On Saturday, October 10, 2020 at 4:52:43 AM UTC-7, Gene Filatov wrote:
(snip)
Post by Gene FilatovCan't believe I'm that stupid... :) 60 degrees.
Gene
Looking along a cube diagonal, a cube has hexagonal symmetry.
If you pack spheres into a single layer, they form in equilateral triangles,
and the layer has hexagonal symmetry.
If you stack such layers, such that spheres on each layer go into the
spaces between spheres on the layer below, there are three ways to
stack each over the previous layer. If you name those three A, B, C,
then the stacking patterns can be named.
If you stack A, B, C, A, B, C, etc. the result has hexagonal symmetry,
and the crystal form is named HCP for hexagonal close packing.
In this case, the layer spacing can be, but isn't required to be,
such that the spacing between adjacent atoms on different layers
is equal to that within a layer.
If you stack A, B, A, B, etc., the result is face-centered cubic.
This is the symmetry of looking at a cube along its diagonal.
Cubic crystals have the same index of refraction in any direction.
Others don't, and are then birefringent, which causes different
light polarizations to have different index of refraction.
Cubic zirconia is used for jewelry instead of the alternative,
hexagonal form, as it isn't birefringent. Diamond has
cubic symmetry, but it is possible to put carbon atoms
together in a diamond-like structure with hexagonal
symmetry.