It's been a few weeks since my last post, and this is because I've spent 4 weeks "in the lab". Remember the 60kg of rocks I brought back from Greece? I've got to actually do something with those rocks now rather than just looking at them in boxes on the end of my desk.
So what do us geologists mean when we say we're "in the lab"? These last few weeks I have been crushing, milling, igniting, weighing, melting (using a flame thrower!), pressing, getting equipment stuck, and now I have 42 pressed pellets and 42 fusion beads!
What exactly is the point of all this, what am I trying to achieve? With this run of being "in the lab" I am preparing my rocks for XRF analysis. XRF stands for x-ray fluorescence. Elements have a really fun and useful property, when you fire an x-ray at them, each one will emit a specific wavelength of secondary x-ray, which we can measure using fancy equipment. We can measure the wavelength and the amount of secondary x-rays that the material gives back, so we can find out what's in it, and how much. However you can't just stick a lump of rock in the XRF machine, as the goal is to find the overall average composition of the material. This is the bread and butter of geochemistry, finding out the average "whole rock" composition, which (in theory) tells you the composition of the lava/magma before it cooled and crystallised into a rock.
If you remember from my previous posts, rocks are made up of minerals, and if you fire an x-ray at a mineral, you'll just get the composition of that mineral, and not of the entire rock. Somehow we have to average out the rock, so that we bypass this mineralogical effect. This is where the crushing and milling and pressing and melting comes in. For all of this work we wear lab coats and varying PPE, feeling like a proper scientist! The first step is crushing the rock sample, using a flypress (weighted, big, clumsy machine, when you spin the handle the crushy bit goes down):
With the flypress I crushed my rocks to a reasonably small level, as shown below, but this still isn't enough.
The crushed rock then needs milling, which is basically like milling flour. It comes out of the mill all warm and fluffy. The mill is a machine that spins the crushed rock around really fast with some agate balls (hard and don't shed anything), and it makes a lot of racket, especially when you don't tighten one of the holders in correctly.
After it's been milled down to a fluffy soft powder, two different lots of prep are required, fusion beads and pressed pellets. The fusion beads are for "major" elements, things like silicon, potassium, sodium, and calcium, elements which make up the majority of Earth's crust. The pressed pellets are for "trace" elements, elements which make up less than 0.1% of the Earth's crust, things like rubidium, strontium, copper, zinc, and cobalt (but there are many many more).
Let's start with pressed pellets. These are used for trace element analysis, because for trace elements, the mineralogical effect which I talked about earlier doesn't have as big an effect on trace elements as it does on major elements. Because they are "trace" elements, there aren't as many of them as there are of the "major" elements, so more material is needed for analysis. Pressed pellets are made in (you guessed it) a press, which exerts 20 tonnes of pressure on your milled powder which is mixed with a bit of glue (which has no trace elements in it). And out pops a pellet! Unless you get it stuck of course, which happened to me in several different ways on several different occasions. But they all came out in the end.
Fusions beads next. This is where the flame thrower comes in. Fusion beads are for "major" elements, and this is because using this method we can really get rid of the mineralogical effect completely. This is by melting the sample and cooling it quickly, too fast for any minerals to form, a bit like volcanic glass.
Firstly, the milled powders need drying out in the oven (only at about 100 degrees), as we don't want to be measuring any stray water that's got in the powders.
The powders then need "igniting". Which basically means heated up until just before they melt, which gets rid of the water which bound in the rock itself, and measuring how much water we got rid of. This furnace is a lot more terrifying than the first oven, 900 degrees is no joke! Absolutely terrifying. But I did it, and didn't light anything on fire. When you "ignite" rock powders, they change colour, so they're no longer the pretty colours that you'd want to make paints out of.
After I survived the fiery oven of death, I had to weigh my powders to 4 decimal places. You know how hard it is to measure exactly 75g of pasta, or 500g of flour? This involves weighing exactly 0.8000g of rock powder. Not 0.8002g, or 0.7998g, EXACTLY 0.8000g. Not an easy task, especially when the breeze from the open window (covid ventilation) can have an effect on the scales.
Once I'd painstakingly weighed out all of my powders, I could then finally use the flame thrower! You used to have to do it by hand, but now they have a fancy flame thrower that does it for you. It melts, mixes, pours and cools, all in one go.
Once the fusion beads have cooled, they're ready to come out of the flame thrower and get labeled up, ready for analysis!
And there you have it, all prepped for XRF analysis, just need to wait for the part to come for the XRF machine so that it can be fixed. This happens a lot with lab equipment, things break, parts need to be ordered, engineers need to come out, there's always something.
Next time I'll delve you into the world of thin sections and polarising microscopes, as I've now got some thin sections ready to go!
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