Bauxite


Bauxite is an aluminum-rich sedimentary rock. It is the principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite4. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks (usually igneous rocks).

Bauxite is a result of intense leaching in hot and humid climate with alternating wet and dry seasons and good downward drainage. The climate promotes vegetation that provides organic acids which help to dissolve rocks in percolating water that carries more soluble components away, leaving only aluminum and also often iron as the least mobile common ions behind. This process is known as lateritization. Hence, bauxite is an aluminum-rich laterite. The accumulation of aluminum-rich residuum, as opposed to one enriched in iron, is a function of higher rainfall, but also lower average temperature (around 22 °C rather than 28 °C)6.

However, there are actually two types of bauxites. The genesis of only one of them can be adequately explained by the lateritization process. Lateritization usually means that aluminum-bearing igneous rocks (like granite) are slowly weathering to clay minerals which are then altered to aluminum hydroxides by continuous leaching of silica. The other type is associated with limestones. This type of bauxite is known as karst bauxite because it accumulates in karst solution depressions within limestone plateaux. These depressions act as sediment traps where either eolian or fluvial aluminum-bearing sediments can accumulate to be later altered to bauxite. This material is bentonitic clay (weathered volcanic ash) in Jamaica, for example6. The chemical process of bauxite formation is similar in both cases (clay is leached to aluminum hydroxides) but the source material and the general geological situation of the two types are different.

Bauxite is either pale pink, orange, or reddish rock which often contains rounded concretionary masses of aluminum hydroxides and iron (hydr)oxides. These are known as pisoliths. They resemble ooids but are generally less regular in shape although their genesis is similar — there is a nucleus around which mineral matter started to accumulate. Width of the sample is 8 cm.

The chemical reactions that constitute the lateritization process5:

2KAlSi3O8 + 2H+ + 9H2O = Al2Si2O5(OH)4 + 2K+ + 4H4SiO4 (K-feldspar dissolves in acidic groundwater to form clay (kaolinite) plus potassium and silicic acid dissolved in water and carried away.)

Then the process continues to take silicon from kaolinite and turn it to aluminum hydroxide:

Al2Si2O5(OH)4 + 5H2O = 2Al(OH)3 + 2H4SiO4

Bauxite deposits in southern Europe, Jamaica, Haiti, and Turkey are associated with karst. Extensive bauxite deposits in Guinea (largest in the world), Australia, Brazil, India, Surinam, and Indonesia are lateritic deposits. Lateritic bauxites constitute more than three-fourths of the world’s bauxite resources. The largest producer is Australia which provides roughly one third (125 million metric tons annually) of the world’s total output1.

Bauxite is usually considered to be a rock type but considerable amount of poorly consolidated material is often also named that way. Bauxitic or lateritic soil is scientifically known as oxisol2. Sometimes “bauxite” has been used as a mineralogical term. In this case it refers to mixed aluminum hydroxide minerals of uncertain identity, analogous to the terms “limonite” and “wad” for iron and manganese hydroxides, respectively3.

Bauxite deposits form close to the surface. Hence, all the mines in the world are open pit. About 8-14 tons of bauxite is needed to produce one ton of aluminum1. So the ore of aluminum needs to be fairly rich in this metal to be worth mining. Aluminum is extracted solely from bauxite but that does not mean that there are no alternatives. Aluminum is the most widespread metal in the crust. That is why we can be so picky and choose only the best possible ore. Only two chemical elements (silicon and oxygen) are more abundant in the crust (here is a longer post about the composition of the Earth’s crust). Aluminum could be extracted from clay minerals or even feldspars. It is not currently being done because there are adequate deposits of bauxite and these deposits are mostly located in more or less normal countries which are not likely to play ridiculous political power games. One of the largest consumers of aluminum is USA which has no metallurgical-grade bauxite mines. But this does not seem to cause much worrying because of the reasons mentioned above.

Karst bauxite from the Urals in Russia. It is composed of gibbsite and hematite. Width of the sample is 14 cm.

References

1. Murray, Haydn H. (2007). Bauxite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 2. 654.
2. Stow, D. A. V. (2005). Sedimentary Rocks in the Field: A Color Guide. Academic Press.
3. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.
4. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.
5. Hale, Martin (1999). Ore Deposits. In: Encyclopedia of Geochemistry (Encyclopedia of Earth Sciences Series) (Ed. Marshall, Clare P. & Fairbridge, Rhodes W.). Springer. 447-453.
6. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.


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