Occurrence
Titanium dioxide occurs in nature as well-known minerals rutile, anatase and brookite, and additionally as two high pressure forms, a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form, both found recently at the Ries crater in Bavaria. It is mainly sourced from ilmenite ore. This is the most wide spread form of titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore.The metastable anatase and brookite phases convert to rutile upon heating.
Titanium dioxide has eight modifications – in addition to rutile, anatase, and brookite three metastable phases can be produced synthetically (monoclinic, tetragonal and orthorombic), and five high pressure forms (α-PbO2-like, baddeleyite-like, cotunnite-like, orthorhombic OI, and cubic phases):
| Form | Crystal system | Synthesis |
|---|---|---|
| rutile | tetragonal | |
| anatase | tetragonal | |
| brookite | orthorhombic | |
| TiO2(B) | monoclinic | Hydrolysis of K2Ti4O9 followed by heating |
| TiO2(H), hollandite-like form | tetragonal | Oxidation of the related potassium titanate bronze, K0.25TiO2 |
| TiO2(R), ramsdellite-like form | orthorhombic | Oxidation of the related lithium titanate bronze Li0.5TiO2 |
| TiO2(II)-(α-PbO2-like form) | orthorhombic | |
| baddeleyite-like form, (7 coordinated Ti) | monoclinic | |
| TiO2 -OI | orthorhombic | |
| cubic form | cubic | P > 40 GPa, T > 1600 °C |
| TiO2 -OII, cotunnite(PbCl2)-like | orthorhombic | P > 40 GPa, T > 700 °C |
The cotunnite-type phase was claimed by L. Dubrovinsky and co-authors to be the hardest known oxide with the Vickers hardness of 38 GPa and the bulk modulus of 431 GPa (i.e. close to diamond's value of 446 GPa) at atmospheric pressure. However, later studies came to different conclusions with much lower values for both the hardness (7–20 GPa, which makes it softer than common oxides like corundum Al2O3 and rutile TiO2) and bulk modulus (~300 GPa).
The oxides are commercially important ores of titanium. The metal can also be mined from other minerals such as ilmenite or leucoxene ores, or one of the purest forms, rutile beach sand. Star sapphires and rubies get their asterism from rutile impurities present in them.
Titanium dioxide (B) is found as a mineral in magmatic rocks and hydrothermal veins, as well as weathering rims on perovskite. TiO2 also forms lamellae in other minerals.
Spectral lines from titanium oxide are prominent in class M stars, which are cool enough to allow molecules of this chemical to form.
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