Thursday, 31 December 2009

Ho Ho Ho!

Dear readers one and all,

A Merry Christmas (slightly delayed, but what's a fwe days to a geologist?) and a pre-emptive jolly New Year as well. It should be quite a year, too... it's possible that I'll finish my gallery, and can finally present it to the world (well, Leeds anyway).

The currently bald sheep. Poor thing.

In the spirit of general festivity, what better things to give to one's dear ones than some sparkly things? Diamonds, I hear you ask? Rubies and sapphires? No, don't go giving her ideas. A woolly sheep will do for starters. For, you see, my dear Dr. Sock is a dab hand with the old knitting needles, and has been threatening to get a sheep for ages. In an attempt to forestall this, I've got her one first, and yes the wool really does grow. Although the poor thing is a touch on the chilly side at the moment... I'll let you know how the fleece grows, and how she gets on with the knitting with it.

For my dad, what better way to get him to take out the geological hammer than with geodes? These wonderful little things are sufficient to assuage anyone's gambling urges, and at the same time... well, they're rocks. Can't say fairer than that.

You see, you can never be sure what's inside a geode until you look. We know how they form, but it's all a bit of pot luck as to whether you have a good one. It all starts with bubbles in lava. Volcanoes erupt because of bubbles - it's the air pockets that makes the magma light enough to rise up to the surface. Once it gets there, the poor lava finds that it should have brought an extra woolly jumper, and promptly freezes - trapping bubbles in mid-escape.

Over the years (and lots of them), water filters through cracks, gets heated by the volcano, and deep underground, strange things begin to happen. First, bits of the solid lava start to dissolve (especially the quartz). Some time later, when the water cools down in the crack and the bubbles, it starts to de-dissolve... and hey presto, a geode! All ready for the smashing.

What you find when you crack them open are normally layers of crystals (often coloured quartz) growing in from the edges. Sometimes the crystals are too small to see, and you just have coloured bands of chalcedony (agate) instead. The best ones are hollow in the centre, so you can see the crystal shapes pointing inwards, but quite often they're solid. The colours are due to other things dissolved in the water, which formed impurities in the growing crystals - iron, magnesium, uranium, kryptonite...

So how did he do? Well... it could have been worse. The first two were virtually solid, but you can at least see layers of blue-grey chalcedony, surrounded by less pure brown stuff. It looks like the second geode has been cracked while forming, with the clearer bit in the middle forming later. The third one was better - almost clear blue-grey chalcedony with a hollow centre coated in tiny quartz crystals. Not bad for a beginner.
Of course, to get the more interesting crystals is more of a challenge. Quartz is all very well, and it's vital for our technology (that's where silicon comes from) - but the stuff in these geodes just isn't pure enough to be useful. The only reason for collecting these geodes is because they look pretty. There's nothing wrong with that, of course - it's how most people start getting interested in minerals. On the other hand, it's not going to solve all humanity's problems. You need lots of different minerals to do that...

1 comment:

  1. Diamonds? Not even really useful for hardness tests, as almost everything is softer... A sheep is much more useful, although there might be a few practical problems in knitting with something that tends to dissolve!

    Dr Sock