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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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Atacamite |
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click here to see a key to this data bar
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octa |
50% |
fcc |
3-d |
6
edges |
A-
6c; X- 3c,4c/fcc |
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structure |
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In the atacamite
structure the anions are packed FCC with the cations occupying
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alternatively 3/4 and 1/4 of the octahedral
interstices of successive occupied layers. |
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Ex. Cu2(OH)3Cl
(copper hydroxychloride, atacamite ) ( Cu - A; OH, Cl - X
) |
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Crystal lattice |
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The FCC lattice model
of atacamite shows that the occupied AX6 groups form |
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hexagonal rings in layers with 75% occupancy.
Compare this net with that for |
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corundum shown previously. The model
clearly shows the variation in occupancy |
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rate for successive layers. |
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Figure 83 - Atacamite lattice |
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click image to enlarge |
structure |
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The AX6
groups can be envisioned as stacking together to form a tetrahedral |
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pyramid (outlined above). This pyramid
encompasses a large tetrahedral shaped |
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area of the lattice that is devoid of cations.
This region is large enough to |
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accommodate large metal cations as will be shown
with the spinel structure later. |
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Polyhedral framework |
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linking surfaces |
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Figure 84 -
Atacamite polyhedral |
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framework model |
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( 128 T, 162 pinges) |
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6
edges |
click image to enlarge |
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The polyhedral
framework details the structure of the pyramid including the |
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tetrahedral shaped area devoid of cations in the
center. |
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Cleavage |
Crystal habit |
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(100) - perfect |
prismatic, tabular |
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Isostructural minerals/compounds |
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Mg2(OH)3Cl, Mn2(OH)3Cl,
Fe2(OH)3Cl, Co2(OH)3Cl, Ni2(OH)3Cl |
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Page
55 - Structure matters - Atacamite |
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