Bismuth, the most fabulous of metals.

That's the kind of thing I want to show people who are all like "Nature doesn't have any hard lines or angles, only smooth curves"

That is epic.

what i said up there^^^
alien technology and whatnot CALL THE DOCTOR

How does it do that and why?

When cooling from liquid to solid state, some materials form what are called "dendrites", which are crystalline structures that vary in shape depending on the material. This phenomenon occurs because heat is being drained from the material on the surface and therefore cools the walls down faster. Once the first bits of material on the wall cool and become solid, they become a surface to which other molecules can attach themselves to form a solid. And since some materials have a specific configuration in which the molecules can be packed together to form the tightest fit (called a crystal matrix), they will naturally bond in that form to minimize potential energy. And since they formed a crystal which has "slots" where more molecules can attach themselves, the crystal grows faster in specific directions where the slots accept a molecule with more ease. The result is what you see in the post.
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Pic related, its dendrites forming in water (snow flakes are just dendrites made of water).

You're almost correct with your explanation, I'll just stick with the basics (or try my best) to explain it though. Dendritic growth in only one mechanism of growth in solidification. With dendrite growth, it is a localized concentration gradient at the dendrite surface that gives rise to constitutional undercooling which gives the temperature gradient you mentioned. In the case of a meteorite, the widmanstatten structure is a lamellar formation in the Fe-Ni phase diagram that occurs under ungodly slow cooling rates which yields diffusion along lamellar boundary giving the dark and light colors in the meteorite (caused by different compositions of iron and nickel). Also, when solidifying, atoms do not always pack in the tightest form, iron is notable for transitioning between FCC and BCC at really unusual temperatures. The direction and length of crystal growth is a balance between the energy of the system and energy change by formation in a given direction because crystal surfaces are less stable than one at the center. If you have any questions let me know!

Overall, I'm very impressed with what you know about solidification, do you study this in school?

Well, you're right about the meteorite however i was addressing the gentleman's question about the growth of the bismuth crystal seen above. I could be wrong about that too but i tried to make the best i could out of my course in metallurgy and crystallography (that mostly covered materials and alloys commonly used in industry). Of course, since i was explaining in layman's terms, i did away with some of the background in crystallography, alloys and the energy principles in crystal lattices as you have pointed out. Since I am a mechanical engineering student, i would be delighted to hear your explanation on the formation of large crystals in bismuth.

It's all good, I got excited when I finally found someone who knows about this stuff. Plus, I think diffusion won this discussion anyway. Bismuth is a pain in the ass cause it is thought to have at least 5 growth morphologies in its peritectic reaction and a pretty awesome rainbow oxide coating that messes with diffusion. So in the end, friedgreenpomatoes said it better than all my comments combined

For future reference, Necrotising = Cytotoxic and the anti-coagulant venoms are haemotoxic. They're technically classified into Neurotoxic, haemotoxic and cytotoxic.

Most species of vipers have haemotoxic or cytotoxic venoms, most elapids (cobras, coral snakes, brown snakes, taipans etc) have neurotoxic venoms, although a couple of species of cobras have cytotoxic venoms, too.


Hope you learned something today.

**collegedood rolled a random image posted in comment #115 at cactus **
Damnit doooublebaass! I'm an engineer, not a zoologist!

Nor am I, I keep snakes as a hobby. I plan to move into venomous snakes soon, so this is all the sort of stuff I need to learn about.

Let's cut the shit here, all we need to know is when we get bit by a snake we're fucked.

Not really. Most snake bites don't even need hospitalisation (I say need because it's still recommended) - the more famous species, like Western Diamondbacks, Fer-de-lance, Monocled cobras etc can do some damage, but in most cases antivenom isn't even DJ 4DM1Nistered. All you need is to watch your heart rate, blood pressure etc.

It's cytotocxic bites you really want to watch for. I think the venom used in the picture is from a B. Atrox, mostly because they use one of the enzymes in the venom to test how well someone's blood coagulates.

That is an understatement.