Unity and WDS: a powerful combination for geological analysis

11^th June 2024 | Author: Lucia Spasevski

A while ago, I wrote a blog about an exciting new approach for the analysis of geological samples using Oxford Instrument’s new, high-throughput BEX (Backscattered Electron and X-ray) detector, Unity and WDS. This combination gives the best of both worlds; the speed of Unity and the high spectral resolution and detection limits of WDS.
This blog is a continuation of that story, showing some new examples where this combination benefits the analysis of geological material. In order to make it more engaging, I have inserted a couple of videos where you could see what I was doing!
I would like to show you some results from a diabase (altered dolerite, microgabbro) sample from the Island of Jabuka in Croatia, (fun fact; Jabuka means apple in English). Jabuka is an uninhabited volcanic island located in the Adriatic Sea. Jabuka, along with Brusnik, are the only two Croatian islands that are completely volcanic in origin, the results of volcanic eruptions. Dolerites/microgabbros contain minerals like plagioclase, pyroxene (augite) and olivine but may also contain minor amounts of ilmenite, magnetite, titanite and apatite. Here, we will look at the ilmenite.

1. Finding ilmenite using BEX

In this video, you can see how I found the accessory mineral ilmenite in this sample:

Figure 1: Ilmenite grain found using Unity (left) showing layered BEX data and (right) Unity backscattered electron signal.

We spotted some low levels of vanadium in this data. It’s known that vanadium overlaps with titanium in EDS. In order to deal with this, I applied AZtecLive’s TruMap option to deconvolute these elements and therefore check where the V signal was coming from. Results showed that the signal remained but was at a low level. I noticed that there were exsolution lamellae present that appeared different in the Unity BSE image, so I extracted spectra from dark lamella (Ti rich) and light lamella (Fe rich) regions of the map to compare V intensity. From these EDS spectra, I could conclude that the darker lamella contained more V. Given the EDS overlap I mentioned above, however, I wanted to check that this information was correct. Ti rich lamella could seem to have more V because Ti peak could give us false positive for V.

Figure 2: Extracted spectra from the lighter and darker lamellae comparing V levels.

2. Ilmenite mapping using WDS

You can check this part also in video:

In order to confirm these results, I decided to use WDS. I could check the relative signals for Ti and V using the WDS scan functionality. This allows us to collect a WDS spectrum from a selected portion of an EDS spectrum. Figure 3 shows the corresponding EDS spectrum (top figures) and the peak profiles for both V and Ti. The bottom figure is highlights how higher spectral resolution of WDS can resolve Ti Kb and V Ka overlap, while EDS spectrum in yellow cannot distinguish them. In Figure 3, the theoretical WDS spectrum is shown in purple and shows us what kind of resolution we can achieve with WDS spectral scan even before starting any acquisition.

Figure 3: Peak profiles for V and Ti in the EDS sum spectrum showing V and Ti overlaps (top) and virtual WDS scan (bottom) showing how Ti and V can be resolved even though EDS data is showing below 1 wt% V concentration.

The alternative approach is to use WDS mapping to map V intensity at every pixel within a field of view - and this is what I did! In WDS mapping, we are measuring peak intensity at exact peak position, and since spectral resolution is so high, Ti signal is not an issue; Ti is well separated from V. EDS mapping reaches its limitations when there is a need to investigate the spatial distribution of trace elements, as typically the detection limit of EDS is greater than ~1000 ppm (depending on element). Also, as you can see in this example, another challenge for EDS mapping is overlaps in the EDS spectrum.

WDS immediately revealed the true distribution of V and confirmed that the V rich region was also Fe rich and came from the lighter part of the lamella – this finding is opposite to that from window integral (minimally-processed) mapping with EDS (read on to find out how EDS wasn’t actually wrong because of our spectrum processing algorithms…).

When we perform WDS mapping, we simultaneously acquire EDS for a much longer time than we would in a single frame acquisition, as in Figure 2. This means that we can build even better EDS statistics, so that when we switch on TruMap correction, we have excellent confidence when applying corrections for the overlaps (Ti and V), and therefore know that we will get the correct V distribution (Figure 4).

Figure 4 (Left) WDS mapping data highlights the true V distribution, but (Right) EDS TruMap corrections also display the correct distribution.

3. Nothing stays hidden from BEX - looking into cracks

And lastly, a video showing BEX imaging:

I continued exploring the sample and decided that I wanted to collect some EDS information from within the cracks. This is not usually possible with conventional EDS due to the angle that the detector is mounted at – this causes shadowing. You can see in the video below how quickly I was able to collect high quality maps within the crack. I found this really quite amazing, and it made me feel almost like an artist (Figure 5). Analytical Microscopy can be a truly artistic technique!!

Figure 5: BEX mapping inside of cracks

I hope this blog has highlighted how our tools can help you when analysing geological materials. In particular, how quickly you can find grains of interest or specific mineral phases with minimum effort. I also talked about why WDS is so important when you need that extra confidence in analytical results; for example when you are dealing with difficult spectral overlaps, it can really come to the rescue. You could see from the example that with WDS, it is possible to map the distribution of trace elements, as well as produce accurate and representative maps for elements affected by X-ray peak overlaps in EDS. This combination of Unity and WDS is unique to Oxford Instruments, and it can gives you the best performance from a single instrument, making your workflow easier than ever.

Acknowledgment: Special thank you goes to Marin Šoufek (curator at Croatian Natural History museum and my father) for providing the sample.

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