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.). It is also sometimes called hydrargillite (or hydrargyllite ) .as
It 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.a
It 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 .a
Ball-and-stick model of the part of the crystal structure of gibbsite
Doyleite with Gibbsite
Structure
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.a
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.a
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 it 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.a
This 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.a
Gibbsite is named after George Gibbs (1776–۱۸۳۳), an American mineral collector. G. Gibbs’ collection was acquired by Yale University early in the nineteenth century.a