Bauxite is a aluminum oxide/hydroxide-rich rock. It is the richest source of aluminum compared with any other common rock, and is the best aluminum ore. Bauxites are mixtures of several Al-rich minerals, including gibbsite (Al(OH)3 - aluminum hydroxide), boehmite (gAlO·OH - aluminum hydroxy-oxide), and diaspore (aAlO·OH - aluminum hydroxy-oxide). They also may have lepidochrosite (FeO·OH), an iron hydroxy-oxide, and other minerals. X-ray analysis is usually required to determine what minerals are present in any given bauxite sample. Many bauxites have an oolitic or pisolitic structure. Oolites are small, concentrically layered, spherical to subspherical structures. Pisolites are larger, pedogenic (soil-related) versions of oolites.
Aluminum is abundant in felsic and mafic igneous rocks (e.g., in the feldspars), but considerable heat & energy is required to get the aluminum metal out. Processing of bauxite requires much less heat & energy to produce Al metal. Bauxite forms in tropical and subtropical environments, so most bauxite aluminum ores come from tropical countries.
Bauxites are essentially fossil laterite soils - highly leached tropical soils. Strong chemical weathering in such areas leaches out many components of most bedrock lithologies, leaving behind aluminum hydroxy-oxides and some iron oxides.
Pisolitic bauxite (field of view 2.9 cm across) from Arkansas, USA. Published analyses indicate that Arkansas bauxites are generally dominated by gibbsite, kaolinite, and various iron minerals.
Pisolitic bauxite (cut surface; field of view 1.6 cm across) from Arkansas, USA.
Pisolitic bauxite (4.1 cm across at its widest)
Chertified pisolitic bauxite (wet, cut surface; 11.7 cm across) from Arkansas, USA.