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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
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CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES |
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. |
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HgI2 |
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click here to see a key to this data bar
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tetra |
25% |
fcc |
layer |
4
vertices |
A-
4c, X- 2c/fcc |
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. |
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structure |
. |
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. |
. |
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In the HgI2
structure fifty percent of the tetrahedral voids of every other layer of
|
. |
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FCC packed X atoms are occupied with A atoms
resulting in an overall occupancy |
. |
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rate of twenty-five percent. These layers
are oriented parallel to the (100) plane. |
| |
. |
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Ex. HgI2
(mercuric iodide, coccinite ) - red form
( Hg - A , I -
X ) |
. |
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. |
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Crystal lattice |
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The
AX4
groups in an occupied layer of the FCC lattice are colored differently |
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than the tetrahedrally coordinated groups of
close packed iodine atoms whose voids |
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are empty (blue). |
| . |
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 |
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Figure 67 - HgI2 lattice
structure |
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click image to enlarge |
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| . |
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The lattice model shows
that the AX4 groups of atoms are joined by their vertices |
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only due to the fifty percent occupancy rate of
each occupied layer. The (100) |
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orientation of the occupied layers can be
extrapolated from the lattice also. |
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. |
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Polyhedral framework |
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linking surfaces |
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Figure 68 -
HgI2 polyhedral |
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framework model |
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( 69 T, 80 pinges) |
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4
vertices |
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The polyhedral
framework model shows the layers of red, vertex linked AX4 |
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groups run parallel to the (100) plane.
(note: the black triangles are only used to |
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brace the structure and orient the layers
properly) |
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Isostructural minerals/compounds |
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a-ZnCl2,
g-ZnCl2 |
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. |
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Page
46 - Structure matters - HgI2 |
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