|
|
MINERALS AND SYNTHETIC COMPOUNDS, ABmXn, W/ CLOSE
PACKED X ATOMS, |
|
AND
A ATOMS OCCUPY TETRAHEDRAL AND B ATOMS OCTAHEDRAL INTERSTICES |
| |
|
. |
|
Spinel |
|
click here to see a key to this data bar
|
|
tetra/octa |
1/8,1/2 |
fcc |
3-d |
vertices, edges |
A-
4c,B- 6c; X- 4c/fcc |
|
. |
|
structure |
. |
|
. |
. |
|
In the spinel structure
the X atoms are close packed and the A atoms occupy 1/8 |
. |
|
of the tetrahedral interstices and the B atoms
occupy 1/2 of the octahedral voids. |
. |
|
Like the atacamite structure shown previously
the occupancy rate of the octahedral |
|
voids in successive layers alternates from 25%
to 75%. And, like atacamite, the BX6 |
|
coordination groups stack together so as to
encompass large tetrahedral shaped |
|
sections of the lattice where the octahedral
voids are not occupied. However, in |
|
spinel an A atom occupies the central
tetrahedral interstice present within these |
|
sections. As a result each X atom is tetrahedrally
coordinated with one A atom and |
|
three B atoms. |
| |
. |
|
Ex. MgAl2O4 (
magnesium aluminum oxide, spinel ) ( Mg - A , Al -
B , O - X ) |
. |
|
. |
|
Crystal lattice |
|
The lattice can be
modeled identical to atacamite with the addition of the
AX4 |
|
groups that are colored differently (orange in
this image). The location of an AX4 |
|
group in the center of the tetrahedral stack of
BX6 groups is clearly shown by the |
|
lattice model. The percent occupation of
the tetrahedral and octahedral voids can |
|
also be deduced. |
| . |
|
|
|
| |
 |
|
|
Figure 107 - Spinel lattice structure |
|
click image to enlarge |
|
| |
|
| |
| . |
|
|
|
|
|
Polyhedral framework |
| . |
|
|
|
|
linking surfaces |
 |
|
|
  |
Figure 108 - Spinel polyhedral |
|
framework model |
|
(
152 T, 186 pinges ) |
|
AX4
BX6 |
click image to enlarge |
| . |
|
|
|
|
|
The polyhedral
framework model is especially useful for showing the linkage and |
|
location of the AX4 and BX6
groups. |
|
. |
|
Cleavage |
|
|
|
(111) separation plane (parallel to the plane of the BX6
coordination groups) |
| |
|
Crystal habit |
|
|
|
octahedral,
dodecahedral (rare), cubic (rare),
twinning on (111) |
|
. |
|
|
|
Isostructural minerals/compounds |
|
Normal spinel structure |
|
FeAl2O4, ZnAl2O4,
MnAl2O4, ZnFe2O4, FeCr2O4, |
|
MgAl2O4, MgCr2O4,
SiMg2O4 ( silicate spinel ) |
|
Inverse spinel structure:
the inverse spinel structure is identical to spinel except |
|
that both the A and B atoms occupy, in a
1/1 ratio, the tetrahedral interstices only |
|
occupied by A atoms in spinel; and both
the A and B atoms, in a 1/2 ratio |
|
respectively, occupy the octahedral voids
only occupied by B atoms in spinel. |
|
Fe2MgO4, Fe2MnO4,
Al2MgO4, Fe2TiO4, Fe3O4,
Mg2SiO4 ( silicate spinel) |
|
. |
|
|
 |
Page
70 - Structure matters - Spinel |
 |
