Fig.2.
A view of a stoichiometric HAp crystal down the c, [001] axis, showing an OH group (small green circle) in its center, surrounded by hexagonal (blue circles) and columnar (purple circles) Ca atoms and phosphate tetrahedra: each green pyramid is a P5+ ion and four red circles in its corners are four O2- ions. Notice the misrepresentation though: O2- ions are not the smallest ions in the lattice, as it is being shown here, but, in fact, the largest, occupying most space in it. As parts of the PO4 sublattice, they determine the stacking order of the unit cell. All other ions could be imagined as if merely occupying interstices created by oxygen atoms.