In ccp and hcp lattices, there are two tetrahedral holes per packing atom. A stoichiometry of either M2X or MX2 gives a structure that fills all tetrahedral sites, while an MX structure fills only half of the sites. An example of an MX2 structure is fluorite, CaF2, whose structure is shown in the figure at the left. The packing atom in fluorite is Ca2+ and the structure is composed of three interpenetrating fcc lattices. It should be noted that the Ca2+ ion (gray spheres) as a packing atom defies our "rule" that anions are larger than cations and therefore must be the packing atoms. The fluorite structure is common for ionic MX2 (MgF2, ZrO2, etc.) and M2X compounds (Li2O). In contrast, the hcp relative of the fluorite structure is quite rare because of unfavorable close contacts between like-charged ions.