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Miniaturization in SEM–Tools & Analysis

Dr. Robert Linnen discusses the impact a compact SEM has had on his research.

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Dr. Robert Linnen is a professor and the Robert Hodder Chair in Economic Geology in the Department of Earth Sciences at Western University in London, Ontario, Canada. His research focuses on the behavior of metals in magmatic-hydrothermal systems. Dr. Linnen’s approach is to combine field and experimental studies in order to identify the mechanisms that are important for concentrating metals and controlling mineralization and then quantifying these processes in order to develop ore deposit models.

Q: Can you tell me a bit more about your work at Western University?

A: Largely, I study the origin of mineral deposits—if we understand how mineral deposits form, we can come up with better models to explore for new mineral deposits. In studying mineral deposits, there are a variety of tools. I’m a geochemist and a mineralogist, so I tend to use geochemical tools. One of the things that I look at is the compositions of minerals and how they help us understand ore-forming processes or just identify ore.

Q: I see you’re using a benchtop scanning electron microscope [SEM] to look at problems in mineral exploration and mining. What are some of the specific problems you’re looking at?

A: An example of where the JEOL NeoScope SEM comes in is with gold mines. In a typical gold mine, the ore, which is what you are recovering to make a profit out of the process, may only be a few ppm. It may be between 5 and 10 ppm. So when you look at the rock, you may not see any gold in it because the particles of gold are too small. We’re dealing with very low concentrations that you’re still making money off of. So I can have an assay of a rock that comes back at, say, 10 ppm, but I can’t see any of the gold in it. The SEM allows me to go to a much, much lower scale—down to the 1μm or less than 1μm scale—and observe where the actual gold is in the rock. Is the gold in a particular mineral or is it by itself ? There’s pure gold, but there are also gold minerals like gold tellurides and things like that, so I can identify the minerology of the gold using the EDS [energy-dispersive X-ray spectroscopy] on the SEM. While a lot of my work involves understanding the genesis of ore deposits, I also spend some time on technique development. One of my current projects involves combining pXRF [portable X-ray fluorescence] and the benchtop SEM to map igneous stratigraphy [correlating different layers of rock], which is a key for platinum group element exploration. This is the first study of its kind, where the benchtop SEM is applied to either mineral exploration or studying mineral deposits or mines.

Q: How have these technologies made things easier for geologists?

A: With pXRF, instead of having to take a rock and ship it to a lab, and then wait weeks to get an analysis back, you can just take one of these pXRF units in the field with you and start analyzing rocks. That’s pretty cool. You don’t have to send everything back to the lab now; you can start analyzing samples in the field. If you want to know chemical compositions of rocks and you want to do it quickly and do it in the field, this pXRF has revolutionized things; it’s very widespread in the mining and mineral exploration industry. But for minerology, you can’t really do that. 

Q: What techniques have you had to use for the minerology side?

A: Historically, what we’ve done is bring a rock back, cut what’s called a section of it, and then analyze it with the JEOL electron microprobe that we have in our department. I’m one of the principal investigators on that instrument. With the electron microprobe, you can analyze mineral compositions, but it takes a long time. You have to bring the rock in from the field, cut up the rock, get the section prepared, and then do the analyses. That can take weeks to months. 

Q: What does the miniaturized benchtop SEM allow you to do that you couldn’t do before?

A: With the benchtop SEM, I don’t even have to make one of these sections—I can just take a piece of rock and put it in the benchtop SEM and start analyzing my minerals. I can see whether there’s gold or whether there are things like platinum group elements. I can see how the metals are distributed in the rock and the chemical compositions of the minerals that are associated with the metals. Benchtop SEMs have been around for a while, but nobody has really been doing this in geology. At our lab we’re the first ones who are doing this, so we’re developing new techniques as to how to use the benchtop SEM for mineral exploration and, for that matter, mineral processing as well. This is a new field of research. We haven’t been bringing [the benchtop SEM] into the field with us, but there’s no reason why we can’t. We’re doing analyses with it in my lab for gold, platinum, and other projects, and the EDS is the system that does the chemical analysis under the microscope. That’s the real key part of the scanning electron microscope—the EDS on it. We use that to get mineral compositions. Everything that can be done with an electron microprobe, we’re pretty much doing with the benchtop SEM at a much lower cost and with a much quicker turnaround.

Q: How has that work with the benchtop SEM gone so far?

A: Excellent. Most of my students use it, and they’re applying it in what I call the “triage approach.” In triage you look at what’s most critical and you deal with what’s most critical first and then what’s less critical afterward. With rocks, we’re not operating on them, but you do winnow things out—so you might collect 100 rocks in the field, and then of those 100 rocks in the field maybe 30 of them go for a particular kind of analysis, and then out of those 30 you’re going to select three for another type of analysis. There’s a hierarchy of decisions that you make on what you want to analyze. The benchtop SEM is not the highest precision or the highest accuracy, but it gives you an idea as to what’s in the rock. So we use that philosophy in this triage method, and a lot of my students are using it as a first-pass evaluation of the rock to make the decision, for example, on which sample out of ten is the best one to select for further analysis, because that might be all I can afford. The benchtop SEM is very well suited to screen what samples go to the next stage of analysis.

Q: What are the main ways that the miniaturization of SEM instrumentation has impacted your work?

A: With a regular SEM, and the electron microprobe is the same way, there are waiting times. So if I want to use the [regular laboratory] SEM, I might need to wait two or three weeks. Whereas with the benchtop SEM, because it’s relatively low cost—an order of magnitude less than a full-scale laboratory SEM—my students have instant access to it. If I have something right in front of me and I want to answer that question, I can go down to the lab right now. I’m not booking an appointment. Obviously, different people from the department use it, but it’s the accessibility as well—it’s that all my students and students of others can use it without having to have a specific time booked.

Q: What, if any, are some of the challenges with the benchtop instrument?

A: If we had a wish list as to what the next generation is going to look like, then a larger sample size would be useful. But if you work with a larger sample size, then you need to make the entire piece of equipment bigger too, and then it becomes less portable. It’s a trade-off between the small size of the benchtop SEM and the size of the samples that we’d like to analyze with it. The only other challenge will be making it truly portable. We haven’t gotten around to designing a travel case for the SEM. That’ll be something that’ll have to come down the road. In coming up with a travel case, there are screws that have to come in and out of the machine, and that sort of stuff. That’s an engineering thing that we haven’t started looking at yet.

Categories: Ask the Expert

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Published: March 11, 2016

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