Anatase

Anatase is a metastable mineral form of titanium dioxide (TiO₂) that crystallizes in a tetragonal system. In its pure state, it is colorless or white, but natural samples are typically black due to impurities. It is one of four natural polymorphs of TiO₂—the others being brookite, akaogiite, and rutile—with rutile being the most common and stable. Anatase forms at relatively low temperatures and occurs in small amounts in igneous and metamorphic rocks. A thin film of TiO₂ on glass provides antifogging and self-cleaning properties under ultraviolet light.

The mineral always appears as small, isolated, sharply developed crystals. Like rutile, it belongs to the tetragonal system, but despite having the same degree of symmetry, the two minerals share no relation in their interfacial angles except in the prism-zone at 45° and 90°. Anatase’s common octahedral habit has four perfect cleavage planes, with a polar edge angle of 82°9', compared to rutile’s 56°52½'. This steeper angle gives anatase crystals a longer vertical axis and a skinnier appearance. Anatase is also softer (Mohs 5.5–6 vs. 6–6.5), less dense (specific gravity ~3.9 vs. 4.2), optically negative (rutile is positive), and has a more strongly adamantine or metallic-adamantine luster. Although anatase is metastable at all temperatures and pressures (rutile is the equilibrium phase), it often forms first in many processes due to its lower surface energy, converting to rutile at elevated temperatures.

The name “anatase” was introduced by René Just Haüy in 1801, though the mineral was known earlier. It comes from the Ancient Greek ἀνάτασις (“stretching out”), referring to the crystals being elongated along an axis compared to other dipyramidal forms. An older name, “octahedrite,” was given by Horace Bénédict de Saussure for the common acute octahedral habit. Other obsolete names include oisanite (by Jean-Claude Delamétherie) and dauphinite, from the French locality of Le Bourg-d'Oisans in Dauphiné.

Two crystal habits are distinguished. The more common type forms simple acute octahedra with an indigo-blue to black color and steely luster. These are abundant at Le Bourg-d'Oisans, where they occur with rock-crystal, feldspar, and axinite in crevices in granite and mica schist. Microscopic crystals of this type are widespread in sedimentary rocks like sandstones, clays, and slates, and can be separated by washing away lighter constituents. The (101) plane is the most thermodynamically stable surface and thus the most exposed facet in natural and synthetic anatase. The second habit has numerous pyramidal faces, producing flatter or sometimes prismatic crystals that are honey-yellow to brown. These resemble xenotime and were once thought to be a special form of it, called wiserine. They occur attached to crevice walls in gneisses in the Alps, notably at Binnenthal near Brig in Valais, Switzerland. Anatase is metastable near room temperature and converts to rutile between about 550 and 1000 °C; the transformation temperature depends strongly on impurities, dopants, and sample morphology.

Synthetic anatase is often prepared for semiconductor applications. Common laboratory methods include the sol-gel process, via controlled hydrolysis of titanium tetrachloride (TiCl₄) or titanium ethoxide. Dopants are frequently added to control morphology, electronic structure, and surface chemistry.