Gibbsite, Al(OH)₃, is one of the mineral forms of

aluminium hydroxide Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic and ...

. It is often designated as γ-Al(OH)₃ (but sometimes as α-Al(OH)₃.). It is also sometimes called hydrargillite (or hydrargyllite). Gibbsite is an important ore of

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

in that it is one of three main phases that make up the rock

bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH ...

. Gibbsite has three named structural polymorphs or

polytype In materials science, polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon. Allotropy refers to polymorphism ...

s: bayerite (designated often as α-Al(OH)₃, but sometimes as β-Al(OH)₃), '' doyleite'', and nordstrandite. Gibbsite can be monoclinic or triclinic, while bayerite is monoclinic. Doyleite and nordstrandite are triclinic forms.

Structure

The structure of gibbsite is interesting and analogous to the basic structure of the micas. The basic structure forms stacked sheets of linked

octahedra In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

. Each octahedron is composed of an

ion bonded to six hydroxide groups, and each

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

group is shared by two aluminium octahedra. One third of the potential octahedral spaces are missing a central aluminium. The result is a neutral sheet: with aluminium as a +3 ion and hydroxide a –1 ion, the net cationic charge of one aluminium per six hydroxides is (+3)/6 = +1/2, and likewise the net anionic charge of one hydroxide per two aluminium atoms is (-1)/2 = -1/2. The lack of a charge on the gibbsite sheets means that there is no charge to retain ions between the sheets and act as a "glue" to keep the sheets together. The sheets are only held together by weak residual bonds and this results in a very soft easily cleaved mineral. Gibbsite's structure is closely related to the structure of

brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists ...

, Mg(OH)₂. However the lower charge in brucite's magnesium (+2) as opposed to gibbsite's aluminium (+3) does not require that one third of the octahedrons be vacant of a central ion in order to maintain a neutral sheet. The different symmetry of gibbsite and brucite is due to the different way that the layers are stacked. It is the gibbsite layer that in a way forms the "floor plan" for the mineral

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pre ...

, Al₂O₃. The basic structure of corundum is identical to gibbsite except the hydroxides are replaced by oxygen. Since oxygen has a charge of -2 the layers are not neutral and require that they must be bonded to other aluminiums above and below the initial layer producing the framework structure that is the structure of corundum. Gibbsite is interesting for another reason because it is often found as a part of the structure of other minerals. The neutral aluminium hydroxide sheets are found sandwiched between silicate sheets in important clay groups: the illite, kaolinite, and montmorillonite/ smectite groups. The individual aluminium hydroxide layers are identical to the individual layers of gibbsite and are referred to as the ''gibbsite layers''. The lattice parameters for gibbsite depending upon the particular method used to measure or calculate them and are therefore displayed as ranges below. An Al-Al interlayer spacing of 0.484 or 0.494 nm has been reported.

Mineralogical Properties

Etymology

Gibbsite is named after George Gibbs (1776–1833), an American

mineral collector Mineral collecting is the hobby of systematically collecting, identifying and displaying mineral specimens. Mineral collecting can also be a part of the profession of mineralogy and allied geologic specialties. Individual collectors often ...

.Gibbsite Mineral Data on Webmineral.com
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Structure

Mineralogical Properties

Etymology

References

Further reading