Anatase

Anatase is one of three natural polymorphs of titanium dioxide (TiO2), along with rutile and brookite. It crystallizes in the tetragonal system and is best known for sharp, steep dipyramidal crystals, an older name, octahedrite, refers to this near-octahedral habit.

Colors range from deep indigo-blue and black to brown, honey-yellow, and rarely near-colorless, often with a brilliant adamantine to metallic-adamantine luster. The blue varieties are especially sought by collectors.

Though chemically identical to rutile, anatase is the lower-temperature, less dense polymorph and converts to rutile when heated, which is one reason large anatase crystals are uncommon.

Anatase forms mainly as an accessory and secondary mineral in alpine-type fissures, hydrothermal veins, and metamorphic rocks such as schists and gneisses, where it can develop sharp small crystals on fracture surfaces. It also occurs as detrital grains in sediments because of its resistance to weathering.

It is commonly associated with quartz, brookite, rutile, adularia, and chlorite in alpine clefts. Classic localities include the French Alps (Le Bourg-d'Oisans, hence "oisanite"), the Swiss Alps, Norway, Brazil, and various pegmatite and metamorphic terrains.

Look for small, sharp, steeply pointed dipyramidal (near-octahedral) crystals with a brilliant adamantine to sub-metallic luster, frequently deep blue, brown, or black, perched on quartz or rock in alpine-style fissures. The streak is white to pale yellowish.

Hardness is moderate (Mohs 5.5-6). Distinguishing it from its polymorphs requires habit and crystallography: rutile forms slender prisms and is denser, while brookite forms tabular orthorhombic crystals. The tetragonal bipyramidal habit and often blue color help separate anatase from rutile and from dark spinels or cassiterite.

Bulk titanium dioxide (mostly from rutile and ilmenite, but the anatase form too) is the world's most important white pigment, used in paint, paper, plastics, and sunscreen, and as a photocatalyst. Synthetic anatase nanoparticles are widely used in photocatalytic and solar applications.

Natural anatase crystals, however, are valued almost entirely as mineral specimens; the sharp blue crystals from alpine and Brazilian localities are prized by collectors. It is rarely faceted because crystals are small, and it has little metaphysical tradition.