A mineral of composition CaF₂ and an important structure type with space group Fm3m, cell edge = 0.546 nm. The fluorite structure type is written RX₂, where the R cations are in cubic (eightfold) coordination by the X anions. Alternatively, the X atoms are tetrahedrally (fourfold) coordinated by the R atoms. The fluorite arrangement may occur when the ratio of r(R), the ionic radius of the R atoms, to r(X), the ionic radius of the X atoms, is equal to or greater than 0.73. The arrangement XR₂ is termed the antifluorite arrangement. Here the anions are eight-coordinated and the cations are four-coordinated. Halides of larger divalent cations, oxides of larger quadrivalent (4+) cations, some intermetallic compounds, and XR₂ alkali univalent oxides frequently belong to the fluorite structure type. Approximately 100 synthetic compounds are known to have this arrangement; distortions may result in similar arrangements but with lower symmetry, such as in ZrO₂, which is monoclinic. A vast number of complex structures can be derived from the fluorite arrangement by ordering vacancies and different cations or anions over the fluorite atomic positions.