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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES |
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Litharge |
click here to see a key to this data bar
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tetra |
50% |
fcc |
layer |
4
edges |
A-
4c/fcc, X- 4c |
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structure |
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Litharge is an uncommon
simple oxide. It has a unique structure derived from |
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the fact that the relatively larger cations
(A) are approximately close packed in a |
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layer structure rather than the anions (X).
One hundred percent of the tetrahedral |
interstitial voids of every other
layer are occupied with X atoms for an overall |
occupancy rate of fifty percent. These layers
are oriented parallel to the (100) plane. |
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Ex. PbO ( lead oxide, litharge )
( Pb - A , O - X ) |
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Crystal lattice |
The FCC lattice
structure of litharge is modeled so that the XA4 groups of lead |
and oxygen atoms are colored
differently than the unoccupied tetrahedral groups |
of lead atoms (blue). |
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Figure 62 - Litharge lattice structure |
click image to enlarge |
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The lattice model demonstrates that the occupied
XA4 groups form layers oriented |
parallel to the (100) plane that alternate with
layers of the lattice that have |
unoccupied tetrahedral voids. |
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Polyhedral framework |
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linking surfaces |
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Figure 63 -
Litharge polyhedral |
framework model |
(86 T, 99 pinges) |
4
edges |
click image to enlarge |
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The polyhedral
framework model clearly shows the layering and orientation of |
the occupied layers. Note that the
structure of the completely unoccupied layer is |
not modeled because, again, the polyhedral
framework model is only concerned |
with coordinated groups of A and X atoms.
(note: the black triangles only serve as |
braces to separate and properly orient the
occupied layers) |
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Cleavage |
Crystal habit |
(110) |
scaly - reflects layer structure |
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Isostructural minerals/compounds |
LiOH, SnO (OH and Sn are close packed with Li
and O interstitial) |
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Page
44 - Structure matters - Litharge |
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