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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.Gibbsite has three named structural polymorphs or polytype s: bayerite, doyleite, and nordstrandite. Gibbsite and bayerite are monoclinic, whereas doyleite and nordstrandite are triclinic forms.
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.
Gibbsite's structure is closely related to the structure of bruciteBrucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH). It is pearly white or pale green in colour, translucent, with perfect cleavage, and tabular crystals or massive. It has a hardness of 2. Brucite is often found in hydot, 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 corundumCorundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. Corundum is naturally clear, but can have different colors when impurities are added. Transparent specimens are used as gems, called ruby if red, while all other col, Al2O3. The basic structure of corundum is identical to gibbsite except the hydroxides are replaced by oxygenOxygen is the chemical element in the periodic table that has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe. Molecular oxygen (O, often called free oxygen on Earth is thermodynamically un. 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 , kaoliniteKaolinite is a mineral with the chemical composition Al Si O( OH). It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen molecules to one octahedral sheet of alumina octahedra. It is also known as china clay and kaolin ( in pi, 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.
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Minerals
Oxide minerals
