THE MINERAL GIBBSITE

• Chemical Formula: Al(OH)3, Aluminum Hydroxide
• Class: Oxides and Hydroxides
• Uses: A minor source of metallic aluminum.
• Specimens

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Gibbsite is an important ore of aluminum 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 aluminum 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's structure is interesting and analogous to the basic structure of micas. The basic structure forms stacked sheets of linked octahedrons of aluminum hydroxide. The octahedrons are composed of aluminum ions with a +3 charge bonded to six octahedrally coordinated hydroxides with a -1 charge. Each of the hydroxides is bonded to only two aluminums because one third of the octahedrons are vacant a central aluminum. The result is a neutral sheet since +3/6 = +1/2 (+3 charge on the aluminums divided by six hydroxide bonds times the number of aluminums) and -1/2 = -1/2 (-1 charge on the hydroxides divided between only two aluminums); 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 aluminum (+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 aluminums 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 aluminum hydroxide sheets are found sandwiched between silicate sheets in important clay groups: the Illite, Kaolinite and Montmorillonite/smectite groups. The individual aluminum hydroxide layers are identical to the individual layers of gibbsite and are referred to as the "gibbsite layers".

PHYSICAL CHARACTERISTICS:

• Color is white or colorless with shades of gray, blue and green.
• Luster is vitreous to dull; cleavage surfaces have a pearly luster.
• Transparency Crystals are translucent and rarely transparent.
• Crystal System is monoclinic; 2/m.
• Crystal Habit is usually massive but rare crystals are found in flattened tabular crystals. In many bauxite specimens gibbsite is found in a pisolitic habit. Also found as botryoidal encrustations, concretionary, stalactitic and foliated masses.
• Cleavage is perfect in one direction, basal.
• Fracture is uneven.
• Hardness is 2.5 - 3.5
• Specific Gravity is 2.4 (slightly below average)
• Streak is white.
• Other Characteristics: When breathed on, gibbsite gives off a noticeable clay smell.
• Associated Minerals are boehmite, diaspore, azurite, hydrozincite and aurichalcite.
• Notable Occurrences include Vogelsberg, Germany; Gant, Hungary; Les Baux, France; Lavrion, Greece; Guyana and Arkansas, USA.
• Best Field Indicators are crystal habits, hardness, smell and associations.