Wednesday, 31 October 2012

How Science Works: seeing through rock

Amendment to the last blog: Marcello Ruta is now at the University of Lincoln.

Last week I was back in Leicester sorting out the samples that we collected in the field. It was not the most exciting week, mostly consisting of re-boxing the 250 rock samples and answering lots of emails. At least I was away from the wet weather for a while… but not for long, as now I am back in Scotland in the field! So a short blog this time about what we do with all the rock collected.

The most simple method you can use to study sedimentary rock samples is to ‘cut and polish’ the rock. What this means is that you cut the sample with a rock saw and then polish up the surface you have cut, giving a shiny, polished rock surface. The finer the rock polish you use the more detail you can see. 

How to see through rock?
The rock below is one we samples in August 2012, to give an example of this method. 

Step 1. Take a sample of the rock. Here you can see the rock in the field and in the lab. It is a grey/green sediment that is well cemented and looks pretty uniform. This type of rock in the Ballagan Formation is called a cementstone.

Step 2. Cut the rock in half using a rock saw. Don’t do this at home! Here is the outside of the rock once it has been cut in half, again not much to see.

Step 3. Polish the cut half. Here you can see the polishing wheels we use in Leicester, along with different size grades of polishing powder. The powder is a synthetic carbon-silicate called carborundum, which is very hard (hardness of 9 on Moh’s scale). It takes at around an hour to get a good polish on a rock sample.

Step 4. Photograph polished rock surface. This is best done on a scanner at high resolution. Now you can see the rock interior.
The result: In the field it is hard to tell what it is made of as it all looks very much the same, and quite boring. But when we have cut into it, you can see a lot of detail. Some of the features revealed in this sample are 1) a grey/green colour change from base to top, the boundary of which is burrowed or rooted (see the close up picture to the right); and 2) a muddy rock at the bottom of the sample, which may contain microfossils. There are also some as yet unidentified crystal filled voids…. And none of this was visible before the cutting and polishing, so a great result! 

What is the next step?
I will be using the cut and polished rock surfaces of complex specimens as a guide to select which parts of the rock are most interesting and will merit further analysis. The next step is to move onto making thin sections (thin slices) of the rock, which give another, more in depth level of detail. You can then study the thin sections using various microscopes, the most powerful of which is a Scanning Electron Microscope. We have some preliminary thin sections of our samples, which have come out really well. I will blog more about the thin sections and how we analyse them in future posts.

Until next time

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