of icosahedral quasicrystal with a composition similar, but not identical, to icosahedrite. No one had expected that.
More than thirty years after their discovery, hundreds of thousands, perhaps even millions, of quasicrystals had been created in labs all over the world. They were known to be hard and resilient but it had always been thought that they needed to be created with great care and under the most strictly controlled conditions. The violent smashing together of a combination of minerals in the cannon shot was nothing like the chemically pure, low-pressure conditions under which synthetic quasicrystals were normally made.
It was another incredible step forward in the investigation. And our unexpected success inspired a series of shock synthesis experiments with Paul’s cannon. One of them was designed to see if we could produce decagonite, the second natural quasicrystal that had been discovered in our samples. For this, we changed the mineral compositions in our stack to include nickel, an ingredient of decagonite.
This, too, was a success. The impact produced a series of flowerlike arrangements, as shown in the image above. The grayish petals are decagonal quasicrystals. Most astonishing was the bright white substance that formed in the middle of the flowers. That, curiously enough, has the same composition as steinhardtite.
The shock experiments were now so successful that they began taking on a life of their own. Occasionally, they created quasicrystals and other crystals with compositions that had never been seen before, either in nature or in the lab.
That result has led Paul Asimow and me to consider using the gas gun to collide many other combinations of elements together, which will be a new and exciting way to search for new materials. We might be able to find examples of quasicrystals with particularly useful combinations of physical properties, including strength and electrical conductivity. Or we might discover materials with a different kind of orderly arrangement of atoms that has never been contemplated before.
THE MOST AMAZING QUASICRYSTAL YET
The iron beads were just one of several surprises we discovered in Grain #126A. By carefully identifying each type of mineral and taking note of which was connected
to which, we were able to reconstruct in detail what had happened during the enormous impact Khatyrka experienced hundreds of millions of years ago.
In particular, we now began focusing on whether icosahedrite and the other aluminum-copper minerals were produced during the impact or whether they had existed beforehand. Despite all of our testing, neither possibility had yet been ruled out.
To get the answer to that question, we first had to determine if there was any icosahedrite to be found in Grain #126A. Chaney had spent weeks searching through the complicated islands of metal in the sample. Almost all of the metal minerals that he found were either crystalline khatyrkite or other aluminum-rich phases. He could never find any icosahedrite. But we were not about to give up.
As a last resort, we sent Chaney to Pasadena to work with mineralogist Chi Ma, who had access to an electron microscope with even finer resolution than the one Chaney was working with. Chi soon found a tiny fleck of metal that had been much too small for Chaney to resolve. And incredibly enough, it revealed a remarkable combination of metallic alloys in contact with icosahedrite.
Now we could finally report that, within the very same grain of material, one could see examples of icosahedrite alongside evidence of the chemical reactions between metal and silicate. I was thrilled because I knew that the finding truly cemented our scientific discovery. There could no longer be any question that silicate and metal existed together in space and experienced the same physical conditions, adding yet another type of direct proof that our quasicrystals were made in space.
The fresh round of discoveries also included three new crystalline minerals composed of different combinations of aluminum, copper, and iron that had never been observed in nature before. All three have now been officially accepted by the IMA. The three new minerals are named hollisterite, after my Princeton colleague Lincoln Hollister; kryachkoite, after our Russian colleague Valery Kryachko; and stolperite, after Caltech’s former provost Ed Stolper, who provided me with crucial insights and encouragement in the early days of our investigation. Ed also paved the way for me to work with several of the highly accomplished scientists in Caltech’s geophysics department.
The most remarkable of all the new minerals discovered to date in Grain #126A is designated with the provisional name “i-phase II” (our proposed official name is “quintesseite”). It is indicated by the arrows in the image on the following page. It forms little ellipsoidal shapes that are arranged like petals on a flower and surrounded by a complex arrangement of other minerals. A section of Grain #126A had looked like a turkey to me. This one looked like a barking dog. Its head is top center, facing right, with its mouth open in mid-bark.
The discovery of i-phase II represents the completely unanticipated third natural quasicrystal to be found in the Khatyrka meteorite samples.
The provisional name, i-phase II, signifies that it is the second icosahedral quasicrystal phase of matter. Just like icosahedrite, the third natural quasicrystal has icosahedral symmetry and is composed of the same elements—aluminum, copper, and iron. But it has a distinctly different mix of those three elements, thus making it chemically and structurally distinct.
By analyzing the shapes of the icosahedrite and i-phase II and the minerals surrounding them, Lincoln and Chaney were able to fill in some of the remaining blanks about what had happened to Khatyrka hundreds of millions of years ago. They determined that the tiny fragment of metal containing i-phase II had liquefied as a result of the impact and then solidified to form the complex of metallic alloys seen in the barking dog image. That meant i-phase II had definitely formed after the impact. On the other hand, the configuration of icosahedrite and the metal that surrounded it showed that they had definitely not been melted by the impact. That meant icosahedrite definitely existed before the impact.
After and before the impact? How could both things be possible?
The answer appears to be that the massive shock that Khatyrka experienced produced incredibly wild variations in pressure and temperature. Within a few millionths of a meter, which is approximately the diameter of a red blood cell, there are certain regions of material that melted and certain regions that did not. As a result, Khatyrka included two different quasicrystals with icosahedral symmetry that are composed of distinctly different combinations of the same elements and that formed at different times. It was a truly astonishing discovery.
An important implication was that we now knew for sure that that icosahedrite, the first natural quasicrystal we ever identified in Khatyrka, predated the impact. That was consistent with Glenn’s theory that it may date back 4.5 billion years to the beginning of the solar system and disproved Lincoln’s idea that it formed after impact.
From my point of view, the discovery of i-phase II has been the most significant discovery to date for another reason. It is the landmark discovery I have been hoping for ever since 1984, the year that my student Dov Levine and I first published our theoretical proof. That was when I first began flirting with the idea of searching for a natural quasicrystal by scouring display cases at prominent mineral museums.
My goal had always been two-fold. First, I wanted to prove that quasicrystals were stable enough to have formed in nature, as I long suspected. Secondly, I wanted to know if finding a natural quasicrystal could open the door to discovering types of quasicrystals that had not been known before.
With the discovery of i-phase II, my dream came true. For me, it is more important than any of the other natural quasicrystals we have discovered because it is the first one found in nature before being synthesized in the laboratory.
Scientists have barely scratched the surface when it comes to learning the unique properties and potential applications of quasicrystals. More than a hundred different compositions have been synthesized in the laboratory over the last three decades. But most are closely related chemically to the original quasicrystals discovered by Dan Shechtman and An-Pang Tsai.
The lack of variety is due to the fact that there is no theoretical guidance for deciding which particular combinations of atoms and molecules can make this unique and fascinating form of matter. Finding new examples is usually done by trial and error. The simplest approach adopted by many scientists is to make a small change in the chemical composition of a synthetic quasicrystal that is already known to exist.
But that limits the possibilities. If one is interested in finding quasicrystals with properties that are more interesting, both from a practical perspective and a scientific point of view, one can improve the odds by searching to see what nature has made without human intervention. Toward that end, Paul Asimow and I are currently planning more cannon experiments. Experimenting with new methods of fabrication will be another way to advance the science.
Despite all our success, there is still a huge question about Khatyrka that remains unanswered and continues to intrigue me.
By some mysterious process, nature has somehow managed to form quasicrystals with metallic aluminum in direct contact with nonmetallic minerals rich in oxygen, despite the fact that aluminum has a voracious affinity for oxygen. For reasons we cannot yet explain, the aluminum in our natural quasicrystals did not react with the nearby oxygen in the silicate. Normally, the chemical forces would be sufficient to cause the oxygen to react with the aluminum to make corundum, an extremely hard version of aluminum oxide. If we could understand nature’s process, it might teach us a new, more efficient way to make both ordinary crystals and metallic aluminum-bearing quasicrystals.
PHOTONIC QUASICRYSTALS
But do we have any indication that any quasicrystal might have novel and useful properties for science and industry?
Yes, we do. We can either simulate quasicrystals on a computer or create artificial samples using a 3D printer, as shown below. The example pictured here was constructed in 2005 at Princeton by Weining Man and Paul Chaikin, who collaborated with me to study the “photonic” properties of quasicrystals.
The study of photonics is directly
comparable to electronics. Electronics involves the passage of electrons through materials. Photonics involves the passage of light waves through materials. If we could replace electronic circuits with photonic ones, the speed of transmission would be increased and the heat loss due to resistance would be reduced. One of the challenges is to find a way to use photonics to reproduce the effects of semiconductors like silicon, germanium, and gallium arsenide. Those are the materials that comprise transistors and other electronic components used for the amplification and transmission of signals in computers, cell phones, radios, and televisions.
The defining property of a semiconductor is that electrons are totally blocked from propagating through it if their energy lies within a certain band of energies. Engineers take advantage of the so-called “electronic band gap” to control the flow and the information carried by electrons.
Something similar exists in photonics. It is possible to make a material with a “photonic band gap” that blocks light waves within a certain band of energies. The first examples were photonic crystals, introduced and developed twenty-five years ago.
By shining microwaves through our 3D printed structure, Weining Man, Paul Chaikin, and I have shown that quasicrystals have some of the same features as photonic crystals. They also have photonic band gaps. Most importantly, the band gap properties of quasicrystals are superior to those of photonic crystals because they have higher rotational symmetry. That makes their photonic band gaps more spherical, which is advantageous in practical applications.
The photonic quasicrystal example illustrates the point that there may be advantages to quasicrystals over regular crystals in some applications due to their distinctive symmetries, provided we can find examples with the right combination of chemistry and symmetry. We may hit upon good examples by trial and error in the laboratory, but now, we can also imagine discovering useful examples in nature.
The Second Kind of Impossible Page 30