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MINERALS AND SYNTHETIC COMPOUNDS, ABmXn, W/ CLOSE
PACKED X ATOMS, |
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AND
A ATOMS OCCUPY TETRAHEDRAL AND B ATOMS OCTAHEDRAL INTERSTICES |
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Olivine |
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click here to see a key to this data bar
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tetra/octa |
1/8,1/2 |
hcp |
3-d |
vertices, edges |
Si-
4c,B- 6c; X- hcp |
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structure |
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In the olivine
structure the O ions are HCP with Si ions occupying 1/8 of the |
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tetrahedral interstices and metal ions occupying
1/2 of the octahedral voids. |
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Ex. Mg2SiO4 (
magnesium silicate, forsterite ) ( Si - A , Mg - B ,
O - X ) |
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Crystal lattice |
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The dispersion of the
occupied AX4 and
BX6 coordination groups throughout
the |
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crystal structure is demonstrated by the HCP
lattice model. The BX6 groups are |
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linked by edges into zigzag shaped rows that
are separated by corridors containing |
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no octahedral voids. The discrete AX4
groups are located at the site of occupied |
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tetrahedral voids in this corridor (all of the
other tetrahedral voids of the structure are |
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unoccupied). Note how the BX6 and AX4
groups of successive layers are staggered |
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| Figure -
109 |
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click image |
Olivine lattice |
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to
enlarge |
structure |
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relative to each other while those of
alternating layers eclipse each other HCP like. |
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Polyhedral framework |
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linking surfaces |
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Figure 110- Olivine |
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polyhedral framework |
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(
138 T, 152 pinges ) |
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AX4 |
BX6 |
click image to enlarge |
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The linkage of the
coordination groups is demonstrated by the framework model |
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as is the location of the AX4 groups
(yellow tetrahedra). |
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Cleavage |
Crystal habit |
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(100) - good to perfect |
tabular, prismatic |
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Isostructural minerals/compounds |
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Fe2SiO4, Mn2SiO4,
Ca2SiO4, CaFeSiO4, CaMgSiO4 |
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Page
71 - Structure matters - Olivine |
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