Gibbsite, or hydrargillite, is a natural mineral and a part of bauxite ores, which are the source of alumina used to produce aluminium.
The chemical formula of gibbsite is Al(OH)3. The mineral contains 65.4 percent of alumina (Al2O3). The main admixtures are iron and gallium. It occurs in nature associated with such minerals, as diaspore, boehmite, corundum, kaolinite, goethite, chlorites and hydroxides of iron and silicon.
Gibbsite Characteristics and Origin
Chemically pure gibbsite is colorless or white; sometimes it can also be transparent. Natural coloring is grey, light blue, greenish, yellowish-pink, reddish-white or reddish-yellow. The color of powder is white. It forms matted fine-grain aggregates, granular accumulations, spherulites or ball-like sags. Crystals have a glass shine; pearly shining appears along cleavage planes.
The mineral has a layered crystalline structure, similar to that of mica, a prismatic symmetry and is characterized by a monoclinic system. Cleavage plates are fragile, hardness is equal to 2.5-3. It does not react with water and acids and dissolves in hot alkali; not radioactive. The density is 2.42 g/cm3, molecular weight – 78 a. m. u. It decomposes at 150°C into boehmite and water.
Gibbsite is mainly formed in the process of weathering of alumina-containing silicates by way of hydrolysis of feldspars, as a result of chemogenic sedimentary processes or low-temperature hydrothermal mineral formation. It mostly occurs near or on the ground surface. Gibbsite is stable on the surface; at significant depths, when exposed to high temperatures and pressures, metamorphic changes occur, which transform gibbsite into diaspore and then into corundum.
Industrial Applications of the Mineral
Aluminium is one of the most wide-spread metals used in various branches of the modern industry. It accounts for the value of gibbsite, a rock-forming mineral in bauxites, which are the principal commercial aluminium ores. Bauxites are considered to be the most high-quality and valuable raw material for producing aluminium. The world’s reserves of aluminium ores are quite significant; yet, aluminium consumption is ever growing, which makes bauxites strongly sought-for and scarce ores. Some types of gibbsites are also used in jewelry, after processing and coloring, as ornamental stones.
The correlation between the type of bauxites and their age has been established. Gibbsite bauxites are young and soft, while diaspore ones are the oldest and hardest ones. Boehmite bauxites occupy the intermediate position in terms of their age and hardness. These characteristics determine processing conditions and the cost of ready-made products. Given equal production costs, alumina of a gibbsite origin is the cheapest one, while dispore alumina is the most expensive one.