The Power of Five
Mar. 10th, 2009 08:23 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
threemonkeysIts funny what excites some people. If you are interested in the science of crystallography, any mention of five-fold symmetry in crystal formation is exciting stuff. The following certainly perked up my interest:
A team of UK researchers has grown ice crystals in which the water molecules are arranged into pentagons rather than the hexagons found in every natural snowflake.
The unusual ice was grown on a copper surface under a vacuum at -173°C. It owes its pentagonal form to the way the water molecules bond with the underlying copper.
The article in New Scientist is here.
If you are wondering why crystals with five-fold symmetry are so unusual, think about floor tiles. They can have 2-fold symmetry like rectangles, three fold like triangles, 4-fold like squares or 6-fold like a hexagon. Its a bit more complex than that because you can, for example, combine 2 triangles into a rectangle. Overall there are 17 different symmetry arrangements possible for tessalating tiles ("plane space groups") and none of them have 5-fold symmetry. Just try fitting a bunch of pentagons together without gaps and you will see the problem.
The same is true in three dimensions. So while individual molecules and living things can have five-fold rotational symmetry, it is very unusual in crystals. It has only been in the last 20 - 30 years that they have been found at all and new examples don't come along very often - never for something as simple as water ice. In science, new and unusual is exciting.
A team of UK researchers has grown ice crystals in which the water molecules are arranged into pentagons rather than the hexagons found in every natural snowflake.
The unusual ice was grown on a copper surface under a vacuum at -173°C. It owes its pentagonal form to the way the water molecules bond with the underlying copper.
The article in New Scientist is here.
If you are wondering why crystals with five-fold symmetry are so unusual, think about floor tiles. They can have 2-fold symmetry like rectangles, three fold like triangles, 4-fold like squares or 6-fold like a hexagon. Its a bit more complex than that because you can, for example, combine 2 triangles into a rectangle. Overall there are 17 different symmetry arrangements possible for tessalating tiles ("plane space groups") and none of them have 5-fold symmetry. Just try fitting a bunch of pentagons together without gaps and you will see the problem.
The same is true in three dimensions. So while individual molecules and living things can have five-fold rotational symmetry, it is very unusual in crystals. It has only been in the last 20 - 30 years that they have been found at all and new examples don't come along very often - never for something as simple as water ice. In science, new and unusual is exciting.