Gibbsite


Gibbsite

Gibbsite, Al(OH)3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al(OH)3 (but sometimes as α-Al(OH)3 [N.N. Greenwood and A. Earnshaw, "Chemistry of Elements", 2nd edition, Butterworth and Heinemann, 1997] .). It is also sometimes called hydrargillite (or hydrargyllite).

Gibbsite is an important ore of aluminium in that it is one of three main phases that make up the rock bauxite. Bauxite is often thought of as a mineral but it is really a rock composed of hydroxide and oxyhydroxide minerals such as gibbsite, boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), as well as clays, silt, and iron oxides and hydroxides.

Gibbsite has three named structural polymorphs or polytypes: bayerite (designated often as α-Al(OH)3, but sometimes as β-Al(OH)3), doyleite, and nordstrandite. Gibbsite and bayerite are monoclinic, whereas doyleite and nordstrandite are triclinic forms.

tructure

The structure of gibbsite is interesting and analogous to the basic structure of the micas. The basic structure forms stacked sheets of linked octahedrons of aluminium hydroxide. The octahedrons are composed of aluminium ions with a +3 charge bonded to six octahedrally coordinated hydroxides with a -1 charge. Each of the hydroxides is bonded to only two aluminium atoms because one third of the octahedrons are vacant a central aluminium. The result is a neutral sheet since +3/6 = +1/2 (+3 charge on the aluminiums divided by six hydroxide bonds times the number of aluminiums) and -1/2 = -1/2 (-1 charge on the hydroxides divided between only two aluminiums); thus the charges cancel. 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, 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, 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". [ [http://www.galleries.com/minerals/oxides/gibbsite/gibbsite.htm GIBBSITE (Aluminum Hydroxide) ] ]

Etymology

Gibbsite is named after George Gibbs (1776-1833), an American mineral collector [ [http://webmineral.com/data/Gibbsite.shtml Gibbsite Mineral Data ] ] . G. Gibbs' collection was acquired by Yale University early in the nineteenth century [ [http://www.mindat.org/min-1689.html Gibbsite: Gibbsite mineral information and data ] ] .

Further reading

*Hurlbut, Cornelius S.; Klein, Cornelis, 1985, "Manual of Mineralogy", 20th ed., ISBN 0-471-80580-7
* [http://webmineral.com/data/Gibbsite.shtml Webmineral Gibbsite]

References


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Look at other dictionaries:

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  • gibbsite — ● gibbsite nom féminin (de G. Gibbs, nom propre) Hydroxyde naturel d aluminium, de formule Al(OH)3, qui constitue une forme fréquente d accumulation de l aluminium dans les sols des pays tropicaux. ● gibbsite (synonymes) nom féminin (de G. Gibbs …   Encyclopédie Universelle

  • Gibbsite — Gibbs ite, n. [Named after George Gibbs.] (Min.) A hydrate of alumina. [1913 Webster] …   The Collaborative International Dictionary of English

  • gibbsite — /gib zuyt/, n. a mineral, hydrated aluminum oxide, Al2O33H2O, occurring in whitish or grayish crystals and masses: an important constituent of bauxite ore. Also called hydrargillite. [1815 25; named after George Gibbs (d. 1833), American… …   Universalium

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  • gibbsite — noun A mineral with monoclinic crystals, usually white but can have different colors, Al(OH) …   Wiktionary

  • gibbsite — gibbs·ite …   English syllables

  • gibbsite — /ˈgɪbzaɪt/ (say gibzuyt) noun a mineral, hydrated aluminium oxide, Al2O3.3H2O, occurring in whitish or greyish crystals and masses, an important constituent of bauxite ore. {named after G Gibbs, died 1833, US mineralogist. See ite1} …   Australian English dictionary

  • gibbsite — noun white crystalline mineral consisting of aluminum hydroxide; a constituent of bauxite and a source of alumina • Hypernyms: ↑mineral • Substance Holonyms: ↑bauxite • Substance Meronyms: ↑aluminum hydroxide, ↑aluminium hydroxide, ↑ …   Useful english dictionary

  • Kaolin Deposits of Charentes Basin, France — The geological unit called Charentes basin is composed of Eocene and Oligocene deposits, laid above karstic limestone formations of the Campagnien, in the North of the Aquitaine Basin. The kaolin clays of Charentes belong to this mainly… …   Wikipedia


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