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                                  

tetra

50%

fcc

layer

4 edges

A- 4c/fcc, X- 4c

.

  structure

.
. .
     Litharge is an uncommon simple oxide.  It has a unique structure derived from .
the fact that the relatively larger cations (A) are approximately close packed in a .
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.
  .
Ex. PbO ( lead oxide, litharge )    ( Pb - A , O - X ) .
.
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).
.      
 

litharge.gif

 
    Figure 62 -  Litharge lattice structure

click image to enlarge

                       
   
 
.      
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
.      

linking surfaces

litharge.gif

 

   Figure 63 -   Litharge polyhedral
              framework model
               (86 T,  99 pinges)

4 edges

                    click image to enlarge
.      
     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
   
Isostructural minerals/compounds
LiOH, SnO (OH and Sn are close packed with Li and O interstitial)
 

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