Mineral Description

Gibbsite is an aluminum hydroxide mineral of the oxides and hydroxides group, with structural formula [Al(OH)₃]. Gibbsite’s structure is made up by the stacking of octahedral sheets of aluminum hydroxide. Each layer consists of octahedrally (six-fold) coordinated Al³⁺ cations sandwiched between two closed-pack layers of OH^–. Because Al is a trivalent cation, each of the hydroxyls is bonded to only two aluminums, and thus, only two-thirds of the available octahedral sites are occupied. This type of octahedral occupancy is called dioctahedral.

For similar structures with divalent cations (e.g., brucite), each hydroxyl is bonded to three cations and all octahedral sites are filled. This is called a trioctahedral mineral. This arrangement results is a neutral sheet with no charge excess or deficit. Therefore, there is no interlayer charge to retain ions between the sheets and to strongly hold the sheets together. The sheets are only held together by weak hydrogen and van der Waals bonds and this results in a very soft and easily cleavable mineral.

In normal gibbsite, the hydroxyl ions are facing each other in successive layers. Layers are slightly offset, to produce monoclinic symetry. Minor substitutions of Fe³⁺ for Al³⁺ are common in gibbsite. Crystals of gibbsite are typically-very small (< 2 µm in diameter), tabular and often, foliated, showing pseudohexagonal outline. They may occasionally be granular. Compact lamellar aggregates, or rarely fibrous masses, may occur as whorls or as stalactitic forms. The color may be white, gray, yellow, red, and brown, though most colors other than white or gray are due to traces of iron (hydr)oxides.

Occurrence and Use

Gibbsite is a secondary mineral mainly of tropical and subtropical occurrence. It is an alteration product of many aluminous and alumino-silicate minerals under intense weathering conditions. As such, it is commonly found in lateritic formations, highly-weathered soils and clay deposits. It is a major mineral in Bauxite rocks, in association with other aluminum oxide and hydroxide minerals such as diaspore [AlOOH], boehmite [AlOOH], and minor proportions of kaolinite and iron oxides and hydroxides. Gibbsite, diaspore and boehmite may occur in varying proportions, and any one of the three minerals may dominate to the almost complete exclusion of the other two. The fine-grained and tabular form of gibbsite make it most likely to be confused with other clay minerals, like kaolinite.

Bauxites are the principal ore of aluminum and typically contain high quantities of one or more of these Al (hydr)oxides. Bauxite deposits occur in many areas of the tropics and subtropics zones. They may also exist in other areas where ancient climates (paleoclimates) were sufficiently intense and landscapes sufficiently stable to allow the formation of gibbsite and other oxides and hydroxides. Notable occurrences include Brazil, Central Africa, Germany, Hungary, France, Guyana and USA (mainly Alaska).

Importance in Soil Environments

Gibbsite is the most common crystalline Al (hydr)oxide in soil environments. It is a major mineral in soils of the tropics and subtropics where it may be the dominant mineral in the clay fraction. Soils may have formed under intense weathering from a variety of parent materials, either of igneous, metamorphic or sedimentary origins. It may also exist in less weathered soils where it was inherited from the parent materials, and its content usually deceases, with a concomitant increase of clay minerals such as kaolinite. Gibbsite is very stable under most earth surface conditions, but may alter in special conditions to clay minerals, such as the kaolinite minerals.

Crystallographic Data

Composition

Al(OH)₃

Classification

Oxide and Hydroxide Group

Structural Parameters

a=8.6840, b=5.0780, c=9.7360 Å
alpha=90, beta=94.54, gamma=90°
Space Group: P 2(1)/n

Reference

Saalfeld, H., and M. Wedde. 1974. Z. Kristallogr. 139:129-135.

Saalfeld, H., and M. Wedde. 1974. Z. Kristallogr. 139:129-135.