Product Code Database
Anatase is a metastable mineral form of titanium dioxide (TiO2) 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 rock and Metamorphic rock rocks.Page 419 Deer, Howie and Zussman "An Introduction to the Rock Forming Minerals" ISBN 0 582 44210 9 Glass coated with a thin film of TiO2 shows Anti-fog and self-cleaning properties under ultraviolet radiation.
Anatase is always found as small, isolated, and sharply developed , 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 Octahedron 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.
Another name commonly in use for anatase is octahedrite, which is earlier than anatase and was given by Horace Bénédict de Saussure because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, are oisanite (by Jean-Claude Delamétherie) and dauphinite, from the well-known French locality of Le Bourg-d'Oisans in Dauphiné.
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 in the Alps, a well-known locality being the Binnenthal near Brig in canton Valais, Switzerland.
While anatase is not an equilibrium phase of TiO2, it is metastable near room temperature. At temperatures between 550 and about 1000 °C, anatase converts to rutile. The temperature of this transformation strongly depends on impurities, or Dopant, as well as the morphology of the sample.
|
|
