Polymorf - Knowhwere- Antiflourite

MINERALS AND SYNTHETIC COMPOUNDS, A_mX_n, IN WHICH X ATOMS ARE

CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES

Antifluorite

click here to see a key to this data bar
tetra	100%	fcc	3-d	6 edges	A- 4c, X- 8c/fcc

structure

In the antifluorite structure the X atoms are packed in the FCC arrangement and

the A atoms occupy 100% of the tetrahedral interstices.

Ex. Ag₂S ( aluminum sulfide, argentite ) - high temperature form ( Ag - A , S - X )

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.

Polyhedral framework

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linking surfaces
		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.

Cleavage
(100), (110) - poor		8 LT, 24 RT, 30 SS
		118 pinges
Crystal habit
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