In the calcium fluoride structure the co-ordination number of the cation and anions are  respectively

Ca2+ ions are arranged in (ccp) arrangement. i.e Ca2+ ions are present at all corners and tat the c.entre ofeach face of the cube. the fluoride ions occupy all the tetrahedral sites. This is 8:4 arrangement i.e., each Ca2+ion is surrounded bySF- ions and each F-ion by four Ca2+ ions.