When you hear STEM students discussing their degrees you will often hear the word 'practicals' or 'labs'. This term however means different things for the different sciences. Chemists may be performing chemical reactions, engineers may be building things and medics may be doing dissections. But what actually happens in a geology practical?
A photo taken in one of my first year practical classes. Looking through a handlense at a sandstone with some fossilised burrows in it (the tube like things at the top of the image).
Our practicals so far at undergrad have consisted of either us being given a problem set and sitting at the benches in the lab to complete it, being told to code something, usually on the software MatLab, or looking at hand specimens/thin sections of rocks. Since answering a set of questions at a desk is not particularly out of the ordinary, the hand specimens and thin sections will be what I am going to discuss here.
Hand Specimens:
Let's start with hand specimens, the clue is in the name with these ones - hand specimens are rock samples that you can hold in your hands. Our practicals involving hand specimens often consist of us being given the chance to look at a number of samples each showing something that we have discussed in lectures. We learn how to identify different minerals, fossils or sediments and how they can tell us about how the rock formed. This then helps us when we are in the field as we know what we are looking for. In the lab we examine hand specimens with our eyes, handlenses and sometimes through the use of acid or scratch tests to figure out certain properties. For example quartz and calcite are both clearish minerals but a key way to tell the difference is that calcite can be scratched with metal whereas quartz cannot - it will leave a scratch mark on the metal instead. Another example is calcite (a calcium carbonate mineral) will fizz if you put some acid on it.
Here are some examples of hand specimens I have seen in the lab:
Some fossils in hand specimen. From left to right they are some tabulate coral, a mold of a brachiopod and some scleractinian corals.
We learn how to identify different fossils (invertebrates in first year) and we can use that information to figure out how old the rock they are in must be. The rock must have been deposited at the time that the fossil was alive and we know what organisms lived at what point in Earth's history.
Some igneous/metamorphic rocks in hand specimen from practicals towards the end of my first year. From left to right they are quartzite (you can tell by the sugar like texture), granite (igneous and very common to see in the field), and gneiss (you can tell by the compositional banding).
Image on the left shows the top segment of a trilobite which is a marine invertebrate (think woodlouse that lives on the bottom of the sea) that lived between the Cambrian and the Permian.
Thin Sections:
Next is thin sections. Thin sections are bits of rock cut into really fine (30 micrometre thick) sections so you can shine light through them and be able to examine them under a microscope. In microscope practicals you learn how to identify key minerals and fossils in thin section. You learn what different minerals look like, their properties and what this can tell you about how/where a rock formed. Thin section work is useful for specimens brought back from the field as you are able to give them a much more in depth look. The microscopes we use in the lab can magnify between x4 to x40 resolution so you can see very fine detail, much more than you would be able to with only a handlense.
Here are some examples of thin sections from the lab (taking a photo down a microscope is actually much harder than you would think) and what they can show/tell us:
Looking at a thin section of an igneous rock. The image on the left is the sample in XPL (cross-polarised light), the middle image is my incredible sketch (don't worry I've had more practice since then), and the right image is the same sample in PPL (plane polarised light).
In labs we learn how to use a microscope and to identify features in both plane and cross polarised light.
Above are a bunch of fossils seen in thin section under the microscope at a range of different magnifications. Starting from top left going across the top row there are large benthic foraminifera, an echinoid (sea urchin) spine and some green algae. Left to right on the bottom row is a bunch of worm tubes, a gastropod shell (the icecream shaped thing) and lastly some red algae.
Being able to identify these types of fossils can tell us about the environment in which the rock we are looking at was deposited. For example if we can see algae that we know was marine then the rock must have been formed from deposits in the sea.
Above we have some thin sections from igneous (first two) and sedimentary (last one) rocks. They all have the mineral plagioclase feldspar (commonly called plag) which is one of the first ones you learn to identify because of its distinctive black and white stripes (lamellar twinning) when viewed in XPL.
The bright coloured minerals in the first image all have a higher birefringence which is what is causing them to been brightly coloured in XPL.
The last image has a few different minerals in. It is an example of a sandstone seen under a microscope in x4 magnification. If you look closely you can see a striped crystal of plag near the middle.
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