Anatase
Anatase is a metastable mineral form of titanium dioxide (TiO) 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 know 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. Thin films of TiO-coated glass show 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 have a polar edge angle of 56°52½'. The steeper angle gives anatase crystals a longer vertical axis and skinnier appearance than rutile, which led French mineralogist René Just Haüy to name the mineral "anatase" in 1801, from the Greek "anatasis" ("extension"). 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. Crystal habit. Two growth habits of anatase crystals may be distinguished. The more common occurs as simple acute octahedra with an indigo-blue to black color and steely luster. Crystals of this kind are abundant at Le Bourg-d'Oisans in Dauphiné, France, where they are associated with rock-crystal, feldspar, and axinite in crevices in granite and mica schist. Similar crystals of microscopic size are widely distributed in sedimentary rocks such as  sandstones, clays, and slates, from which they may be separated by washing away the lighter constituents of the powdered rock. The (101) plane of anatase is the most thermodynamically stable surface and thus the most widely exposed facet in natural and synthetic anatase. 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. While anatase is not an equilibrium phase of TiO, 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 dopants, as well as the morphology of the sample. 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) or titanium ethoxide. Often dopants are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of an anatase sample. Alternate and obsolete names. Another name commonly in use for anatase is "octahedrite", which is earlier than anatase and given because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, are "oisanite" and "dauphinite", from the well-known French locality of Le Bourg-d'Oisans in Dauphiné.