THE LARGEST CRYSTAL OF GOLD

Left:  Crystals and Hand.jpg -- Neutron diffraction data collected on the single-crystal diffraction (SCD) instrument at the Lujan Center, from the Venezuelan gold sample, indicate that the sample is a single crystal. Los Alamos National Laboratory photo.

FROM:  LOS ALAMOS NATIONAL LABORATORY 
World’s Largest Single Crystal of Gold Verified at Los Alamos
Lujan Center neutron diffraction team confirms structure

LOS ALAMOS, N.M., April 7, 2014—When geologist John Rakovan needed better tools to investigate whether a dazzling 217.78-gram piece of gold was in fact the world’s largest single-crystal specimen—a distinguishing factor that would not only drastically increase its market value but also provide a unique research opportunity—he traveled to Los Alamos National Laboratory’s Lujan Neutron Scattering Center to peer deep inside the mineral using neutron diffractometry. Neutrons, different from other probes such as X-rays and electrons, are able to penetrate many centimeters deep into most materials.

“The structure or atomic arrangement of gold crystals of this size has never been studied before, and we have a unique opportunity to do so,” the Miami University professor said.

Revealing the inner structure of a crystal without destroying the sample—imperative, as this one is worth an estimated $1.5 million—would allow Rakovan and Lujan Center collaborators to prove that this exquisite nugget, which seemed almost too perfect and too big to be real, was a single crystal and hence a creation of nature. Its owner, who lives in the United States, provided the samples to Rakovan to assess the crystallinity of four specimens, all of which had been found decades ago in Venezuela.

During the past Lujan Center user run cycle, Heinz Nakotte, New Mexico State University professor and lead scientist for the single-crystal diffraction (SCD) instrument, and Sven Vogel, instrument scientist for the high-pressure/preferred orientation (HIPPO) instrument, helped Rakovan probe the stunning pieces at Los Alamos. The authors are preparing a scientific report.

Three of the four samples turned out to be single-crystal pieces of gold, rather than the commonplace multiple-crystal type. Of particular interest was a golf-ball-shaped nugget that at one time was believed to be the world’s largest trapezohedral gold crystal. In 2006 the crystal had been rejected at auction over questions of authenticity, and indeed, the Los Alamos instruments confirmed that it was not a world-record trapezohedral crystal.

Further interpretation of the results will also provide an understanding of how the rare pieces may have formed before they were slightly deformed while being washed down in ancient stream sediments. The ability of the HIPPO instrument to also show how far away a specimen is from being a single crystal helps with these interpretations.

The SCD instrument is a neutron single crystal diffractometer used to determine the periodic atomic arrangement or crystal structure of single crystals, both natural and synthetic. While one of the workhorse-instruments at the Lujan Center, HIPPO is a general-purpose powder diffractometer that measures both the crystal structure and orientation distribution of crystals (or texture) making up a poly-crystalline material from the powder pattern of the crystals. It is the only time-of-flight neutron instrument in the world that routinely measures texture, with single crystals being the ultimate textured samples.

“The gold single crystals are so far the largest single crystals characterized on HIPPO,” Vogel said. HIPPO handles a wide range of materials including rocks, battery materials, alloys, and nuclear fuel mock-ups.
History of Rakovan and the Lujan Center

The big-crystal question is not the first mystery to be solved using the Lujan Center tools: In 2006, Rakovan had been given a collection of several dozen gold crystals to study with X-ray diffraction. One crystal out of the batch was puzzling, showing a single crystal pattern in one orientation but a polycrystalline nature in all other orientations. He hypothesized that weathering and erosion had altered the exterior of the nugget, but that the overall single crystal morphology was intact. “To test this we needed to look at the interiors of the crystals but without cutting them in half,” Rakovan said.

Twelve years before, Rakovan had used the SCD instrument at the Lujan Center to characterize OH ordering in natural apatite crystals. “…Through that experience I learned about other potential applications of neutrons in studying materials. Thus, it dawned on me that neutron diffraction would be ideal to ‘see’ the crystallinity of the interior of these samples without having to destroy them,” he recounted.

While using the SCD instrument for this problem, Nakotte and Rakovan realized that the HIPPO instrument, allowing for texture measurements among other applications, would be able to provide additional data on the gold samples. Several samples were also measured on the HIPPO beamline and the larger probed volume on this instrument indeed provided valuable additional information.

In 2009, the journal Rocks & Minerals published the study, which demonstrated neutron diffraction is the best non-destructive method to establish gold crystallinity of samples that have been formed under the most extreme conditions. At that time, they examined a selection of museum and private collection pieces, discerning which ones were frauds.

While these unusual gold studies open new avenues for geologists, the work underscores a proven capability relevant to other fields, too. Researchers looking to understand the properties of single crystals that are several cubic centimeters in size may need to establish first whether their sample is indeed a single crystal using neutron diffraction. Additionally, researchers that require understanding of single crystal growth procedures or who need single crystals for a specific application, such as scintillators, can find the answers using Lujan Center instruments and expertise.

NNSA funds the production of neutrons at Los Alamos Neutron Science Center (LANSCE), and the U.S. Department of Energy’s Office of Science funded the Lujan Center user program.

THE CHANGES IN SEASONAL CARBON DIOXIDE LEVELS

FROM:  NATIONAL SCIENCE FOUNDATION 

Seasonal carbon dioxide range expanding as more is added to Earth's atmosphere
Northern Hemisphere land-based ecosystems "taking deeper breaths," scientists find

Levels of carbon dioxide in the atmosphere rise and fall each year as plants, through photosynthesis and respiration, take up the gas in spring and summer, and release it in fall and winter.

Now the range of that cycle is expanding as more carbon dioxide is emitted from burning fossil fuels and other human activities, according to a study led by scientists at the Scripps Institution of Oceanography (SIO).

The findings come from a multi-year airborne survey of atmospheric chemistry called HIAPER Pole-to-Pole Observations, or HIPPO.

Results of the study are reported in a paper published online this week by the journal Science.

The National Science Foundation (NSF), along with the U.S. Department of Energy, the National Center for Atmospheric Research (NCAR), the National Oceanic and Atmospheric Administration (NOAA) and the Office of Naval Research funded the study.

"This research provides dramatic evidence of the significant influence the land-based biosphere can have on the amplitude [amount of change] in seasonal trends of carbon dioxide exchange," says Sylvia Edgerton, program director in NSF's Division of Atmospheric and Geospace Sciences, which funded the research.

Observations of atmospheric carbon dioxide made by aircraft at altitudes between 3 and 6 kilometers (10,000-20,000 feet) show that seasonal carbon dioxide variations have substantially changed during the last 50 years.

The amplitude increased by roughly 50 percent across high latitude regions north of 45° N, compared with previous aircraft observations from the late 1950s and early 1960s.

This means that more carbon is accumulating in forests and other vegetation and soils in the Northern Hemisphere during the summer, and more carbon is being released in the fall and winter, says study lead scientist Heather Graven of SIO.

It's not yet understood, she says, why the increase in seasonal amplitude of carbon dioxide concentration is so large, but it's a clear signal of widespread changes in northern ecosystems.

"The atmospheric carbon dioxide observations are important because they show the combined effect of ecological changes over large regions," says Graven.

"This reinforces ground-based studies that show that substantial changes are occurring as a result of rising carbon dioxide concentrations, warming temperatures and changing land management, including the expansion of forests in some regions and the poleward migration of ecosystems."

Adds Peter Milne, a program director in NSF's Division of Atmospheric and Geospace Sciences, "We can easily measure the greenhouse gas budget from a single smokestack, but somewhat less well for a stand of trees. Knowing that for the entire planet is much more challenging.

"Taking advantage of the long-duration and high-altitude-profiling capabilities of the NSF Gulfstream V aircraft [also known as HIAPER], the HIPPO project was designed to take a 'snapshot' of the global troposphere [Earth's lowest atmospheric layer] to see whether we can explain and model greenhouse gas distribution."

In the study, the scientists compared the recent aircraft data with aircraft data gathered from 1958 to 1961 using U.S. Air Force weather reconnaissance flights.

The older data were analyzed by SIO geochemist Charles David Keeling, the father of Ralph Keeling, also an SIO scientist and a member of the research team.

These aircraft measurements were done at the time Charles Keeling was beginning continuous carbon dioxide measurements at Mauna Loa, Hawaii.

While the Mauna Loa measurements are now widely recognized as the "Keeling Curve," the early aircraft data were all-but-forgotten.

Carbon dioxide concentrations in the atmosphere have varied between 170 and 280 parts per million during the last 800,000 years.

When Charles Keeling began collecting data at Mauna Loa in 1958, the concentration had risen to about 315 parts per million.

In May, 2013, daily carbon dioxide measurements at Mauna Loa exceeded 400 parts per million--for the first time in human history.

Recent observations aboard the Gulfstream V were made during regular flights conducted during the HIPPO campaign, from 2009 to 2011.

The aircraft repeatedly ascended and descended from a few hundred meters to roughly 12 kilometers (40,000 feet) in the skies between the North Pole and Antarctica. The goal was constructing a unique snapshot of the chemical composition of the atmosphere.

Additional recent data comes from regular flights conducted by NOAA at a network of locations.

Increasing carbon dioxide amplitude since 1960 had already been observed at two ground-based stations: Mauna Loa and Barrow, Alaska.

Other stations operated by Scripps and NOAA only began measuring carbon dioxide in the 1970s to 1990s.

The aircraft-based observations uniquely show the large area in northern high latitudes where carbon dioxide amplitude increased strongly since 1960.

The exact reasons for the wider seasonal swings in carbon dioxide concentration remain to be determined, say the researchers.

Although plant activity can increase with warmer temperatures and higher carbon dioxide concentrations, the change in carbon dioxide amplitude over the last 50 years is larger than expected from these effects.

Carbon dioxide concentration has increased by 23 percent, and average temperature north of 30°N has increased by one degree C, since 1960.

Other factors may be changes in the amount of carbon in leaves, wood or roots; changes in the extent or species composition of ecosystems; or changes in the timing of plant photosynthesis and respiration.

Simulating complex processes in land-based ecosystems with models is a challenge, scientists have found.

The observed change in carbon dioxide amplitude is larger than that simulated by models used by the Intergovernmental Panel on Climate Change (IPCC).

While this underestimate does not call into question the response of climate to carbon dioxide concentration in the IPCC models, the researchers say, it does suggest that a better understanding of what happened during the last 50 years could improve projections of future ecosystem changes.

The bottom line, according to Graven, Ralph Keeling and colleagues, is that Northern ecosystems appear to be behaving differently than they did 50 years ago.

In addition to Graven and Ralph Keeling, Science paper co-authors include Stephen Piper, Lisa Welp and Jonathan Bent of SIO; Prabir Patra of the Research Institute for Global Change in Yokohama, Japan; Britton Stephens of NCAR; Steven Wofsy, Bruce Daube and Gregory Santoni of Harvard University; Colm Sweeney of NOAA and the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder; Pieter Tans of NOAA; John Kelley of the University of Alaska, Fairbanks and Eric Kort of the Jet Propulsion Laboratory in Pasadena, Calif.