12th January 2023 | Author: Matt Hiscock
Geological materials are incredibly important for a number of reasons – two major ones of which are:
1) They provide the raw materials for everything that is manufactured on earth – (of course, even recycled materials were originally created from raw materials that have subsequently been used and reused). Essentially, they are the resources from which everything is made.
2) Their composition and structure hold a huge amount of information on how they were formed and re-formed and this in turn provides us with the insights that we need to understand how our planet (and celestial bodies!) form, develop and continue to operate to this day. This has direct implications for everything from the resources mentioned above to understanding natural hazards and having the necessary information to know how to build and where.
The information needed to answer these questions is held at a range of scales in the rocks themselves – everything from km scale structures that help to define the topography of the planet down to features on a nanoscale. How we go about extracting this information is the topic of this blog and a tutorial which I will be giving on Thursday 19th Jan 2023.
Perhaps unsurprisingly, given that this is an Oxford Instruments NanoAnalysis blog, I’m going to argue that an analytically equipped electron microscope is the ideal tool for understanding the centimetre, millimetre, micrometre and nanometre features of geological materials. Modern instruments can host multiple analytical techniques at once and can also support high levels of automation – meaning that the return on investment of this equipment can be maximised by using it all hours of the day.
Perhaps the best place to start is with Energy Dispersive X-ray spectrometry – EDS. This approach is perfect for the comprehensive characterisations of samples – particularly unknown ones where we might not have any pre-knowledge. This is because it is both fast and flexible – able to detect a wide range of elements both quickly and accurately. Our first step would be to use EDS in its most immediate and live incarnation with Live Chemical Imaging to rapidly navigate around our sample, gaining an immediate understanding of the elements that are within it and their distribution – you can see an example of this in the video below:
Once we’ve had an initial look at our sample with Live Chemical Imaging, we could move on – perhaps performing some quantitative EDS analysis – allowing us to start identifying the mineral phases that are present or investigate the presence of important impurities or other elements. Alternatively (or in addition!), we might acquire a large area map of several cm2 of our sample – giving an overview of a whole large area and allowing structures and compositional variation on this scale to be seen - but with the ability to do a google maps style zoom-in to see the fine detail.
Acquiring individual spectra from grains in a geological sample with Point and ID
The questions that we then have of our samples might then move on to include things like “What are the relative proportions of the different minerals present?” or “How does grain size relate to mineral species?”. We can answer these questions by performing an automated feature analysis. This approach will identify “features” – which could be grains in a solid material or perhaps particles mounted in a resin – and will combine morphological and compositional data on a feature-by-feature basis. The great advantage of Feature analysis with a product like AZtecFeature is that one spectrum is acquired per Feature – rather than per pixel – making it a far more efficient way of analysing samples with lateral variations. This approach is extended in a mineralogy specific implementation in our AZtecMineral product – which goes one step further and produces outputs on parameters such as mineral associations and liberation in particulated samples.
Example outputs from AZtecFeature (left) and AZtecMineral (right) – information is gathered on a feature-by-feature basis. AZtecFeature gives information on particle classifications, size distributions etc. whereas AZtecMineral gives additional information on particle associations, liberation etc.
Having now developed a detailed understanding of the sample, it is more than likely that even more questions will be generated. Perhaps we are particularly interested in a certain mineral and want to understand the levels of trace elements within it. Or perhaps a compositional understanding of the sample doesn’t tell the whole story and we need to understand its crystalline texture – to understand the stresses and strains or cooling history that it has been subjected to. Thankfully, we can easily perform all of these more in-depth investigations on the same system, in the same analysis session.
Wavelength Dispersive X-ray Spectroscopy (WDS) provides exacting quantitative compositional measurements – using a combined approach with EDS to perform optimised analyses – using EDS for the major elements and WDs for the minor. The approach gives Electron Probe Micro-Analyser (EPMA) level quantification on the same platform as all of the other analyses that I’ve already mentioned. The advantages of not having to go to another instrument are clear!
Finally, we could go even further by adding in structural information from Electron Backscatter Diffraction (EBSD) – giving an insight into the granular structure of the material.
The applications here are many and varied – from looking at deformation in a structural geology context to phase identification (in combination with EDS) in metamorphic rocks – enabling an understanding of the relationships between metamorphic minerals and for key phases to be identified and located – e.g. for subsequent use in geochronology.
A garnet-bearing pelitic gneiss – with phases identified by EBSD in combination with EDS
And of course, many more! You can learn more about the EBSD technique at our EBSD.com
website.
Of course, all of this has been an exceptionally short run-through of what is possible on an analytically equipped SEM.
I’ll go through real-life examples of all of how all of these approaches can be used in a single workflow in the tutorial that I mentioned earlier – in far more detail than I can in a single blog. Please do join me!
Dr Matt Hiscock,
MAG Head of Product Science, Oxford Instruments
Dr Matt Hiscock is the MAG Head of Product Science at Oxford Instruments and holds an MSci in geology from the University of Bristol and a PhD in geochemistry from the University of Edinburgh. He has also worked in the mining industry in Australia and been involved in the running of an academic SEM facility. He joined Oxford Instruments NanoAnalysis in 2013 and works with customers across a wide range of applications to understand their needs and how Oxford Instruments products can address them, as well as overseeing the development of products designed for specific applications and industries, particularly in the field of feature analysis.
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