Bauxite - Encyclopedia

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BAUXITE, a substance which has been considered to be a mineral species, having the composition Al 2 0(OH) 4 (corresponding with alumina 73.9, water 26.1%), and thus to be distinct from the crystallized aluminium hydroxides, diaspore (A10(OH)) and gibbsite (= hydrargillite, Al(OH) 3). It was first described by P. Berthier in 1821 as "alumine hydratee de Beaux," and was named beauxite by P. A. Dufrenoy in 1847 and bauxite by E. H. Sainte-Claire Deville in 1861; this name being derived from the original locality, the village of Les Baux (or Beaux), near Arles, dep. Bouches-du-Rhone in the south of France, where the material has been for many years extensively mined as an ore of aluminium. It is never found in a crystallized state, but always as earthy, clay-like or concretionary masses, often with a pisolitic structure. In colour it varies from white through yellow and brown to red, depending on the amount and the degree of hydration of the iron present. The specific gravity also varies with the amount of iron; that of the variety known as wocheinite (from near Lake Wochein, near Radmannsdorf, in northern Carniola) is given as 2.55. The numerous chemical analyses, which have mostly been made for technical purposes, show that material known as bauxite varies very widely in composition, the maximum and minimum percentages of each constituent being as follows: alumina (Al 2 0 3) 33.2-76.9; water (H 2 0) 8.6-31.4; iron oxide (Fe203) 0.1-48.8; silica (S10 2) o. 3-37.8; titanic acid (TiO 2) up to 4. The material is thus usually very impure, being mixed with clay, quartz-sand and hydroxides of iron in variable amounts, the presence of which may be seen by a microscopical examination. Analyses of purer material often approximate to diaspore or gibbsite in composition, and minute crystalline scales of these minerals have been detected under the microscope.

Bauxite can therefore scarcely be regarded as a simple mineral, but rather as a mixture of gibbsite and diaspore with various impurities; it is in fact strikingly like laterite, both in chemical composition and in microscopical structure. Laterite is admittedly a decomposition-product of igneous or other crystalline rocks, and the same is no doubt also true of bauxite. The deposits in Co. Antrim occur with pisolitic iron ore interbedded with the Tertiary basalts, and similar deposits are met with in connexion with the basaltic rocks of the Westerwald in Germany. On the other hand, the more extensive deposits in the south of France (departments Bouches-du-Rhone, Ariege, Herault, Var) and the southern United States (Georgia, Alabama, Arkansas) are often associated with limestones; in this case the origin of the bauxite has been ascribed to the chemical action of solutions of aluminium sulphate on the limestones.

Bauxite is of value chiefly as a source of metallic aluminium; the material is first purified by chemical processes, after which the aluminium hydroxide is reduced in the electric furnace. Bauxite is also largely used in the manufacture of alum and other aluminium salts used in dyeing. Its refractory qualities render it available for the manufacture of fire-bricks and crucibles. (L. J. S.)

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