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Concept
Titanium dioxide
Summary
Titanium dioxide, also known as titanium(IV) oxide or titania taɪˈteɪniə, is the inorganic compound with the chemical formula TiO2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble in water, although mineral forms can appear black. As a pigment, it has a wide range of applications, including paint, sunscreen, and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million tonnes. It has been estimated that titanium dioxide is used in two-thirds of all pigments, and pigments based on the oxide have been valued at a price of $13.2 billion.
In all three of its main dioxides, titanium exhibits octahedral geometry, being bonded to six oxide anions. The oxides in turn are bonded to three Ti centers. The overall crystal structures of rutile and anatase are tetragonal in symmetry whereas brookite is orthorhombic. The oxygen substructures are all slight distortions of close packing: in rutile, the oxide anions are arranged in distorted hexagonal close-packing, whereas they are close to cubic close-packing in anatase and to "double hexagonal close-packing" for brookite. The rutile structure is widespread for other metal dioxides and difluorides, e.g. RuO2 and ZnF2.
Molten titanium dioxide has a local structure in which each Ti is coordinated to, on average, about 5 oxygen atoms. This is distinct from the crystalline forms in which Ti coordinates to 6 oxygen atoms.
Synthetic TiO2 is mainly produced from the mineral ilmenite. Rutile, and anatase, naturally occurring TiO2, occur widely also, e.g. rutile as a 'heavy mineral' in beach sand. Leucoxene, fine-grained anatase formed by natural alteration of ilmenite, is yet another ore. Star sapphires and rubies get their asterism from oriented inclusions of rutile needles.
Titanium dioxide occurs in nature as the minerals rutile and anatase. Additionally two high-pressure forms are known minerals: a monoclinic baddeleyite-like form known as akaogiite, and the other has a slight monoclinic distortion of the orthorhombic α-PbO2 structure and is known as riesite.
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