What do we do when we look at rocks? This post is about the three days I spent looking at rocks in the lab, one of the inspirations behind starting this blog! Before I started my undergraduate degree, I could stare at a rock for as long as I liked, and still come up with: rock. I had my epiphany that my degree had actually taught me something when I went back to Guernsey (where I lived between the ages of 10 and 16) after completing my degree, and I could see things in the rocks! I could pick out faults and dykes and xenoliths and all kinds of weird and wonderful geology words, just by glancing at the cliffs. A big change from: rock.
I spent 3 days last week staring at the rocks I collected on fieldwork last month (from Greece, a post about this later). There are various modes of staring at rocks: When you're in the field in blinding sunlight/pouring rain (depending on whether you're in Greece or Scotland), possibly having forgotten your hand lens (although mine is attached to my compass so I don't lose either of them); down a binocular microscope (just a regular microscope) in the lab; with the naked eye and squinting a little; and using a handlens in the lab (I will explain why this is different to in the field in a bit). You can even make a really thin slice of a rock and stick it on a glass slide ("thin section"), and look at it under a petrographical microscope (this is a whole other post).
In the field: it's good to look at your rock in the field and make some notes about what it looks like and where it came from, because you will not remember anything you're thinking about at this exact moment when you get back from the field. It also comes in handy when you have a minor sample mix up, you can match your sample to your notes (within reason, some all look VERY similar). I will elaborate on fieldwork in another post.
Now we're in the lab, and there's 60kg of rocks to wade through. This equates to 42 samples in my case. Where do we start? It might seem like we're just duplicating the notes we made in the field, but this is not the case. It might be 30 degrees, pouring rain, or you've hiked 17 miles that day and you're completely done, so your field notes might not be of the best caliber. Not to mention that rocks look different depending on which way the wind is blowing (sometimes literally). Remember that scene in National Treasure 2 where they're pouring water on the rocks to find the hidden symbol? This could be legitimate in theory. When you pour water on a rock, it changes completely. Sometimes the water vapour from your breath is enough, so you don't have to waste water from your bottle or create a flood in the process. I know you're all wondering, do we lick the rock? For health and safety reasons I am going to have to say, no, DO NOT LICK THE ROCK. It literally could be anything, radioactive, a dog could have peed on it, and it is almost never salt (if you're by the sea every rock will taste salty, definitely not from experience). If you're struggling to see anything in the rock, then breathe on it or something, and things might start jumping out (probably not hidden bird symbols though). Another factor which affects how you see the rock is the light conditions (this is why looking through a handlens in the field and in the lab are so different). Nothing is quite as good as nice natural light (although not blinding 30 degree sunlight, that is definitely a hindrance). In the artificial lights of the lab, rocks will look ever so slightly different to when you were in the field, some more so than others. And they definitely all look different when there's a power cut (this happened last week, one of my colleagues was operating a rock saw when the power went out, but he is still in possession of all of his extremities). If the rock has lots of lichen or a horrible weathered (starting to crumble and looks different to the rest) outside bit on it, or is just being irritating to look at, get that rock split in half (remember the rock saw?). Rock saws are great fun, they don't cut skin! Just rocks. And fingernails. Getting it cut in half gives you a smooth and fresh surface, so you might be able to see more minerals (or still not be able to see anything).
So what exactly am I looking for in my rocks? Colour is a good place to start; what is the overall colour of the rock? Red? Purple? White? A vague shade of dark grey? Next we move onto minerals. You probably want to go to the binocular microscope or hand lens for this, as they tend to be 1-5mm in diameter (in the rocks that I'm looking at, the granite in the photo below has much larger minerals), sometimes more, sometimes less, all still fairly small. All rocks are made of minerals (quick definition: a fixed crystal structure and fixed chemical composition), some (hopefully) more well known examples of minerals are: quartz, garnet, tourmaline, pyrite, malachite, but not opal or agate (no fixed crystal structure!), although these do play a part in forming some rocks. What minerals can you see? If you can see any at all; some rocks are too fine grained to identify minerals with the naked eye. Minerals in rocks rarely form the lovely crystal shapes that you find in rock shops. Quartz for example, won't have a nice pointy end to it, it'll just appear as a mostly transparent blob. Some minerals, such as amphibole and pyroxene (two common rock forming minerals) are difficult to tell apart in hand specimen (they're both dark and often elongate, honestly who knows), so I just write: dark elongate mineral, possible amphibole or pyroxene. And that's great! You'll be able to know for sure what they are once you've got your thin section. You'll often find a white opaque mineral which is more often than not plagioclase feldspar (disclaimer, I'm talking about igneous rocks (lavas and stuff), not sedimentary rocks, I don't do those).
Now here's a fun subject to broach: cleavage. Some minerals have cleavage. Quartz does not have cleavage. Amphibole and pyroxene both have cleavage, but at different angles. Plagioclase feldspar has cleavage. Have I said it enough that you've stopped laughing yet? It took my cohort at uni a year and a half to stop laughing, so feel free to keep giggling. Cleavage is a good thing to use to aid mineral identification, as certain cleavages can be unique to certain minerals. What is cleavage? Cleavage is where a mineral will naturally split down a plane. To see cleavage in a mineral in a rock, you will often need to use a hand lens or binocular microscope. If you dropped a cube of galena (the really dense purple coloured mineral, lead sulphide), it will shatter into smaller cubes. This is down to the cleavage, galena has 3 cleavage planes, all at right angles. Hence, shattering into cubes. Struggling to work out whether you've got a plagioclase feldspar or a quartz? Does it have cleavage? There's your answer (I mentioned the cleavages of both of these minerals earlier if you were paying attention).
So you've noted the colour of the rock, you've identified (or had a vague guess at) your minerals, now what? Does the rock have any thing weird or special? Does it contain a blob of something else? If no, then great, move on, if yep, then write it down. We've looked at our rocks again in a different environment to the field, made notes, we're done, right? Incorrect.
Remember that weird day glow green sample that you found in a vein in a lava flow on that really sunny outcrop?
Too fine grained to see any minerals at all, just a crumbly day glow green rock or mineral of some description. This next part is the beauty of not working from home (not that I could do all of this from home anyway, but still). I wander out into the corridor to see who I can find who might have a clue what my day glow green mineral is. The first person I find, the collections manager at BGS: *surprised and curious noises*. "Is it radioactive?" - something I probably should have checked a lot earlier. Luckily, there's a radioactivity detector lying around, so he aims and fires, it is luckily not radioactive; "unless it's a pure alpha emitter, because this only detects beta and gamma". But pure alpha emitters are fairly rare, so we're probably safe. The next question which is fairly easy to check, is it a carbonate (think limestone, chalk type rocks)? Put a little bit of acid on it, no fizzing: not a carbonate (carbonates fizz on contact with acid, the reaction releases CO2 gas, which is the fizz). So now we're out of ideas.
I wander up to my office with my mysterious green rock, and one person suggests another person to look at it (geologists are always interested in mysterious rocks), and before you know it my sample is undergoing XRD analysis (X-ray diffraction, it shows you the crystal structure of the minerals you're looking at, and most minerals have a unique crystal structure (some have similar ones), so is a quick and easy way to identify minerals present). My day glow green sample is: a magnesium or iron rich smectite clay (yes clays are minerals), mixed with opalised silica. It's day glow green because of the magnesium or iron rich clay (their crystal structures are too similar to tell apart). And that's all there is to it. This sample will have formed from a hydrothermal (hot) fluid, busting its way through the pre-existing rock, and dumping stuff (like when sea water evaporates and leaves salt) down in the cracks it created. We know this because both the clay and the silica are known to be hydrothermal in origin, and the sample was found in a vein. What a wild ride.
Anyway, back to "looking at rocks". One of the most important thing about having 60kg of rocks to look through, is ORGANISATION. You absolutely do not want sample mix ups. If you don't know which sample is which, any data collected and any observations are worthless. All of my samples were in their own individual sample bags (hefty bois, you don't want a rock busting right through the side of the bag), the bags themselves labelled 3 times with a sharpie, and a flash card with the sample number on it in the bag with the sample. Sharpie rubs off of plastic bags, if you hadn't discovered this already. Each bag was folded over and stapled 4 times. Each sample has to have a unique sample number too, duplicate sample numbers do not do wonders for your spreadsheets. I spent most of the last day of "looking at rocks" just reorganising my rocks. Note: I did not have more than one sample out of its bag at any one time, recipe for disaster.
Checking, relabeling, resealing and boxing up and labeling boxes (of weights that I can lift, 7 boxes in total) for 42 samples is no small undertaking. And to top it all off, the lift in my office building is broken, so I had to carry each box up the stairs (lift with your legs not your back) to store in my office before I decide what to do with them next, which is another story. There you have it, now you know what us geologists do when we look at rocks!
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