The crystal structure of armalcolite is close to that of distorted brookite. Brookite and rutile may grow together in an epitaxial relationship. Terminology for pseudomorphs is ""replacer" after "original"", as in "brookite" after "rutile". Two other naturally occurring mineral forms of TiO are known, brookite and rutile. Common titanium-containing minerals are anatase, brookite, ilmenite, perovskite, rutile, and titanite (sphene). Brookite exhibits very weak pleochroism, yellowish, reddish and orange to brown. Brookite is usually brown in color, or sometimes yellowish or reddish brown, or even black. Brookite is doubly refracting, as are all orthorhombic minerals, and it is biaxial (+). Brookite belongs to the orthorhombic dipyramidal crystal class 2/m 2/m 2/m (also designated mmm).