bauxites

bauxites Bauxites are aluminous ores containing more than about 50 per cent of Al₂O₃, formed under humid tropical climates by the weathering of aluminium-rich minerals. The parent materials can be igneous or metamorphic rocks (all basic or moderately acid) as well as sedimentary rocks: shales marls, limestones, or sandstones (rich in clay minerals but relatively poor in quartz). Bauxites are lateritic materials either remaining in situ (lateritic bauxites) or located in karstic areas within which limestones have been dissolved in ground and surface waters (karstic bauxites). They are chiefly composed of aluminium (gibbsite, Al₂O₃·3H₂O; boehmite or diaspore, AlOOH), but may contain significant amounts of iron (goethite, FeOOH or haematite, Fe₂O₃) and small amounts of silica (kaolinite, Al₂O₃·2SiO₂·2H₂O or quartz, SiO₂) and sometimes titanium (anatase, TiO₂). Three major classes of bauxite are distinguished. (1) Orthobauxites are gibbsitic, of massive structure, red in colour, relatively rich in iron, and overlain by an iron-rich hardcap;(2) metabauxites are orthobauxites subsequently transformed under arid conditions; they are boehmitic, white in colour, poor in iron, exhibit a pisolitic structure, and are underlain by kaolinitic nodular ferricrete;(3) cryptobauxites are orthobauxites that evolved under very humid conditions and are covered by a kaolinitic soft horizon of a biological origin. Bauxites are good continental indicators of palaeoclimatic changes.

Y. Tardy