Formation of two close-packed polytypes (hcp and ccp)

The formation of two close-packed polytypes.
(a) A single close-packed layer, A.
(b) The second close-packed layer, B, lies in dips above A.
(c) The third layer reproduces the first to give an ABA arrangement structure (hcp).

(d) The third layer lies above the gaps in the first layer, giving an ABC arrangement (ccp). The different colours identify the different layers of identical spheres.