In the bleak mid-winter...

Hello all,
There's one type of crystal that's been at the forefront of everyone's mind over recent weeks - at least in the UK. I am, of course, talking about the glorious hexdendritic dihydrogen monoxide. Crystals are normally things you have to go out looking for to find, but not at the moment. You could hardly avoid them, whizzing over your head in powdery ballistic spheroids, or joining forces in a bold attempt to stop the trains going anywhere.
Of course, I'm talking about that delightful thing called snow. When you see satellite photographs showing the entire country covered by a white blanket, you get a sense of the scale of it. Crystals, crystals, everywhere...
So does ice count as a mineral? It seems like a tricky one, at first. According to the International Mineralogical Association's official definition, a mineral is a solid, chemically homogeneous and normally crystalline substance that formed geologically. Well, ice is certainly solid (when it's not melted, of course), and it's certainly a homogeneous crystal... in fact, ice does fit all the criteria.
It turns out that ice is indeed defined as a mineral by the IMA. It could hardly not be, really. If it melted at a couple of hundred degrees, instead of zero, then you'd never think to question it (it wouldn't rain either, of course, and there wouldn't be any oceans, tea, or indeed living things; so you'd probably have better things to not worry about). Where was I? Oh yes: ice. It even forms familiar types of crystal growth... here are some stalactites, for example:

Ok, so snowflakes are a bit weird, though. It's said that every snowflake is unique, and it's probably true... but the shape still follows simple rules. Like apatite, beryl and wurtzite (I like that one!), ice has hexagonal crystal symmetry. It tends to grow more quickly at the tips of the hexagon, though. Each spine often branches repeatedly, leading to the typical snowflake shape:

You can see immediately it's not a random branching pattern, because it's the same on each spine. So what controls it, and why is each one unique? It's all down to the conditions it forms in. When atoms join onto the tip of a crystal, the place they attach to depends on the exact humidity and temperature. Under some conditions, the crystals will form almost perfect hexagons (although I've never seen these myself):

If the temperature is quite cold where they form (-3 to -8C), it's common for all except one or two rays of the crystal to not form at all. I was a bit surprised to find little snowy needles falling on me the other day, so I'm very glad to find out what was happening.

The normal pattern that we see is called a dendrite, from the greek word for tree - it branches. (People often think scientific words are really high-brow, but it's really not true; you just need to look at species names, like Horridonia horridus... scientists really do know how to have a bit of fun when they can get it!) Dendrites are common structures in crystals, whenever it grows mostly in one direction, and occasionally branches. Here's some pyrolusite (manganese dioxide) I found earlier.

Anyway, back to ice. It would be much more obvious that we should think of it as a mineral if we had, say, pebbles of ice in our rivers. Well, if you go to Titan, the moon of Saturn, that's exactly what you get - pebbles and boulders of water ice, in ethane or methane rivers. Water can even form the solid surface of some planetary bodies, and I must confess I'd really like to visit some of them...

Take Europa, for example - a fabulous place. The crust is made of water ice, probably with a deep ocean underneath. Where the crust has cracked, more water has frozen in the gaps, and some of this is coloured brown. What does that mean? Probably that there are minerals dissolved in the ocean, and circulating around with it. And where there are minerals and water, there might be life. A pet rock from Europa would really be something... I wonder if I could keep it in my freezer?

Thanks to NASA for putting these pictures in the public domain. I must go and do some more about putting my exhibition in the public domain too, now. I've just got hold of some flint to try my hand at knapping now... which reminds me, I must go and buy some plasters. Toodle-pip!