MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X ATOMS ARE
          CLOSE PACKED AND A ATOMS OCCUPY OCTAHEDRAL INTERSTICES
 
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Corundum

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octa

2/3

hcp

3-d

1 face, 3 edges

A- 6c, X- 4c/hcp

.

  structure

.
. .
     In the corundum structure metal atoms (A) occupy 2/3 of the octahedral interstitial .
voids of each successive HCP packed layer of oxygen atoms (X). .
  .
Ex. Al2O3  ( corundum )     ( Al - A, O - X ) .
.
Crystal lattice
     The lattice demonstrates that the occupied AX6 octahedral coordination groups
form hexagonal shaped rings that surround a central unoccupied octahedral void. 
The hexagonal nets of successive layers stack together in the HCP arrangement so
that two occupied AX6 groups overly each unoccupied void in the structure.  This is
thought to permit the two cations of these overlying groups to move as far apart as
possible to reduce electrostatic repulsion.
.      
 

corundum.gif

 
    Figure 77 -  Corundum lattice

click image to enlarge

                     structure
   
 
.      
     The 2/3 occupancy rate of the octahedral voids can also be seen.  The AX6 groups
share three edges and one face with adjacent groups.
.
Polyhedral framework
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linking surfaces

corundum.gif

 

   Figure 78 -  Corundum polyhedral
                framework model
               ( 112 T,  144 pinges)

1 face, 3 edges

                       click image to enlarge
.      
     The polyhedral framework model shows a hexagonal ring of AX6 octahedra on
the bottom with sections of the rings of the next two layers on top.  It should be noted
that the coordination polyhedra in the actual crystal structure are slightly distorted
from the ideal shown here (approximately 5% for corundum and 10% for hematite).
Cleavage Crystal habit

hexagonal_prism.gif

 
     none      prismatic       hexagonal
              prism
     
Isostructural minerals/compounds
a-Fe2O3, a-Cr2O3, a-FeTiO3, LiNbO3, MgTiO3, Mn(Fe,Sb)O3, Cr2O3, Ti2O3, V2O3

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