Lattice structure of corundum

Jewelry Discussion
1

I am trying to find the lattice structure of corundum and not
having much luck. I’m after this for molecular
modeling but cannot find any references for corundum. Can you
point me in the right direction?

Regards,

Vik Olliver

2

Hi Vik, Corundum’s lattice structure is HEXAGONAL Ref: Simon &
Schuster’s Guide To Rocks & Minerals (1978) Regards…Bob
Williams

3

Vik, Space group Rbar3c; a(rh) 5.120, alpha 55deg. 17 min; a(o)
4.751, c(o) 12.97; c(o)/2 : a(o)=1.365:1; Al4O6 in rhombohedral
unit. From Dana 7th edition. Try the news group
sci.techniques.xtallography for more info. Will Estavillo

4

“I am trying to find the lattice structure of corundum”

Vik, the crystal system for Corundum is the trigonal system
(rhombohedral system). The axes and angles are similar to the
hexagonal system and are sometimes combined into one system.
The difference is symmetry. In the hexagonal system, the cross
sections of the prism base is six sided. In the trigonal
system, they are three sided. The six sided hexagonal shape is
formed by the cutting off process of the corners of the
triangles. Typical crystal forms of Corundum are hexagonal
prisms, hexagonal pyramids and rhombohedrons.

Ref. Walter Schumann, Gemstones of the World, Sterling
Publishing Co., New York, NY, 1997

Charles Heick

5

Howdy Vik, Although I’m fairly ignorant about this subject, I
have a few links bookmarked (due to research on COE of facetable
materials) which may help you or lead you to other links.

	http://www.iumsc.indiana.edu/html
	
	http://ruby.colorado.edu/~smyth/min/spinel.html

	http://minerals.gps.caltech.edu/index.htm

	http://www.micro.magnet.fsu.edu/birthstones/index.html

Carl
1 Lucky Texan

6

Vic, a little more info, from “Gems made by man”, Nassau, 1980n
the bits enclosed by [[double brackets]] are my additions to the
quoted text.

"StructuRe: the (alpha) Al2O3 corundum structure is based on a
near hexagonal close packing of oxygens, which occur in sheets,
as shown in figure 8-9 [[sorry, you’ll have to look this up
yourself if you need the diagram.]] . Between the oxygen sheets
there are somewhat distorted octahedral holes, and two out of
every three of these octahedra are occupied by aluminum in a
regular pattern. Each Al is thus surrounded by six O.

Crystal symmetry: Hexagonal, class [[bar over the 3]] 32/m;
space group R3c [[bar over the 3]]

Optical properties: Refractive index o = 1.770, c = 1.762.
(green sapphire may be up to .01 higher); birefringence 0.008,
dispersion 0.018; unizxial negative.

Hope this helps a bit more.

Peter Rowe