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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
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CLOSE PACKED AND A ATOMS OCCUPY OCTAHEDRAL INTERSTICES |
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Corundum |
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click here to see a key to this data bar
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octa |
2/3 |
hcp |
3-d |
1
face, 3 edges |
A-
6c, X- 4c/hcp |
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structure |
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In the corundum
structure metal atoms (A) occupy 2/3 of the octahedral interstitial |
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voids of each successive HCP packed layer of
oxygen atoms (X). |
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Ex. Al2O3 (
corundum ) ( Al - A, O - X ) |
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Crystal lattice |
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The lattice
demonstrates that the occupied AX6 octahedral coordination groups
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form hexagonal shaped rings that surround a
central unoccupied octahedral void. |
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The hexagonal nets of successive layers stack
together in the HCP arrangement so |
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that two occupied AX6 groups overly
each unoccupied void in the structure. This is |
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thought to permit the two cations of these
overlying groups to move as far apart as |
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possible to reduce electrostatic repulsion. |
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Figure 77 - Corundum lattice |
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click image to enlarge |
structure |
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The 2/3 occupancy rate
of the octahedral voids can also be seen. The AX6 groups |
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share three edges and one face with adjacent
groups. |
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Polyhedral framework |
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linking surfaces |
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Figure 78 - Corundum polyhedral |
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framework model |
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( 112 T, 144 pinges) |
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1
face, 3 edges |
click image to enlarge |
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The polyhedral
framework model shows a hexagonal ring of AX6 octahedra on |
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the bottom with sections of the rings of the
next two layers on top. It should be noted |
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that the coordination polyhedra in the actual
crystal structure are slightly distorted |
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from the ideal shown here (approximately 5% for corundum
and 10% for hematite). |
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Cleavage |
Crystal habit |
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none |
prismatic |
hexagonal |
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prism |
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Isostructural minerals/compounds |
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a-Fe2O3,
a-Cr2O3,
a-FeTiO3,
LiNbO3, MgTiO3, Mn(Fe,Sb)O3, Cr2O3,
Ti2O3, V2O3 |
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Page
52 - Structure matters - Corundum |
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