As promised, and just in time for #thinsectionthursday, today we are delving into the world of rocks under the microscope. Now I'm fairly sure that most of you will know what a microscope is, an instrument which allows you to zoom in on really small stuff. Thin sections might be a bit more of an alien concept, but don't worry, they are basically what they say on the tin. They are a thin (very very very thin, 30 microns thick, 1 micron = 1 thousandth of a millimetre) section of rock which is stuck to a piece of glass, and polished to a very high standard.
I've never personally made a thin section, we have very talented and highly trained people to do this for us, as dealing with something that thin isn't easy. Some rocks can be extremely tricky to deal with, which adds further complication.
So now we've got our thin section, onto the microscope! This is no ordinary microscope, geologists get to use very specialist microscopes that can do all kinds of funky things, which we use for the identification of different minerals (and lots of other things that I have no idea how to use!). This microscope is called a "polarizing petrographic microscope", which basically means that it polarizes (cuts out the light from one or two direction) the light (the same way that fancy sunglasses which are polarized, when you look at your phone it goes a bit psychedelic), and you use it for studying rocks, in greek - petrography.
The main differences between a regular microscope and the fancy one is the polarizing bit, and the way it uses light. It has a light source underneath, so that it can shine light through the thin section and into the microscope lens ("transmitted" light). The microscope I have been using has a fancy computer setup with a camera, so I can take photos of what I'm looking at, which is really useful for making reports, and wondering what on earth your notes are going on about, as well as uploading photos to blogs.
What actually happens when you look at a thin section through a microscope? It blew my mind the first time I looked at a thin section, and still does really. The property of minerals which allow them to transmit light in thin section as they do is their structure. Remember the definition of a mineral? A fixed chemical formula and a regular, fixed repeating structure. It's the latter part which allows minerals to do funky things with light. I've mentioned the polarizing ability of our fancy microscopes, you can either have "plane polarised light" (PPL), which has one polariser, or "crossed polarised light" (XPL), which has two polarisers at 90 degrees to each other; both types of polarisation are are useful. The best way to describe the difference between PPL and XPL is in meme form:
Rocks also do different things under the microscope depending on their orientation (you can spin them and they do disco things). Here's a video of some plagioclase feldspar doing its thing under crossed polarised light:
Plagioclase feldspar isn't one of the most exciting minerals to look at in thin section, but it's a good start.
Properties of rocks in thin section include: relief, cleavage (yep you can see this in thin section as well as hand specimen), birefringance, extinction, twinning, pleochroism, and many more. What do these all mean?
Relief: How much does it look like it jumps out at you? For example quartz has low relief. This is mostly a plane polarised light thing, not crossed polarised light.
Cleavage: Exactly the same as in hand specimen, the minerals can show cracks at certain angles, which can help with identification.
Birefringance: How bright are the colours under crossed polarised light? Are they 1st order greys or are they 3rd order disco yellows and blues? In contrary to hand specimen, the colour of a mineral in thin section can be indicative of the mineral.
Extinction: Twice in a 360 degree rotation under XPL, 99% of minerals go black. This is the mineral being "in extinction". The crystal structure has lined up with the direction of the polarisation and therefore no light can get through, so the mineral goes black.
Twinning: Two or more crystals have grown together, and therefore go extinct at different times (XPL), so you get a stripey effect (which you can see in the video of plagioclase feldspar above).
Pleochroism: In plane polarised light some minerals change colour from one shade of brown or green to a slightly different shade of brown or green (this isn't so much of an exciting disco thing).
Below are two images from one of my thin sections, with what are hopefully helpful annotations, to display some of the many thin section mineral properties. The scale bar in the bottom right is 200 microns across, which is 0.2mm! This makes the entire field of view around 3mm wide. A challenge for you, identify the XPL image and the PPL image below, using the meme above!
This all seems super complicated, which it can be, but once you get used to identifying the common minerals, you soon get your geology spidey senses tuned in, and you can identify most of the minerals at a glance. A wise person informed me that these geology spidey senses I talk about a lot is probably just experience. I'd like to think it's a bit of both. Someone once asked me how I knew the tiny (< 1mm) crystal I found was a garnet. The only answer I could give them was: umm... garnet. And I would put a lot of money on the fact that Geraldine was in fact, a garnet. In reality it had that dodecahedron crystal shape which is indicative of garnet, and the colour also lent itself to garnet. When you see a good garnet crystal (however small) they're fairly unmistakable.
Thanks to the staff and equipment at the Petrography Labs, British Geological Survey, Keyworth, for making this possible!
I hope you've enjoyed the wild ride into rock thin sections and the world of disco microscopy. I will leave you with this video of the weirdest thing I've come across in my samples yet, any suggestions as to what it is are welcome!
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