Anatase is a metastable mineral form of titanium dioxide (TiO<sub>2</sub>) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) of titanium dioxide are known to occur naturally: brookite, akaogiite, and rutile, with rutile being the most common and most stable of the bunch. Anatase is formed at relatively low temperatures and found in minor concentrations in igneous and metamorphic rocks. Glass coated with a thin film of TiO<sub>2</sub> shows antifogging and self-cleaning properties under ultraviolet radiation.

Anatase is always found as small, isolated, and sharply developed crystals, and like rutile, it crystallizes in a tetragonal system. Anatase is metastable at all temperatures and pressures, with rutile being the equilibrium polymorph. Nevertheless, anatase is often the first titanium dioxide phase to form in many processes due to its lower surface energy, with a transformation to rutile taking place at elevated temperatures. Although the degree of symmetry is the same for both anatase and rutile phases, there is no relation between the interfacial angles of the two minerals, except in the prism-zone of 45° and 90°. The common octahedral crystal habit of anatase, with four perfect cleavage planes, has an angle over its polar edge of 82°9', whereas rutile octahedra only has a polar edge angle of 56°52½'. The steeper angle gives anatase crystals a longer vertical axis and skinnier appearance than rutile. Additional important differences exist between the physical characters of anatase and rutile. For example, anatase is less hard (5.5–6 vs. 6–6.5 on the Mohs scale) and less dense (specific gravity about 3.9 vs. 4.2) than rutile. Anatase is also optically negative, whereas rutile is optically positive. Anatase has a more strongly adamantine or metallic-adamantine luster than that of rutile as well.

Nomenclature

The modern name was introduced by René Just Haüy in 1801, but the mineral was known and described before. It derives from 'stretching out', because the crystals are stretched along an axis compared to other dipyramidal ones.

Another name commonly in use for anatase is octahedrite, which is earlier than anatase and was given by Horace Bénédict de Saussure

Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit. Their color is honey-yellow to brown. Such crystals closely resemble the mineral xenotime in appearance and were historically thought to be a special form of xenotime, termed wiserine. They occur attached to the walls of crevices in gneisses in the Alps, a well-known locality being the Binnenthal near Brig in canton Valais, Switzerland.

Synthetic anatase

Due to its potential application as a semiconductor, anatase is often prepared synthetically. Crystalline anatase can be prepared in laboratories by chemical methods such as the sol-gel process. This might be done through controlled hydrolysis of titanium tetrachloride (TiCl<sub>4</sub>) or titanium ethoxide. Often dopants are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of an anatase sample.

See also

alt=|thumb|400x400px|Black anatase crystals on smoked quartz

  • Delustrant
  • Titanium dioxide
  • Rutile
  • Brookite
  • Akaogiite
  • List of minerals

References