3. Calcium fluoride (fluorite, CaF₂), in two different views

(a) Centered on the Ca2+ ions Push for spacefill (note that there are some Ca2+ ions "missing", i.e. not every cube of F– ions has a Ca2+ ion in it) Push for ball-and-stick Highlight one Ca2+ ion and the cube of eight F– ions surrounding it Highlight one F– ion and the tetrahedron of four Ca2+ ions surrounding it

(b) Centered on the F– ions Push for spacefill Push for ball-and-stick Highlight one F– ion and the tetrahedron of four Ca2+ ions surrounding it Highlight a cube of eight F– ions that contains no Ca2+ ion in the center!

4. and 5. Two different structures for zinc sulfide (ZnS)

4. Cubic structure (sphalerite) Push for spacefill (note that now half of the anions have been removed from the CaF2 structure, so now both anions and cations have CN = 4) Push for ball-and-stick Highlight one Zn2+ ion and the tetrahedron of four S2– ions surrounding it

5. Hexagonal structure (wurtzite) Push for spacefill Push for ball-and-stick Highlight one Zn2+ ion and the tetrahedron of four S2– ions surrounding it

6. NiAs (niccolite), a layer structure In this structure there are alternating layers of Ni (blue) and As (pink) atoms. This leads to a lot of close Ni-Ni and As-As contacts, but that's OK because the crystal is highly covalent. Push for spacefill Push for ball-and-stick Highlight one Ni atom and the octahedron of six As atoms surrounding it Highlight one As atom and the trigonal prism (not an octahedron!) of six Ni atoms surrounding it

Thanks to Mike Mehl, of the Naval Research Laboratory, for the wurtzite and NiAs coordinates.