Figure 40 -

Figure 41 -

HCP lattice and

FCC lattice and






click image to enlarge

    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

click to enlarge

( 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

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