The best gold specimen in the world probed with neutrons of Los Alamos

Using neutron characterization techniques, a team of scientists peered inside one of the most unique examples of gold thread, including for the first time the structure specimen and its possible training process. This 263-gram 12-centimeter specimen, called a ram's horn, is part of the collection of the Mineralogy and Geology Museum at Harvard University (MGMH).

"We know almost nothing about the gold wire, apart from the specimen's existence," said Sven Vogel, a physicist at the Los Alamos National Laboratory's neutron science center, LANSCE, a particle accelerator. A kilometer long which provides high and low energies protons and neutrons for a wide variety of scientific research.

Found in 1887 at the Ground Hog pig mine in Red Cliff, Colorado, mysteriously fashioned as a heap of twisted wires instead of the more recognizable golden nugget, the Ram's horn has baffled mineralogists since its discovery. The unknown: what is its basic structure and how is it formed?

"Some native metals, a metal or a metal alloy found in nature, may appear in what is called the morphology of the wire," said John Rakovan, a professor of mineralogy at the University of California. University of Miami, Ohio. "Although more common in silver, the morphology of the yarn is rarely observed in gold samples and this specimen is unquestionably the finest known example."

Due to its rarity and the associated monetary value, it is not possible to cut or open the specimen, and because of its density, low energy X-rays and other diagnostics can not interrogate than the outer surfaces. Therefore, no scientific study has been published on the internal nature of this specimen, until now.

The results of the research suggest that gold wire is very different from silver wire. "The silver wire is a mosaic-like polycrystalline aggregate with several hundred to thousands of crystals in a single wire," Rakovan said. "The gold seems to consist only of a few single crystals.In addition, we discovered that these specimens were not pure gold, but rather gold-silver alloys with up to a few years. at 30% silver substituting gold for the atomic structure. "

"We knew that on the surface this gold sample contained about 70% gold, but nobody knew if it was only the case on the surface," Vogel said. "By using our neutron source, which we normally use to study materials such as uranium alloys or nuclear fuels, we could verify that this sample was homogeneous, which means that all the sample is made up of this 70-30 mixture of gold-silver. "

In another study, Rakovan and his colleagues recently discovered that the growth of silver thread resulted in unexpected enrichment in isotopes. The Los Alamos team uses neutron spectroscopy data to assess whether isotope enrichment also occurs in these gold-silver alloys. Another surprise was the uniformity of the gold / silver blend. "Because the alloy is very homogeneous, it would seem that the silver binds to the gold in the crystal structure at the atomistic level," Rakovan said.

The results of this study will have implications for geoscientists who are trying to understand the geochemical processes involved in the formation of gold deposits, as well as for scientists and materials engineers who can use the unique properties of these materials in technological applications.

The unique capabilities of the LANSCE installation can be used to study the interior of precious specimens like this gold wire without having to open them for analysis. Using LANSCE's neutronic techniques, scientists can "examine" the interior of these large gold specimens in a nondestructive manner and learn more about their texture, atomic structure, and chemistry of elements and materials. isotopes.

The sample will be the centerpiece of a new spring 2020 exhibit at the Harvard Museum of Natural History. "The Rare and Beautiful", highlighting aspects that make the value of an object and how humanity appreciates beauty, rarity, culture has shaped these values. The gold wire is not only the best gold specimen in the world, it also represents unique training circumstances that reveal its training process and, ultimately, the history of the Earth. The exhibition is a rare opportunity for the public to see this specimen of gold, normally kept safe in a bank vault.

"The Harvard Mineralogical and Geological Museum is one of the oldest and still operating in the United States," said MGMH curator Raquel Alonso-Perez. "Because of the strong chemistry department of Harvard University, the University began collecting minerals from the 1850s to better understand their crystalline structure from a chemical point of view." The specimen of gold wire was bequeathed to Harvard in 1947 as part of the A. C. Burrage Collection.

"These academic collaborations really push the boundaries for us," said Vogel. "We usually use the LANSCE Accelerator to characterize materials relevant to the Los Alamos mission, but we sometimes get to apply it to geological samples like this very famous gold specimen." winning solution for the Laboratory and the universities. "

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