The CaF₂ (fluorite) structure can be considered as a fcc array of Ca²⁺ ions with F^- ions occupying the tetrahedral holes.

The Ca²⁺ ions are 8 cordinate and the F^- ions are 4 coordinate.

The unit cell contains

{4x(1/8)}+{6x(1/2)}] = 4 Ca²⁺ ions

and 8 F^- ions