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Wednesday, 18 January 2012

A close look at granite

Some rocks, like granite, contain quite large crystals that can be easy to see, once you take the rock apart.

To do this and see the bits, you will need well slides, some pieces of granite or similar coarse-grained igneous rock, cover slips, tweezers (to pick up the fragments), and a large sheet of paper to bang the rocks together over (mind your fingers!).  Where do you get granite?  Well, if you are like me, and pick up small samples of rock wherever you go, and keep them in a jar like this, you will probably have some granite in there. If not, you will have to go looking.

Here is a closer look at the granite bits:

These samples are quite probably granite: they both have lots of quartz and large crystals of other stuff, but I make no promises.  Let me just say that either of them could be taken for granite.

Still, banging the rocks together is just so Neolithic, but since we invented fire, there is a better way, but you will now need a good pair of tongs as well as the other stuff.  If you have a clean barbecue, you can heat the granite piece on that, and then lift it off with tongs and drop it into water.  That is the safest method of using heat on granite.

You can also heat granite held over the flame of a Bunsen or other burner in tongs and then drop it into a saucepan of water, but if you are a younger reader, this needs adult supervision because of the risks of fire and burns.  Have your adult stand by with a glass of water to pour on the granite if you drop it.  Sounds like a case for outside work, doesn't it?

Once the granite is cooled, lift it out of the water with tongs and put it somewhere safe, because the centre is probably still quite hot.  Think fires (unlikely), scorch marks, small children and pets.  Play safe!

Then take some of the small fragments and look at them under the microscope to see if you can identify different minerals, but it might be interesting to see if heat and mechanical action produce different results.  It might be worth looking at any effects of polarised light here, though that sort of work is best done with thin sections of uniform thickness.

Now here is a quote from the past to set you thinking about making your own thin sections.  I have never tried to do this, but that need not stop you.
.  .  .  in the field some amount of information concerning igneous rocks can be obtained by rubbing down the chip on a grindstone and using a whetstone, carborundum file, or water of Ayr stone for the final grinding.  By these and other methods .  .  .  there are obtained slices of rocks which, though thick, uneven, scratched, and all that is bad, from the point of view of the professional maker of thin sections, are nevertheless capable of yielding much information.  With a pocket lens it is possible to make out from such a 'thin' section the nature of the minerals present, the texture and the nature of the rock.
Frank Rutley, Elements of Mineralogy, 22nd edition, 1915, p.  104.
If you have access to high power magnification, why not try a few other rocks?  Can you see crystals in basalt, for example?  (The answer ought to be "no", but why take my word for it?)  What can you see in limestone and sandstone?

Granite crystals form slowly, as magma cools over long periods, so the crystals have time to grow quite large.  Basalt is molten rock that spills out and cools fast, before large crystals can form.


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