Hexagonal lattice in NaCl?! NaCl forms a cubic lattice!

Yes I'm a crystallographer!

A cube looks like a hexagon, looking down the long diagonal.

The NaCl structure is two interpenetrating FCC lattices of A and X, offset by {1/4, 0, 0} relative to the NaCl cubic unit cell (but by {1/2, 0, 0} relatively between the FCC sub-cells. In space-group symmetry notation, it is *F* m 3-bar m, in terms of its *fundamental* (that is most-basically-expressible, reduced unit cell).

In the same manner that an FCC atomic arrangement could be expressed as having a hexagonal unit cell, with it *not* being the fundamental cell, but instead a multiple formula-unit cell – a supercell – one could do the same with NaCl. I've not had my coffee, but intuitively a supercell double-sized this one could similarly be used to define the crystal structure of NaCl. Not optimal, but possible. Look in the front-matter section of your Space-Groups tome to look up the matrix conversion to transform the atomic positions from one fundamental lattice-type to another.

Put more simply, while HEXAGONAL is ABABAB stacking, FCC is ABCABC stacking of close-packed layers of atoms.

Yes, I am also a crystallographer.