the hexagonal close-packed structure
Start again from the tightest cannonball pile, but this time let the third layer sit directly above the first (an ABAB stacking) instead of shifting to a brand-new position the way FCC does. Boxed up, the repeating unit is a hexagonal prism. This is the hexagonal close-packed structure.
Like FCC, it is close-packed: coordination number 12 and packing factor 0.74, exactly as dense. The cell is described by a base edge a and a height c, with an ideal ratio c/a = 1.633; the conventional hexagonal cell contains 6 atoms. Magnesium, zinc, titanium, cobalt, and alpha-zirconium are HCP.
Here is the honest twist: although HCP is as densely packed as FCC, it has far fewer easy directions to slip on — deformation is largely confined to the basal plane — so HCP metals often behave less ductile and more directional. That is why magnesium and zinc sheet are harder to cold-form than aluminum, and are usually shaped warm.
Magnesium and titanium are HCP. Though as densely packed as FCC (74 percent, coordination number 12), their easy slip is mostly confined to the basal plane, giving fewer independent slip systems — which is why magnesium sheet is usually warm-formed rather than cold-bent.
HCP: ABAB stacking, close-packed but fewer easy slip directions than FCC.
HCP is close-packed (74 percent) yet usually less ductile than FCC. Packing density alone does not set ductility; the number of independent slip systems does.