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

In fluorite structure, ${\mathrm{Ca}}^{2+}$ ions are in the face centred cubic arrangement. Each ${\mathrm{Ca}}^{2+}$ is connected to $4{\mathrm{F}}^{-}$ ions below it and to another set of $4{\mathrm{F}}^{-}$ ions above it

i.e. ${\mathrm{Ca}}^{2+}$ has a coordination number of 8 and each ${\mathrm{F}}^{-}$ ion has a coordination number 4 .