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April 20, 2017

We've got a library full of spectral signatures, like a police fingerprint library for minerals and other substances!

Image shows a side-by-side view of plywood and its spectral signature
This shows the spectral signature (left) of the mineral topaz (right).(Public domain.)

What do rare earth elements, Deepwater Horizon oil spill residue, artisanal paint powders, and coastal vegetation have in common? They’re all part of the newly updated and enhanced USGS Spectral Library.

Spectroscopy is a tool that detects the absorption or emission of light by a material as a function of wavelength. Think of it like a light-based fingerprint for various materials. Each material has a spectral signature that is unique to its chemical structure.

“By expanding the number of spectral signatures, the USGS Spectral Library improves the ability of scientists to locate mineral resources, helping them to make new findings not previously possible,” said USGS geophysicist Raymond Kokaly.

Image shows a side-by-side view of plywood and its spectral signature
This shows the spectral signature (left) of plywood (right).(Public domain.)

The USGS Spectral Library is a reference database containing thousands of these fingerprints, which are also known as reflectance spectra. As scientists study materials using spectroscopy, they can compare their measurements to spectra on file within the library to identify minerals, study the composition of soil for farming or other activities, detect oil or other substances in the environment, quantify the chemistry of plants and microorganisms, examine the authenticity of artwork, or even conduct planetary exploration.

This new version of the library expands an important resource used to analyze imaging spectrometer data, also known as hyperspectral remote sensing or imaging spectroscopy, for many applications, including:

  • mapping the distributions of minerals and vegetation on the landscape 
  • defining mineral deposits and associated ore forming processes 
  • delimiting surface expressions of geologic structures, such as fault zones
  • mitigating hazards from  environmental contaminants

The library includes samples of minerals, rocks, physically constructed as well as mathematically computed mineral mixtures, plants, vegetation communities, microorganisms, and man-made materials.

Image shows an Alaskan Mountain viewed through hyperspectral imaging
Regional mineral classification map overlaying a digital elevation model of the Orange Hill area, Wrangell–St. Elias National Park and Preserve, Alaska. Colors represent the spectrally dominant minerals. Data collected at 6-meter spatial resolution.(Public domain.)

The update adds higher resolution spectra and measurements of new materials:

  • minerals bearing rare earth elements
  • mixed vegetation plots in the coastal wetlands of Louisiana and mountain chaparral of California
  • grain size fractions of mineral samples
  • oil emulsions, residues and oil-contaminated marsh plants from large scale oil spills
  • vermiculite from the four main historical sources (Louisa, Virginia; Enoree, South Carolina; Libby, Montana; and Palabora, South Africa)
  • a new collection of powdered paint pigments spanning the range of classical artisanal colors is also included

Last but not least, the USGS Spectral Library is continuing to expand, by going ultraviolet. Critical minerals such as rare earths are being scanned in the ultraviolet wavelengths. This will broaden the number of known spectral signatures for these important materials in future releases.

Nonfuel mineral resources helped create an estimated $2.8 trillion in value added products in 2016, which contributed 15 percent to the total U.S. Gross Domestic Product.

Many of the minerals critical to the U.S. economy are not as well-understood, requiring more research to understand how they form, where they occur, and how they can be produced. The USGS meets these challenges by employing cutting-edge research tools like these spectral signatures.

For more information on mineral-resource science, please visit the USGS Mineral Resources Program. To keep up to date on USGS mineral research, follow us on Twitter.

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