Gibbsite

Aluminium hydroxide mineral

Gibbsite is a mineral form of aluminium hydroxide, with the chemical formula Al(OH)₃. It is an important ore of aluminium and is one of the three main phases of the aluminium hydroxide minerals, the others being boehmite and diaspore.

Structure

3D ball model of the gibbsite crystal structure

Gibbsite has a layered structure, with sheets of octahedrally coordinated aluminium ions bonded to hydroxide ions. The layers are held together by hydrogen bonds, which gives gibbsite its characteristic platy habit. The structure is similar to that of the brucite mineral, Mg(OH)₂, but with aluminium replacing magnesium.

Occurrence

Gibbsite specimen from the United States

Gibbsite is commonly found in bauxite deposits, which are the primary source of aluminium. It forms in tropical and subtropical climates through the weathering of aluminium-rich rocks. Gibbsite can also occur as a secondary mineral in hydrothermal veins and as a precipitate in some sedimentary environments.

Properties

Gibbsite is typically white, but it can also be colorless, gray, or tinted by impurities. It has a vitreous to pearly luster and a perfect basal cleavage. The mineral is relatively soft, with a Mohs hardness of 2.5 to 3.5, and has a specific gravity of about 2.3 to 2.4.

Uses

Gibbsite is an important ore of aluminium, and it is processed to produce alumina (Al₂O₃) through the Bayer process. Alumina is then used to produce aluminium metal via the Hall–Héroult process. Gibbsite is also used in the production of various aluminium compounds and as a filler in plastics and other materials.

Gallery

• Gibbsite crystals showing typical platy habit
  Gibbsite crystals showing typical platy habit

Related pages

• Bauxite
• Aluminium oxide
• Boehmite
• Diaspore