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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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Boehmite |
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click here to see a key to this data bar
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octa |
50% |
fcc |
layer |
6
edges |
A-
4c; X- 4c, 3c, 2c/fcc |
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structure |
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The boehmite structure
consists of layers of FCC close packed O and OH ions with |
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cations occupying 100% of the octahderal
interstices which alternate with unoccupied |
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layers for an overall occupancy rate of 50%.
In this respect it is similar to the CdCl2 |
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structure except in the boehmite layer the
occupied AX6 groups share a different set |
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of edges resulting in a unique "corrugated"
layer effect. |
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Ex.
g-AlO.OH
( aluminum oxyhydroxide, boehmite ) - high temperature
polymorph |
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Crystal lattice |
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The FCC crystal lattice
model demonstrates that two rows of adjacent AX6 groups |
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are linked together by their shared edges and
alternate with two rows of unoccupied |
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octahedral groups of X atoms to form layers
parallel to the (111) plane. These rows |
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in each successive layer are shown to be
staggered relative to each other. |
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Figure 81 -
Boehmite lattice structure |
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click image to enlarge |
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Polyhedral framework |
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linking surfaces |
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Figure 82 - Boehmite polyhedral |
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framework model |
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(132 T, 163 pinges) |
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6
edges |
click image to enlarge |
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The polyhedral
framework shows the corrugation effect resulting from the |
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staggering of the rows of AX6 groups.
The corrugated layers are parallel to the (100) |
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plane. The OH ions are located on the
ridges of the corrugation (where the black |
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bracing panels are connected to the octahedra).
The OH ions of adjacent corrugated |
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layers form O - H - O bonds with each other as
indicated by the green pinges. The |
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oxygen ions occupy all of the other vertices of
the framework. In reality the |
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octahedral groups are slightly distorted and the
separation distance between the |
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corrugated layers is slightly greater than
shown. |
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Cleavage |
Crystal habit |
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(100) - perfect (corresponding to the |
tabular |
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direction of the corrugated layers) |
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Isostructural minerals/compounds |
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g-FeO.OH,
g-MnO.OH,
g-ScO.OH,
FeOCl (with Cl substituted for OH) |
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Page
54 - Structure matters - Boehmite |
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