Chertified pisolitic bauxite from Arkansas, USA. (wet, cut surface; 11.7 cm across at its widest)
Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.
There are three categories of sedimentary rocks:
1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks.
2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms).
3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.
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.
The Arkansas bauxite sample shown above has been chertified. It has particularly large, well zoned pisolites.