Examples of ionic structures based on the simple cubic packing of anions are CsCl and CaF2(fluorite). In CsCl all the cubic interstitial sites are occupied by caesium cations(Fig. 1.120b) but in fluorite only half the sites are occupied by the calcium cations. The resulting unit cell is not just one simple cube of flubrine anions, but a larger cube with a cell side double that of the simple cube and containing therefore 2 x 2 x 2 8 cubes, four of which contain calcium cations and four of which are empty.