Anatase - Crystal Habit

Crystal Habit

Two types or habits of anatase crystals may be distinguished. The more common occurs as simple acute double pyramids with an indigo-blue to black color and steely luster. Crystals of this kind are abundant at Le Bourg-d'Oisans in Dauphiné, where they are associated with rock-crystal, feldspar, and axinite in crevices in granite and mica-schist. Similar crystals, but 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.

Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit; the color is honey-yellow to brown. Such crystals closely resemble xenotime in appearance and, indeed, were for a long time supposed to belong to this species, the special name wiserine being applied to them. They occur attached to the walls of crevices in the gneisses of the Alps, the Binnenthal near Brig in canton Valais, Switzerland, being a well-known locality. Naturally occurring pseudomorphs of rutile after anatase are also known.

While anatase is not an equilibrium phase of TiO₂, it is kinetically stabilized. At temperatures between 550 and about 1000 °C, anatase transforms to the equilibrium rutile phase, increasing its specific gravity to 4.2. The temperature of this transformation strongly depends on the impurities or dopants present in the material as well as on the morphology of the sample.