Archive for the ‘Crystallography’ Category

So we recently had a ‘blizzard’ here where I am, and while my area didnt get iced, we did get beautiful lovely snow. As a budding crystallographer, I looked at the snowflakes, really big crystals and went, “OMG! Tropochemical twinning! Merohedral twinning! SQUEE SQUEE!” After realizing my inherent nerdyness, I stumbled upon to this website from Caltech talking about the crystallography, physics and chemistry of snowflakes. Utterly and completely fascinating. Everyone should check it out. I’ve been lucky enough to see a 12 sided snowflake. Even the three sided ones. I really dont think these are ‘crystals’ but probably more like quasicrystals that have the ‘forbidden’ 5, 7, 9 and 12 symmetry. Or it could just be twinned up the wazoo. Either way, it’s really interesting. Go read!



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So I put up that earlier post showing the Fourier transforms. For those of you who do know the answer, congratulations. You know the basics of symmetry! Crystals are said to be periodic. As such, they fill all space and have translational symmetry. 2, 3, 4 and 6 fold symmetry shows translation. If you look in the earlier post, then you’ll see that the pictures to the left have translational symmetry in the atoms, while the 5, 7, and 9 fold do not.

5, 7, and 9 fold symmetry does exist however, in the forms of quasi crystals. These are materials that are ordered, but nonperiodic in the short range, but have a long range order. They tend to produce X-ray diffraction patterns that are penrose tiles and all sorts of cool stuff. Quasicrystals can be solved by traditional X-ray crystallography, but for that you need to go into higher dimensional crystallography. That is another topic all together. And for now I’m tired so that’s where I’ll end. Huzzah!

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I’m a crystallographer in training. I love crystals. They’re pretty. Organikers love ’em too, since it’s another away asides from NMR to go through it. But asides from using SHELXTL or CRYSTALS, the math behind crystallography is lost on some people, so here’s an exercise.

So here are a bunch of pictures. To the left is an array of atoms and the right is its corresponding Fourier transform/x-ray scattering. Which ones are allowed by traditional symmetry? Yes, I’m going to talk more about crystallography, but I thought I’d put this out first…

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