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Name: Johann
Status: other
Grade: other
Location: N/A
Country: Germany
Date: N/A


Question:
I have heard that gold can have the crystal structure of a (pentagon-)dodekaeder. Is this in a size eyes can see? Like a pyrite? Or is it an inner structure of an amorphous piece of Gold?


Replies:
I have only heard of this kind of thing in one instance and it only makes sense in one way: gold nano-wires.

There is no way to build something out of an ordered array of atoms (i.e. a crystal) that will have a repeating structure. It is just impossible to build up repeating arrays of pentagons. Anything pentagonal just does not possess the required symmetry to be built up over and over.

However, if there is only one pentagon in the entire cross-sectional area, then you are fine. A while ago people latched onto this idea and about a decade ago it was first experimentally realized (I believe). What you get is a long pentagonal block of atoms, a nano-wire with an area shaped like a pentagon, but that can stretch out with repeated pentagons along perpendicularly. In some cases pentagon is oriented in the same direction, but in other cases the pentagons are rotated by 180 degrees each time you add another 5 to the stack. There are also cases where there's an atom sitting at the center with 5 around it.

Sadly these are not visible to the eye as they are so small, but they are visible in experimental labs using x-ray scattering and other nano-science techniques.

One caveat ~ I have heard of Au pentagonal nano-wires being seen in a lab. And I have seen some of the theoretical papers that make predictions. But after a quick journal search I could not find any solid papers that indicated this. I do know that they have been seen with other metals such as Na and Al.

Second caveat ~ It is always possible to shape a polycrystal to the facets you want, and then to polish them nicely. So in principle you could make pentagonal facets on a piece of gold and make it appear (to the eye) to look like a crystal. In fact, because gold is such a soft metal, shaping it would be easy.

A couple of weeks ago I ran across some images of pyrite and was truly shaken by what I saw... pentagons! My first inclination was to believe them to be false (man-made/cut), but a little searching revealed more than enough to convince me that they were indeed naturally occurring. What I found even more surprising was that the pyrite was referred to as a crystal. How could this be?

In my earlier entry I say that you can't build up a regularly repeating array of atoms that keeps to a pentagonal shape. ie, there is no pentagonally sided unit cell from which to build up a crystal. This is true enough, but I failed to fully appreciate that the unit cell of a crystal and the actual shape of a bulk specimen can in fact be quite different. I had expected the symmetry properties of the crystal to directly transfer over to the large object (the crystal "habit" as it's called). This in fact is not always true and in fact the metal pyrite commonly forms crystals that have the pentagonal-dodecahedron shape despite an underlying isometric crystal structure. This is something I would not have expected!

I'm still not aware of gold ever crystallizing in this fashion, but it's interesting that the behavior is found in pyrite.

cheers,

Michael S. Pierce Materials Science Division Argonne National Laboratory

PS My brain shorted out for a bit and a friend reminded me of "quasi crystals." That is arrays of atoms can be made of an aperiodic (nonrepeating) structure from pentagons (and other things). There are, at least to my knowledge, a few instances of highly engineered gold quasi crystals (or rather gold-alloy quasicrystals). But I think these are also pretty small and not pure in substance.

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