Talk:Close-packing

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There are two regular lattices that achieve this highest average density.

There's actually a few other regular lattices that have just as high average density.

Let's start with a hexagonally-close-packed sheet of atoms (marbles, spheres, whatever): A. (See http://www.kings.edu/~chemlab/vrml/clospack.html for some pretty illustrations). Pack a second sheet B on top of the first. The third layer is where something very interesting happens.

  • We *could* line up the atoms in the 3rd sheet directly over the atoms in A --

and keep repeating A-B-A-B-A-B... . This gives us "hexagonal close packed" hcp.

  • We *could* line up the atoms in the 3rd sheet in so they *don't* line up with the atoms in A. Then we could make the 4th sheet line up with A, and repeat: A-B-C-A-B-C-A-B-C. This is "cubic close packed" ccp.

Are these the *only* possibilities ?

I vaguely remember hearing that some real crystals form such a close-packed lattice, but one different from hcp or ccp. Perhaps it was something like

  • A-B-C-B-A-B-C-B-A
  • A-B-A-B-C-A-B-A-B-C.

Anyone remember exactly what it was that formed those crystals ? If I knew its name, I could google for more information.

--DavidCary 05:10, 8 Jan 2005 (UTC)

Those are certainly close packed, but they're not regular - not all spheres within them are identical under symmetries of the lattice.


I'm having a hard time visualizing the difference. Does anyone have a good image to show it off?

The hexagonal lattice linked from here is not the packing intended; in terms of this article's notation it is AAA, less dense. --Tamfang 23:42, 15 January 2006 (UTC)

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