Chertified pisolitic bauxite (wet, cut surface; 11.7 cm across) from Arkansas, USA. 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.
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