Composition

Bauxite occurs in three main forms depending on both the number of molecules of water of hydration and the crystalline structure. The three structural forms of bauxite are Gibbsite, Böhmite and Diaspore.

Gibbsite is a true aluminium hydroxide and Böhmite and Diaspore are both aluminium-oxide-hydroxides. The main difference between the latter two is that Diaspore has a different crystalline structure to Böhmite, and requires even higher temperatures for rapid dehydration. As can be seen from the table below Gibbsite exists in trihydrate form and the latter two are monohydrate.

	Unit	Gibbsite	Böhmite	Diaspore
Composition		Al(OH)3	AlO(OH)	AlO(OH)
Maximum Alumina Content	%	65.4	85.0	85.0
Crystal System		Monoclinic	Orthorhombic	Orthorhombic
Density	gcm-3	2.42	3.01	3.44
Temp. for Rapid Dehydration	°C	150	350	450

Of the bauxite ores currently being mined the dominant form is Gibbsite, followed by a mixed ore of Gibbsite and Böhmite. Differences in ore composition influence their subsequent processing. Processing conditions are also affected by impurities, chiefly compounds of iron, silicon and titanium.

Deposits

The major bauxite deposits are to be found in the tropics and in the Caribbean and Mediterranean regions. Today most bauxite mines are in the Caribbean area, South America, Australia and Africa. Bauxite is found in four types of deposit: blanket, pocket, interlayered and detrital.

• Blanket deposit

  Large blanket deposits occur in West Africa, Australia, South America and India. These deposits occur as flat layers lying near the surface and may extend over an area covering many kilometres. Thickness may very from a metre or less to 40 metres in exceptional cases although 4-6 metres is average.

• Pocket deposits

  In both Jamaica and Hispaniola, as well as in Southern Europe, bauxite is found in pocket deposits. The size of these depressions range in depth from less than one metre to more that thirty. In some cases each pocket is isolated, while in others areas the depressions overlap and compose one large deposit.

• Interlayered deposits

  Interlayered deposits are found in the United States, Suriname, Brazil, Guyana, Russia, China, Hungary and the Mediterranean area. They originally existed at the surface as other types of rocks or volcanics. These formations are usually more compact than in other deposits because of the additional weight of the overburden.

• Detrital deposits

  The term "detrital" refers to deposits that form as a result of accumulation of bauxite that has eroded from other deposits. Arkansas bauxite in the USA is mined from detrital deposits.

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