Fig. 415.55 Tetrahedral Closest Packing of Spheres:
Nucleus and Nestable Configurations:
 In any number of successive planar layers of tetrahedrally
organized sphere packings, every third
triangular layer has a sphere at its centroid (a nucleus).
The 36sphere tetrahedron with five spheres
on an edge (fourfrequency tetrahedron) is the lowest
frequency tetrahedron system which has a
central sphere or nucleus.
 The threefrequency tetrahedron is the highest frequency
without a nucleus sphere.
 Basic "nestable" possibilities show how the regular
tetrahedron, the 1/4tetrahedron and the 1/8
octahedron may be defined with sets of closest packed
spheres. Note that this "nesting" is only
possible on triangular arrays which have no sphere at
their respective centroids.
