Snowflakes Revisited: Why No Two Alike?

General Questions
#1

We’ve discussed that the crystalline structure of the snowflake is determined by the lattice structure for the crystal of water in the solid form. Yet, there is only one lattice structure for the water’s solid form (or is there*?). So, why should each snowflake be unique? I can only WAG that somehow the flight path of each individual flake is unique and impacts the “integrity” of the crystal’s structure - shaping the crystal this way or that - and hence making each flake unique. Is my WAG close to the scientific theory on this? - Jinx

*Extra Credit: Basic chemistry courses would tell us that the there is only one lattice structure as determined by the molecule’s structure. In contrast, a detailed weather article from Scientific American presents 7 regimes (IIRC) of conditions which determine 7 different geometries for a crystal of H2O. …hmm, how does chemistry explain why there are these regimes?

#2

I am in Structural Biology - specifically Crystallography - & grow crystals everyday. But even so I’m still just speculating (sorry if I’m repeating the other thread responses) - as snowflakes form outward from one H2O molecule, the added molecules will randomly become nucleation points for new H2O molecules. This randomness generates a different snowflake everytime. Keep in mind that the average protein crystal has ~10^15 moleucules (and this is very small, hardly visible to the naked eye), so I’d guess there’s >10^20 molecules of H2O per snowflake. That’s a lot of randomness.

#3

Bob55, I must WAG that (once again) high school and Freshman College Chem 101 have over-simplified the concept of the lattice structure, or has it? Why then do all salt crystals in my salt shaker look alike? - Jinx

#4

Not every snowflake is unique.

#5

And i believe that somewhat similar logic lies behind the idea of fingerprints. We have no way of knowing that no two are alike; rather, we assume that the likelihood of finding two alike is so small that it is a reasonable way of identifying individuals.

#6

I confess that I’ve never quite seen why the six arms are the same on a given snowflake. I know the explanation: “the six arms experience the same growth conditions, therefore grow the same”. But what about random defects?

#7

Jinx, there are 232 ways crystals can be ordered in a “spacegroup”. Your salt crystals always form the same spacegroup though, being a sodium surrounded by 6 chlorides, and each chloride surrounded by 6 sodiums.

in Protein Crystallography, the properties of proteins reduce the number of spacegroups to 65 (like the fact that only L-amino acids are used in proteins, and other symmetries). A spacegroup is where proteins sit in the crystal. Some are “cubic” meaning the protein sits at each corner of a cube (and there are 10^15 of these cubes), or rectangluar. Others are tetragonal, orthrombic, hexagonal, monoclinic, triclinic and rhombohedral. Each can have different angles and lengths. Cubics can also have a 7th protein, sitting right in the middle of the cube. So you can see how quickly space groups can add up. Here is a good explanation, where they also discuss the 14 Bravais Lattices. I couldn’t find any good images, but here is an example (.pdf).

Regarding snowflakes, in my WAG I figured water, like salt, could probably only form a crystal in 1 space group. Therefore my reasoning was that gaps in the crystals had to be missing due to randomness, making each snowflake unique.