Metal dioxides are pervasive, e.g. MnO and rutile (TiO), but these are oxides, not peroxides. In addition, the dioxides of cerium, plutonium and neptunium have the same structures. Like many dioxides, RuO adopts the rutile structure. CrO adopts the rutile structure (as do many metal dioxides). Germanium bonds to all natural single chalcogens except polonium, and forms dioxides, disulfides, and diselenides. A mixture of uranium and plutonium dioxides is used as MOX fuel. The majority of transition metal dioxides (MO) have the rutile structure, seen to the right. Cerium(IV) oxide has the fluorite structure, similarly to the dioxides of praseodymium and terbium. Oxidation of allenes affords allene dioxides or products of intramolecular participation. No other elemental dioxides have the fluorite structure. All tend to hydrolyse to give the so-called oxyhalides and dioxides.