Gibbsite, Al(OH)₃, is an important ore of aluminium and is one of three minerals that make up the rock bauxite. Bauxite is often thought of as a mineral but is really a rock composed of aluminium oxide and hydroxide minerals such as gibbsite, boehmite (AlO(OH)), and diaspore (HAlO₂), as well as clays, silt, and iron oxides and hydroxides. Bauxite is a laterite, a rock formed from intense weathering environments such as found in richly forested, humid, tropical climates. General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... Bauxite Bauxite is a naturally occurring, heterogeneous material composed primarily of one or more aluminium hydroxide minerals, plus various mixtures of silica, iron oxide, titania, aluminosilicate, and other impurities in minor or trace amounts. ... Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with chemical formula Al2O3. ... Diaspore is a native aluminium hydroxide, AlO(OH), crystallizing in the orthorhombic system and isomorphous with goethite and manganite. ... Diaspore is a native aluminium hydroxide, AlO(OH), crystallizing in the orthorhombic system and isomorphous with goethite and manganite. ... Clay is a generic term for an aggregate of hydrous silicate particles less than 4 μm (micrometres) in diameter. ... Silt refers to soil or rock particles of a certain very small size range (see grain size). ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... Hydroxide is a polyatomic ion consisting of oxygen and hydrogen: −O−H It has a charge of −1. ... Laterite is a red-colored clay rich soil found in the tropics and subtropics. ...

Gibbsite has three named structural polymorphs or polytypes: bayerite, doyleite, and nordstrandite. Gibbsite and bayerite are monoclinic, whereas doyleite and nordstrandite are triclinic forms. In general, a polymorph is something that can exist in several states or forms. ... In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. ... In crystallography, the triclinic crystal system is one of the 7 lattice point groups. ...

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 aluminiums 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. rock with mica Mica sheet mica flakes The mica group of minerals includes several closely related materials having highly perfect basal cleavage. ... An octahedron (plural: octahedra) is a polyhedron with eight faces. ...

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. Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg(OH)2. ...

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. Corundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ...

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. Structure of Illite mica - USGS. Illite is a non-expanding, clay-sized, micaceous mineral. ... Kaolin Kaolinite (Aluminium Silicate Hydroxide) Kaolinite is a mineral with the chemical composition Al2Si2O5(OH)4. ... Montmorillonite is a very soft phyllosilicate mineral that typically forms in microscopic crystals, forming a clay. ...

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
• Webmineral Gibbsite