>>12561998Generally speaking the macroscopical shape of the crystal that you can see with your eyes is a reflection of the unit cell of the crystal. So for example if the crystal is a cube, octahedron or a tetrahedron the molecular arrangement will have some sort of cubic unit cell. The unit cells fill up the space without any gaps, that's why 5-fold symmetry is forbiden since you can't pave even a 2D plane without any gaps with pentagons (except in quasicrystals). Same as you can't fill 3D space with spheres or pave a 2D plane with circles without any gaps.