BROOKITE

Brookite crystals are typically elongated, tabular, and striated parallel to their length. They can also be pyramidal, pseudo-hexagonal, or prismatic. Brookite is usually brown in color, but can also be yellowish, reddish brown, or black.

It is the orthorhombic variant of titanium dioxide (TiO2), which occurs in four known natural polymorphic forms (minerals with the same composition but different structure). The other three of these forms are akaogiite (monoclinic), anatase (tetragonal) and rutile (tetragonal). Brookite is rare compared to anatase and rutile and, like these forms, it exhibits photocatalytic activity. It also has a larger cell volume than either anatase or rutile, with 8 TiO2 groups per unit cell, compared with 4 for anatase and 2 for rutile. Iron (Fe), tantalum (Ta) and niobium (Nb) are common impurities.

At temperatures above about 750 °C, brookite will revert to the rutile structure.

It usually occurs in very dark colors, transparent only in small fragments. Cuttable crystals are exceedingly rare, making attractive faceted gems prized collector’s items.

Brookite was named in 1825 by French mineralogist Armand Lévy for HenryJames Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.

USES

Titanium dioxide applications:
It is processed to create titanium dioxide, which is used in many applications, such as:

Pigments: Used in paints, toothpastes, and foods

Thin-film deposition: Used as a reflective coating for decorative gems and dielectric mirrors

Sunscreen: Used to absorb UV light and resist discoloration

Ceramic glazes: Used to make ceramic glazes opaque and to aid crystallization during cooling

Cosmetics: Used in cosmetics because it is oil and water dispersible

Anti-corrosive coatings:
A small amount of brookite is converted back to synthetic brookite, which is used as an anti-corrosive and biocompatible coating on orthopedic implants

MOHS HARDNESS

5 1⁄2 to 6

LUSTER

Submetallic

CHEMICAL COMPOUND

TiO2 (titanium dioxide)

WHERE TO FIND

It typically occurs as brown, metallic crystals in veins in gneiss and schist; it is also found in placer deposits and, less commonly, in zones of contact metamorphism. It is widespread in veins in the Alps; in Fronolen, north Wales, it forms crystals on crevice walls in diabase.

Arkansite is a variety of brookite from Magnet Cove, Arkansas, US. It is also found in the Murun Massif on the Olyokma-Chara Plateau of Eastern Siberia, Russia, part of the Aldan Shield.

DIAGNOSTIC PROPERTIES

Brookite has over the limit (OTL) refractive indices of 2.583-2.740.

A gemstone with a refractive index (RI) reading that exceeds 1.81 is said to be “over the limit” (OTL).

It is ordinarily deep red, reddish-brown, yellowish-brown, brown or black. Less often, it is light yellow-gray to almost white. It is opaque, translucent or transparent, with adamantine to submetallic luster. It exhibits weak pleochroism in shades of yellow, red or orange-brown