What is the rarest form of gold

This is the rarest form of gold in the world. New clues reveal why. - Science & Innovation 2021

As Colorado gold production neared its peak, the Ground Hog mine near Gilman produced an unexpected treasure in 1887: a roped branch of gold that split into a trio of curlers. With a height of just over 10 cm and a weight of about half a pound, the yellow precious metal curlicues are the rarest form of gold that has ever been found.

"It's really a one-of-a-kind property - there's nothing like it," says John Rakovan, a mineralogist at Miami University in Ohio. However, this rarity has a scientific disadvantage: no one wanted to damage or cut up the few known wire gold specimens in order to run the tests necessary to find out how they were made. The tendrils remained a wonderful mystery for decades.

Now, with the help of an 800 mm long particle accelerator in the Los Alamos National Laboratory, the researchers have taken a first look at the exquisite sample known as Ram's Horn. The results showed that the bundle, which appears to be dozens of shimmering gold wires, is either a solid crystal or few crystals grow together.

The new work with wire gold has not yet been published in a journal, but the geology community is excited to see a recent preview from the Los Alamos lab. The results, and the results of future studies, could have far-reaching implications, from contributing to more efficient mining techniques to finding new uses for gold in metal-dependent technologies, says Raquel Alonso-Perez, curator of the Harvard Museum, which currently houses the horn .

"What they have done is fantastic, and no doubt they will continue to work on it," said James Hagadorn, curator of geology at the Denver Museum of Nature and Science, which also houses other wire gold samples from the Colorado Ground Hog Mine. "We thought about doing the same with our copies." (Find out how studying some of the world's oldest rocks revealed that young earth meteors provided gold and other precious metals.)

"It's all very new and pretty exciting," added Aaron Celestian, curator of mineral science at the Natural History Museum in Los Angeles, which also houses wire gold.

Golden toothpaste

Gold mining can evoke mental images of shiny nuggets rounded off by their downward movements. But gold can also grow into breathtaking crystals when it is created from naturally heated, mineral-rich water that flows through crevices in the rock. When these crystals are elongated, they are often called wires.

"It's like a gorilla stepping on a tube of toothpaste - only it's solid gold," says Hagadorn.

While such smooth gold wires are relatively common, real wire gold is extremely rare. You can recognize it by the grooves that run along its length, as if the metal had been piped out of a piping bag, says Hagadorn, earning the name striped wire gold. According to Rakovan's estimate, the number of deposits known to produce such samples could likely be counted on one side.

The ram's horn is currently in the Harvard Mineralogical and Geological Museum. It entered the museum in the mid-20th century as part of an impressive mineral collection left by Harvard graduate Albert C. Burrage. At the time of its discovery, the gold horn was valued at $ 160 by weight - or approximately $ 4,500 in today's currency. Rakovan declined to disclose its current value for security reasons, only saying that it was "worth a very pretty penny".

In February 2018, Alonso-Perez reached out to Rakovan at Tucson's annual Gem and Mineral Show to gauge his interest in studying the golden horn. As a child he remembered visits to see the glittering sample in the museum and did not hesitate. He and his team have teamed up with researchers at Los Alamos National Laboratory to decipher the horn's crystal structure using innocuous techniques.

But that posed a daunting challenge: moving an invaluable asset, subject to a series of security protocols, from Massachusetts to New Mexico.

“We worked for months to get all the papers and guidelines in order,” says Alonso-Perez, who adds that research at a nuclear facility has only increased the size of the pile of papers. But at some point the gold treasure made the journey.

"This is probably the most valuable item that I have ever held or picked up," says Sven Vogel, a physicist at the Los Alamos National Laboratory's Neutron Research Center.

Silver and Gold

In Los Alamos, the team bombarded the sample with neutrons, one of the subatomic building blocks of atoms that could penetrate the gold surface. When these particles hit crystal structures, they hurl away in different directions. In a single crystal sample, the scattered neutrons tend to follow only a few paths. The more crystals there are, the more the paths are scattered, blurring the signals.

The team's analysis found that the golden horn was likely made up of a few crystals rather than the apparent amount. The team also analyzed a much smaller specimen from Colombia that curls in a single spiral, and it also appears to be made up of surprisingly few crystals.

This conclusion is in stark contrast to wire silver, the shimmering cousin of wire silver, which is far more common. In previous work, researchers had sliced ​​these silver samples and found that they consisted of an intertwined bundle of many crystals.

For wire gold, the researchers also looked for the absorbed neutrons, which resulted in another surprise: Both samples are actually gold alloys that contain up to 30 percent silver. Additional work at LANL with powerful X-rays that are over a hundred times more energetic than medical scans confirmed that the density and therefore the composition is similar in each sample.

Complete a mission

What all of this means is still up for debate, but some clues may come from a new study published in Geology examining the mechanism behind silver wire formation. For this work, Rakovan and another team of researchers grew synthetic wire silver from silver sulfide to tell how it was made. This ability to synthetically grow silver wire was documented as early as the 1500s when miners smelting silver ore reported sparkling metal hairs popping open while they were working. A similar method was used in this new work by creating a temperature gradient over a rock made of silver sulfide.

The formation of the wire is based on a special property of silver sulfide known as superionic conduction. This means that silver ions - supported by temperature or electrical gradients - race through solid rock at surprisingly high rates, explains Rakovan. These fast-moving silver atoms collect on the surface of the rock and eventually push the tip of the wire up and away. They keep expanding as more silver forms at its base. In just half an hour, the researchers were able to grow wires 0.3 inches in length.

"You can see it growing before your eyes," says Rakovan.

Perhaps this speed is the reason why silver wire has so many interlocking crystals, the researchers postulate. Gold is much less reactive and does not naturally form sulfides. Therefore, it can grow much more slowly and thus form larger and fewer crystals. Rakovan compares the process to building a wall.

"If you build this wall really slowly, you will get a nice flat, straight wall," he says. "But if everyone were to throw their stones down and cement it, everything would be full of flaws - and basically those flaws are [crystal] grain boundaries." Future high-tech studies on gold and silver might help. Try some of these answers.

9 dazzling discoveries of the old gold treasure

“We have learned more about silver wire in the past two years than we have in the past 400 years,” adds Rakovan. "It's pretty amazing what we can come up with."

For Alonzo-Perez, the work has an even greater meaning: “I have the feeling that we have fulfilled the mission of the museum collections,” she says.

Museums are an important part of public education, but also a repository to protect our natural history for future generations of scientists who can answer more complex questions with increasingly sensitive tests - for example with a particle accelerator to examine a sample of gold discovered a century earlier.

“Most of the museum collections are used for the exhibition. People come and enjoy, ”she says. "But I think it's basically about science and research - and about moving science forward."