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

HgI2

                  click here to see a key to this data bar                                  

tetra

25%

fcc

layer

4 vertices

A- 4c, X- 2c/fcc

.

  structure

.
. .
     In the HgI2 structure fifty percent of the tetrahedral voids of every other layer of .
FCC packed X atoms are occupied with A atoms resulting in an overall occupancy .
rate of twenty-five percent.  These layers are oriented parallel to the (100) plane.
  .
Ex. HgI2 (mercuric iodide, coccinite ) - red form               ( Hg - A ,  I - X ) .
.
Crystal lattice
     The AX4 groups in an occupied layer of the FCC lattice are colored differently
than the tetrahedrally coordinated groups of close packed iodine atoms whose voids
are empty (blue).
.      
 

coccinite.gif

 
    Figure 67 -  HgI2 lattice structure

click image to enlarge

                 
   
 
.      
     The lattice model shows that the AX4 groups of atoms are joined by their vertices
only due to the fifty percent occupancy rate of each occupied layer.  The (100)
orientation of the occupied layers can be extrapolated from the lattice also.
.
Polyhedral framework
.      

linking surfaces

coccinite.gif

 

   Figure 68 -   HgI2 polyhedral
            framework model
            ( 69 T,  80 pinges)

4 vertices

 
.      
     The polyhedral framework model shows the layers of red, vertex linked AX4
groups run parallel to the (100) plane.  (note: the black triangles are only used to
brace the structure and orient the layers properly)
.  
Isostructural minerals/compounds
a-ZnCl2, g-ZnCl2
.

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