In the recent article, “Analysis of Garnet by Laser-Induced Breakdown Spectroscopy—Two Practical Applications,” (Minerals 11, 705 (2021)), Drs. Russell Harmon, Michael Wise, Richard Hark and Ph.D. student Peter Defnet evaluated two practical applications of LIBS that could be implemented with a handheld LIBS analyzer on site in a museum or during geological fieldwork.
In an effort to dig into the significance of the study, Dr. Harmon filled us in on the past, present, and future work with garnets, and the importance of SciAps handheld LIBS to geochemical fingerprinting and museum collections in general. Harmon is adjunct associate professor, Department of Marine, Earth, & Atmospheric Sciences, North Carolina State University.
Q: Can you give us a quick explanation of what geochemical fingerprinting is?
Russell Harmon: Every material has a unique chemical composition, and this can be deciphered from its LIBS spectral signature. Geochemical fingerprinting is not a quantitative analysis, which requires matrix matched standards. Instead, geochemical fingerprinting is a qualitative rapid analysis technique that compares all of the information in LIBS broadband spectra about elemental composition to differentiate between materials.
Based on this idea, a large number of LIBS spectra were acquired for the six different classes of garnets – almandine, andradite, grossular, pyrope, spessartine, and uvarovite – and then we used sophisticated signal analysis for a statistical processing of the spectral data to see if we could differentiate those classes. This approach was highly successful. The next step is to build a spectral library of the different garnet types, so that when presented with a garnet of unknown or uncertain type, its LIBS spectrum can be compared to the library to ascertain whether or not it belongs to one of the six garnet classes in the library.
We can apply this technique to other geological materials as well. Does an obsidian artifact come from a particular source? Does this columbite tantalite sample come from the Democratic Republic of Congo, where it was mined illicitly, or does it come from one of these other ten places in the world that we know the minerals are being mined in a legitimate way? Which limestone formation in a stratigraphic sequence does this isolated outcrop belong to? Field geologists often confront these and many similar questions, and LIBS can be a helpful tool in providing an expedient answer.
Q: Why did you use garnets for this study?
Russell Harmon: Garnet is a mineralogically straightforward mineral with the general formula X3Y2Si3O12, where the divalent X site typically contains either Mg ,Fe2+, or Ca and the trivalent Y site contains either Al, Cr, or Fe3+. Thus, we thought it would be easy to use chemometric analysis of LIBS spectra to distinguish between the six common garnet types – FeAl garnet (almandine), MgAl garnet, MnAl garnet (spessartine), CaAl garnet (grossular), CaFe garnet (andradite), and CaCr garnet (uvarovite).
We wrote the first paper on garnets in 2010, using a laboratory LIBS system and my personal garnet collection. I had a cadet from West Point who needed a summer research project. Dan was interested in the mathematical data processing side of it all, so, at the time, we considered it a bit of a ‘toy problem’ since, although the six garnet types come in many different colors, each specimen had a type label. We published these findings in 2010 in Applied Optics, ”Laser-induced breakdown spectroscopy-based geochemical fingerprinting for the rapid analysis and discrimination of minerals: The example of garnet” (Daniel Alvey, Russell Harmon, et al. https://www.osapublishing.org/ao/abstract.cfm?uri=ao-49-13-C168)
So that’s the basis of what Richard (Hark) and I did a few years later when we used SciAps first handheld analyzer, the Z-500, to see if we could get similar results, which we did. (Geochemical Fingerprinting by Handheld Laser-Induced Breakdown Spectroscopy, 2017).
These two studies then led to the current garnet study published in Minerals (2021), which was initiated when Richard and I began to work with Michael Wise in the Mineralogy Department at the Smithsonian Museum of Natural History and told him about the garnet studies. So, we decided to go through their garnet collection. Michael was using electron microprobe analysis for quantitative garnet and had recognized some misclassifications. So, some 200 garnets were examined by both EMP and LIBS. Much to our surprise, we discovered that some 30 percent of the garnets were misclassified, the labels on the samples were wrong.
Q: That’s fast analysis. And a lot of mislabeling!
Russell Harmon: Well, historically museum collections have relied on paper trails and accepted mineral donations with the label accompanying the specimens. This is what makes the study unique. Handheld LIBS can see what’s really there, right on site in real time. It’s totally a new approach to authentication.
Next, we are planning to take the Z-300 to the Peabody Museum at Yale where there are 800 garnets in the collection and only about a quarter of them actually have labels. Because of the work we did at the Smithsonian, the curator of the mineral collection would like to check the veracity of those labels and identify the others.
Q: Why use the LIBS instead of the other methods?
Russell Harmon: Well, a lot of time is required to prepare mineral samples for analysis by traditional laboratory approaches like EMP analysis. You might only be able to analyze 20-30 in a week. But, LIBS can do hundreds of analyses a day. The LIBS is an analytical tool, without the radiation hazard associated with handheld XRF, and can run through a mineral collection very quickly by just opening a cupboard drawer and rapidly analyzing its contents to see how well the samples are labeled, given that we have a good spectral library as the basis for the comparison. And what’s more interesting is that this can be done with many other minerals in museum collections that have complex chemistries. I think one would find that a lot of the minerals are just not correctly identified. Museums could also use LIBS to analyze many different types of artifacts in their archaeological collections such as wood, ceramics, pottery shards, obsidian, bones, ancient coins, really any kind of artifact.
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