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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
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CLOSE PACKED AND A ATOMS OCCUPY OCTAHEDRAL INTERSTICES |
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Halite |
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click here to see a key to this data bar
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octa |
100% |
fcc |
3-d |
12
edges |
A-
6c, X- 6c/fcc |
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structure |
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In the halite structure
the X atoms are FCC packed and the A atoms occupy all of |
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the octahedral interstices in each successive
layer. In the case of the metal halides, |
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only the lithium halides and NaI
have the proper atomic radius ratios for the halide |
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atoms to be closest packed. That is, the X atoms octahedrally
coordinated around the |
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A atoms of the AX6
groups are in contact with each other. The halite structure is used |
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by ionic compounds in which the cations and
anions have equal but opposite charges |
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and their ionic radii are significantly
different. |
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Ex. NaCl ( sodium chloride, halite
) ( Na - A , Cl - X ) |
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Crystal lattice |
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The halite structure
can be modeled as a simple FCC lattice. Since all of the |
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octahedral voids are occupied, the AX6
groups within each layer can be modeled |
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with the same color. It is understood that
none of the tetrahedral voids are occupied. |
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Figure 73 - Halite lattice structure |
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click image to enlarge |
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The FCC staggered
arrangement of the AX6 groups in each successive layer is |
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evident in the lattice model. |
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Polyhedral framework |
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linking surfaces |
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Figure 74 - Halite polyhedral |
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framework model |
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( 48 T, 60 pinges) |
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12
edges |
click image to enlarge |
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The polyhedral
framework is built up by linking the octahedral AX6 groups |
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together by their edges. |
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Cleavage |
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(100) - perfect |
twinning
on (111), |
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interpenetrating
cube |
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Crystal habit |
click image to enlarge |
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cubic (hopper
common), octahedral (rare) |
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Isostructural minerals/compounds |
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AgBr, AgCl, AgF, BaO, BaS, BaSe, BaTe, CaO, CaS,
CaSe, CaTe, CdO, CoO, CrN, CsF, |
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CsH, KBr, KCl, KF, KH, KI,
LiBr, LiCl, LiF, LiH, LiI,
MgO, MgS, MgSe, MnO, MnS, MnSe, |
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NaBr, NaF, NaH, NaI,
NbC, NbO, NiO, PbS, PbSe, PbTe, PdH, RbBr, RbCl, RbF, RbH,
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RbI,
ScAs, ScN, ScSb, SnAs, SnSb, SnSe, SnTe, SrO, SrS, SrSe, SrTe, TaC, TaO, TiC, |
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TiN, TiO, VC, VN, YAs, YN, YTe, ZrB, ZrC, ZrN,
ZrO, ZrP, ZrS |
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Page
50 - Structure matters - Halite |
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