Residual rocks, in which the alumina trihydrate and monohydrate minerals gibbsite, boehmite, and diaspore predominate are classified as bauxite. Other residual minerals are developed in the unique type of rock weathering which produces bauxites, and these minerals kaolinite, halloysite, goethite, hematite, magnetites, anatase, quartz and some phosphatic and manganiferous minerals may form the lesser constituents of bauxites. Since bauxites have formed from a variety of rock types, minor quantities of residual minerals resistant to weathering. Traces of the elements barium, boron, carbon, bismuth, beryllium calcium, cerium, chromium, cobalt, copper, hafnium, gallium gold, yttrium, zinc, zirconium have been found in bauxites. Some of these elements are presumably present as secondary minerals, while other are present in unaltered mineral remnants. Halloysite is rare in older bauxites, but it is a common constituent of younger bauxites and with quartz is usually the main constituent of the residual clays underlying younger bauxites. The alumina hydrates and hydrous Aluminium silicates of bauxites are derived chiefly from the weathering of felspathoids and felspars of igneous, volcanic, and metamorphic rocks, form Aluminium silicates, felspars, micas and clay minerals of sedimentary and metamoriphic rocks, and from the less soluble minerals in particularly pure calcareous limestone. Some alumina hydrates and hydrous Aluminium silicates are produced also from the breakdown of amphiboles and pyroxenes in igneous, volcanic, and metamorphic rocks. Although certain almost pure calcareous rocks from bauxite, no bauxite has been found which is known to be derived from coarde high silica or ultra basic rocks. The colour of bauxites, which varies from white, cream, yellow, green, gray, pink, brown, red to black, and particle size of the iron minerals in the bauxites and is unrelaled to the alumina hydrate minerals. There is a fair scope for new entrepreneurs to enter in this field.
