(Pentagon-)Dodekaeder Gold

name         Johann
status       other
location     Germany

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?
---------------------------------------
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.

cheers,

Michael

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.
---------------------------
Michael S. Pierce
Materials Science Division
Argonne National Laboratory
====================================================================
addendum April 2008
====================================================================
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
====================================================================