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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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Antifluorite |
click here to see a key to this data bar
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tetra |
100% |
fcc |
3-d |
6
edges |
A-
4c, X- 8c/fcc |
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structure |
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In the antifluorite
structure the X atoms are packed in the FCC arrangement and |
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the A atoms occupy 100% of the
tetrahedral
interstices. |
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Ex. Ag2S ( aluminum sulfide,
argentite ) - high temperature form ( Ag - A , S - X ) |
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Crystal lattice |
The lattice can be
modeled as a simple FCC packing with the understanding that |
100% of the tetrahedral voids formed by the FCC
packed sulfur anions are occupied |
by silver cations thereby forming
AX4 coordination groups of atoms. It is understood |
that none of the octahedral voids are occupied. |
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Figure 58 -
Antifluorite lattice |
click image to enlarge |
structure |
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The above image
demonstrates the staggered FCC arrangement of the occupied |
tetrahedra. The layers are numbered in the
sequence of their vertical order from |
the bottom layer up. The occupied
AX4 groups are colored differently in each layer |
to differentiate the layers from one another. |
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Polyhedral framework |
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linking surfaces |
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Figure 59 -
Antifluorite polyhedral |
framework model |
( 112 T, 112 pinges) |
4
edges |
click image to enlarge |
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The polyhedral
framework model clearly shows that 100% of the tetrahedrally |
coordinated groups of X atoms are occupied with
A atoms. The structure is built up |
by linking all the edges of these tetrahedra
together. |
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Cleavage |
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(100), (110) - poor |
8 LT, 24 RT, 30 SS |
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118 pinges |
Crystal habit |
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cubic, octahedral, dodecahedral |
truncated cube |
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Isostructural minerals/compounds |
Na2O, K20, Rb2O,
Li2S |
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Page
42 - Structure matters - Antifluorite |
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