Polymorf - Knowhwere- FCC symmetry

Notice the subtle differences between these structures. In the HCP lattice adjacent

octahedra are joined together at their faces and edges. Whereas, in the FCC lattice

they only share their edges with each other. Also the tetrahedra of the HCP lattice

share faces and vertices. But in the FCC lattice they only share vertices with each

other. The minor changes in these structures brought about by a slight shift in the

staggering of their sphere packings accounts for the existence of some polymorphs

of elements and minerals that exhibit either the HCP or FCC atomic packing structure

depending on the external environmental conditions existing at the time they

became solid.

Symmetry planes of the FCC lattice

Systematically slicing the FCC lattice parallel to its (100) planes results in the

cubic structure that gives the packing its name. Notice that a sphere is located in the

.

	( 4 ST, 2 REC, 24 RT, 40 pinges )

	click image to enlarge

Figure 42 - Symmetry planes of the FCC packing/lattice
.

center of each face of the cube. This model is useful in relating the orientation and

symmetry of FCC packed atoms to cubic symmetry elements. It can be seen that the

atoms have the closest packed triangulated arrangement parallel to the (111) cubic

plane (green). They have the close packed squared arrangement parallel to the

cube's (100) plane of symmetry (red). And they have a slightly expanded rectangular

arrangement parallel to the cube's (110) plane (blue).

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Page 29 - Structure matters - FCC symmetry