They may occasionally be granular. Compact lamellar aggregates, or rarely fibrous masses, may occur as whorls or as stalactitic forms. The color may be white, gray, yellow, red, and brown, though most colors other than white or gray are due to traces of iron hydr oxides. Gibbsite is a secondary mineral mainly of tropical and subtropical occurence.
It is an alteration product of many aluminous and alumino-silicate minerals under intense weathering conditions. As such, it is commonly found in lateritic formations, highly-weathered soils and clay deposits. It is a major mineral in Bauxite rocks, in association with other aluminum oxide and hydroxide minerals such as diaspore [AlOO H ], boehmite [AlOOH], and minor proportions of kaolinite and iron oxides and hydroxides.
Gibbsite, diaspore and boehmite may occur in varying proportions, and any one of the three minerals may dominate to the almost complete exclusion of the other two. The fine grained and tabular form of gibbsite make it most likely to be confused with other clay minerals, like kaolinite. Gibbsite's structure is interesting and analogous to the basic structure of micas. The basic structure forms stacked sheets of linked octahedrons of aluminum hydroxide.
Each of the hydroxides is bonded to only two aluminums because one third of the octahedrons are vacant a central aluminum.
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.
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 , Al 2 O 3. Gibbsite's structure is closely related to the structure of brucite , Mg OH 2. 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 , Al 2 O 3.
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 individual aluminium hydroxide layers are identical to the individual layers of gibbsite and are referred to as the gibbsite layers.
Gibbsite is named after George Gibbs , an American mineral collector [3].
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