Polymorf - Knowhwere- Litharge

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

CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES

Litharge

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tetra	50%	fcc	layer	4 edges	A- 4c/fcc, X- 4c

structure

Litharge is an uncommon simple oxide. It has a unique structure derived from

the fact that the relatively larger cations (A) are approximately close packed in a

layer structure rather than the anions (X). One hundred percent of the tetrahedral

interstitial voids of every other layer are occupied with X atoms for an overall

occupancy rate of fifty percent. These layers are oriented parallel to the (100) plane.

Ex. PbO ( lead oxide, litharge ) ( Pb - A , O - X )

Crystal lattice

The FCC lattice structure of litharge is modeled so that the XA₄ groups of lead

and oxygen atoms are colored differently than the unoccupied tetrahedral groups

of lead atoms (blue).

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		Figure 62 - Litharge lattice structure
click image to enlarge


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The lattice model demonstrates that the occupied XA₄ groups form layers oriented

parallel to the (100) plane that alternate with layers of the lattice that have

unoccupied tetrahedral voids.

Polyhedral framework

The polyhedral framework model clearly shows the layering and orientation of

the occupied layers. Note that the structure of the completely unoccupied layer is

not modeled because, again, the polyhedral framework model is only concerned

with coordinated groups of A and X atoms. (note: the black triangles only serve as

braces to separate and properly orient the occupied layers)

Cleavage	Crystal habit
(110)	scaly - reflects layer structure

Isostructural minerals/compounds

LiOH, SnO (OH and Sn are close packed with Li and O interstitial)

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Page 44 - Structure matters - Litharge