Brookite

Brookite is the orthorhombic polymorph of titanium dioxide (TiO2), the rarest of the three natural forms alongside rutile and anatase. It typically forms tabular or platy crystals, often brown, reddish-brown, or black, with a brilliant metallic-adamantine luster.

It was named after the English mineralogist Henry James Brooke. A black, lustrous variety from Arkansas is historically known as arkansite.

Like anatase, brookite is metastable and converts to rutile when heated, so large crystals are scarce. Its strong luster and well-formed tabular crystals make it a favorite among collectors of titanium-oxide minerals.

Brookite forms chiefly in alpine-type hydrothermal fissures and veins, and in metamorphic rocks such as schists and gneisses, often perched on quartz or feldspar along with anatase, rutile, and adularia. It also occurs as detrital grains in sediments owing to its hardness and chemical resistance.

Notable localities include the French and Swiss Alps, Snowdon in Wales (where fine bladed crystals occur), Magnet Cove in Arkansas (source of arkansite), Pakistan, and various Brazilian deposits. It commonly forms in association with both of its TiO2 polymorphs.

Look for tabular or bladed crystals, frequently brown to reddish-brown with a striking metallic to adamantine luster and a white to grayish or yellowish streak. Hardness is moderate (Mohs 5.5-6).

The orthorhombic tabular habit distinguishes brookite from tetragonal anatase (steep bipyramids) and prismatic rutile. Thin tabular brown crystals on quartz in alpine fissures are characteristic. Look-alikes include dark titanite and some iron oxides, but the platy form, high luster, and association with rutile/anatase help confirm identification.

Brookite shares the industrial chemistry of titanium dioxide, but because it is rare and converts to rutile on heating, it is not itself an industrial source; bulk TiO2 comes from rutile and ilmenite. Brookite's importance is therefore almost entirely scientific and as a collector mineral.

Well-formed tabular crystals, especially the lustrous black arkansite and bladed Welsh and Pakistani specimens, are prized by collectors. It is essentially never faceted as a gem and carries little metaphysical tradition.