There’s gold in them thar caves! The experimental caves at the Lujan Neutron Scattering Center, that is.
As part of a unique experiment conducted at the Los Alamos Neutron Science Center, Physicist Hienrich Nakotte—a professor at New Mexico State University and the institution’s first LANSCE Professor—used neutrons to examine the crystalline structure of gold. Nakotte worked with Professor John Rakovan of Miami University in Oxford, Ohio.
Unlike X-rays, which only look at the surface structure of the metal, neutrons can penetrate the sample and provide an intricate look at the atomic structure. Depending on the atomic alignment, researchers can determine whether the gold sample is the result of a single crystal of the metal or a composite of many crystals, known as a polycrystal.
The difference is significant for geologists, gem collectors, and museum curators, as well as for scientists striving to understand the behavior of materials in extreme environments—such as the material-rending forces inside the Earth that produce gems or pure precious metals.
In the realm of collectors, all that glitters is gold, although some types of glitter are more highly prized than others. Nuggets formed from a single crystal often fetch 10 times the value they would capture from their weight alone. To illustrate, Nakotte showed off a highly sought after specimen, pyramid shaped, with a base almost an inch wide.
“Here you have a single crystal, which in the case of gold is really, really unique,” he said of the heavy gold-colored sample he hefted in his palm.
The tip off that the specimen may have been a single crystal was the fact that although the pyramid shape was well formed, the facets on each side were not complete. This particular sample was found in Venezuela near the Brazilian border and is one of the largest of its type. Its single-crystal pedigree was later confirmed under the neutron beam, placing its value at an estimated $100,000.
Next to the pyramid specimen, which was securely locked up throughout the experimental process and has since been returned to its owner, sat another unusual looking nugget. The second specimen had distinctive cross-shaped facets on it as well as some embedded quartz, a common impurity in some natural samples. The second sample had been acquired by a collector from a Russian seller for about $70,000. Although the presence of quartz seemed to indicate its authenticity, the truth came out once the sample was subjected to a short burst of neutrons: the sample was a clever forgery, cast replete with quartz impurities to resemble a natural nugget.
“The owner was not too happy,” Nakotte said about having to report the bad news.
The experiment was made possible thanks to the recent acquisition of a single-crystal diffractometer from Argonne National Laboratory’s now-defunct Intense Pulsed Neutron Source. Nakotte said setting up the single-crystal diffractometer for gold nuggets was a snap because the target samples could be easily manipulated and aligned to ensure the best results, unlike many single crystals, which can only be seen under magnification. The only lingering question was how long to subject the samples to the neutron beam.
“Because gold is a neutron absorber, some atoms can become activated by the beam, so we were very conservative in our time estimates,” he said. “Because of the value of the samples, we needed to ensure that they would not remain radioactive and that they could be returned.”
Nakotte was successful in the endeavor and it turns out that an extremely short stint inside the chamber was enough to confirm that the pyramid-shaped nugget was a single crystal. The researcher subjected the cross-faceted specimen to a second test inside a different instrument to confirm that it was a polycrystal.
“Scientifically this was very easy, but it was also very exciting because these are some of the largest single-crystal gold samples ever found,” Nakotte said. “But before you get any ideas about going to Venezuela to find your own samples, you need to know that one of the men who found this one was killed by robbers. It is a very dangerous place.”
Rakovan and Nakotte’s work will be published in an upcoming paper in Rocks & Minerals.
