Bauxite deposits are formed chiefly by weathering of aluminous rock; some have been transported to their present locations, but most are residual accumulations from which most other constituents of the parent rock, other than alumina, have been leached.
Bauxite deposits occur in rocks ranging in age from Precambrian to Holocene.
The processes involved in bauxite formation are more complex than shown below, but the following example for weathering of feldspar is indicative of the overall net formation sequence.
Step 1: Acidification of rainwater
CO2 + H2O → H2CO3
H2CO3 → HCO3 - + H+
HCO3- → CO32- + H+
Step 2: Carbonic and humic acids (from soil) react with feldspar, leaching potassium and silica, and hydrating the alumino-silicate structure to form an illite clay:
3KAlSi3O8 + 2H+ + 12H2O → KAl3Si3O10(OH)2 + 2K+ + 6Si(OH)4
Step 3: Further leaching removes the remaining potassium, transforming illite to kaolinite:
2 KAl3Si3O10(OH)2 +2H+ + 3H2O → 3Al2Si2O5(OH)4 + 2K+
Step 4: Kaolinite is decomposed to form insoluble gibbsite and soluble hydrated silica:
Al2Si2O5(OH)4 + 5H2O → 2Al(OH)3 + 2Si(OH)4
Iron in the minerals is converted to the insoluble forms hematite and goethite. Titanium is transformed to anatase. Quartz and zircon are resistant to weathering.
The mixture of remnant minerals is called ‘laterite’ and is a common surface feature in tropical areas. If it is sufficiently high in alumina and low in silica, it is characterised as ‘bauxite’. Laterite bauxites account for most of the world’s major deposits of bauxite.
Weathering of limestone gives rise to eroded surface and sub-surface features (caves & depressions) which are together known as ‘karst’. If the voids are subsequently filled with minerals containing aluminium (eg. clays or laterite) then further weathering may occur leading to bauxitisation. The resulting deposits are known as karst bauxite. They are important in Europe, China and Jamaica.
Bauxite Chemical Composition
Although bauxite comprises a mixture of minerals, the composition is normally reported as the elemental analysis, expressed as metal oxides. This analysis is usually determined by X-Ray Fluorescence Spectrometry (XRF), though classical methods are also available.
The composition of bauxite samples from a range of deposits are tabulated below.