FTIR and Raman microscopy are powerful analytical techniques in geoscience. They
are used widely both in research and in mining and they help geologists identify
and characterize minerals and rocks. Today, we'll explore how FTIR and Raman complement
each other in this field and take a look at some examples of their applications.
Let's start with banded iron formations (BIFs), one of the most iconic, and may I say,
most beautiful rocks in Earth's history. BIFs are characterized by their distinctive
layering of iron-rich and silica-rich bands, which were formed billions of years ago.
They are of enormous interest for research as they can serve as an archive of Ancient Earth’s
atmosphere, hydrosphere, and potentially its biosphere. Additionally, these rocks are the
biggest source of economic iron on the planet – making them valuable for mining companies as well.
Their economic value results, among other things, from their mineralogical composition and from how
the minerals are associated with each other. Of course, time is money in the mining business and
because of this it is essential to determine these parameters quickly and cost efficient.
This is where FTIR comes into play. There is no need for tedious sample
preparation as would be necessary in a wet chemical or SEM analysis. And even large
sample areas can be scanned quickly creating a detailed chemical image.
Let me demonstrate this with this piece of a South African BIF. We simply take the rock section place
it under the LUMOS II FTIR Microscope and in less than 2 minutes we can start the measurement.
As you can see the creation of the chemical image only takes about an hour.
After the mapping is done the comparison of the
measured spectra with a library allows us to determine the mineral identity.
For an analysis of the spatial mineral association Raman microscopy has been
proven to be particularly useful as it can analyze components that are
present in small quantities or as inclusions in other minerals
Now you may ask why not use Raman for the whole area instead of FTIR?
With Raman the measurement would take not only one hour, like with FTIR but, more than 2 days.
Let us now move to a much faster Raman application that is much faster. For that
we move into the research sector using the SENTERRA II Raman microscope. We are going
to look at another very intriguing sample from early Earth's history:
This sandstone containing ancient microbial mats.
Microbial mats are communities of microorganisms that grow in layers on surfaces. They can have a
variety of different metabolisms one of which can produce a very important gas- oxygen.
The microorganisms in our sample lived 3.2 billion years ago and might have produced early oxygen.
They left behind specific information about the environment they grew in and
about the post-depositional changes they went through after dying.
Let’s have a closer look at how Raman can help in determining the
peak temperature that the rock has been subjected to during its long history.
For this we first identify a spot in the sample that is rich in organic carbon.
Following this we carry out a measurement after which the OPUS software determines
the area ratio of the D and G bands of the carbon, which is temperature sensitive.
Finally the values are inserted into a special temperature dependent empirical equation,
in order to quantitively determine the peak temperature. Our Raman expert Di
created a Python script for this purpose which simplifies the temperature determination for you.
In this case the temperature calculated gives a peak temperature of ca 400°C. This high overprint
most probably caused chemical changes in the microbial remains that make it difficult for
researchers to receive more information about the origin of the organic matter.
I hope we were able to show you how FTIR and Raman
microscopy can be used to study rocks and minerals. By combining these techniques,
geologists are able to gain a wealth of information about the composition
of a rock sample but also about the structure and evolution of our planet.
Thanks for watching, and as we German Geologists say:
Glück Auf!