Anatase is one of the three natural crystal forms of titanium dioxide (TiO₂) (the other two being rutile and brookite). It belongs to the tetragonal crystal system and has an open TiO₆ octahedral structure. It is stable at room temperature but transforms into rutile when heated to 400–600℃.
It is brown, yellow, or black, with a few varieties being blue or greenish-gray; thin films may be colorless, or light yellow or light brown, and can also exhibit uneven or banded distribution. It exhibits weak pleochroism: no-yellow, blue, green; ne-brown, dark blue, sky blue. Darker varieties may have stronger pleochroism. Absorption is usually no < ne, sometimes no > ne. It has a high relief at the positive electrode, with significant roughness on the surface. Refractive index variations are pronounced. For example, yellow anatase crystals have no = 2.501 and ne = 2.431; gray specimens have no = 2.556. Interference colors are very high, reaching order IV or above, with parallel or symmetrical extinction. Twin crystals, rare. Negative ductility. The dark variant may be a biaxial crystal with a very small optic axis angle.
