In calcium fluoride, having the fluorite structure, the coordination numbers for calcium ion (${\text{Ca}}^{2+}$) and fluoride ion (${\text{F}}^{-}$) are:

In the fluorite (${\text{CaF}}_2$) structure:${\text{Ca}}^{2+}$ ions form a face-centered cubic ($fcc$) lattice.${\text{F}}^{-}$ ions occupy all the tetrahedral voids.Each ${\text{Ca}}^{2+}$ ion is surrounded by $8{\text{F}}^{-}$ ions at the corners of a cube. (C.N. $=8$).Each ${\text{F}}^{-}$ ion is surrounded by $4{\text{Ca}}^{2+}$ ions at the corners of a tetrahedron. (C.N. $=4$).