A mixture of three hydrates of alumina, mainly gibbsite, and also diaspore and boehmite, and containing impurities of iron, phosphorus, and titanium; colour is variable from dirty white through grey, yellow, brown, and red; sp. gr. 2.0–2.55; hardness 1–3; it can be compact, earthy, concretionary (see concretion), pisolitic, or oolitic. Bauxite results from the tropical weathering of aluminium silicate rocks under good surface drainage to yield clay minerals which are subsequently desilicated. Minerals associated with the alumina hydrates in bauxites and laterites (ferruginous bauxites) include goethite and lepidocrocite, hematite, and the clay minerals kaolinite and halloysite. Bauxite is the main ore of aluminium and to be commercially exploited should contain more than 25–30% aluminium oxide. The main constraint is the amount of available alumina which can be extracted by the Bayer or similar process. It is named after Les Baux de Provence, in southern France; the major producers are Australia and Brazil.