Laterite

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Cutting of laterite brickstones, Angadipuram, India
Cutting of laterite brickstones, Angadipuram, India

Laterite (from the Latin word "later" meaning brick or tile) is a surface formation in hot and wet tropical areas which is enriched in iron and aluminium and develops by intensive and long lasting weathering of the underlying parent rock. Nearly all kinds of rocks can be deeply decomposed by the action of high rainfall and elevated temperatures. The percolating rain water causes dissolution of primary rock minerals and decrease of easily soluble elements as sodium, potassium, calcium, magnesium and silicon. This gives rise to a residual concentration of more insoluble elements predominantly iron and aluminium. Laterites consist mainly of the minerals kaolinite, goethite, hematite and gibbsite which form in the course of weathering. Moreover, many laterites contain quartz as relatively stable relic mineral from the parent rock. The iron oxides goethite and hematite cause the red-brown color of laterites.

Cretaceous laterite exposed in Makhtesh Gadol, Israel.
Cretaceous laterite exposed in Makhtesh Gadol, Israel.

Laterite covers have mostly a thickness of a few meters but occasionally they can be much thicker. Their formation is favoured by a slight relief which prevents erosion of the surface cover. Laterites occurring in non-tropical areas are products of former geological epochs. Lateritic soils form the uppermost part of the laterite cover; in soil science specific names (oxisol, latosol, ferallitic soil) are given for them.

In geosciences only those weathering products are defined as laterite, which are geochemically - mineralogically most strongly altered. They must be distinguished from less altered saprolite which has often a similar appearance and is also very widespread in tropical areas. Both formations can be classified as residual rocks.

Laterites can be as well soft and friable as firm and physically resistant. Indurated varieties are sometimes cut into blocks and used as brickstones for house-building—the term derives from Latin later = brickstone. Khmer temples were often constructed of laterite, but by the 12th century Khmer architects had become skilled and confident in the use of sandstone as the main building material. Most of the visible areas at Angkor Wat are of sandstone blocks, with laterite used for the outer wall and for hidden structural parts that have survived for over 1000 years. Hardened laterite varieties are also applied for the construction of simple roads (laterite pistes). Nowadays solid lateritic gravel is readily put in aquaria where it favors the growth of tropical plants.

Lateritization is economically most important for the formation of lateritic ore deposits. Bauxite which is an aluminium-rich laterite variety can form from various parent rocks if the drainage is most intensive thus leading to a very strong leaching of silica and equivalent enrichment of aluminium hydroxides above all gibbsite.

Lateritization of ultramafic igneous rocks (serpentinite, dunite, or peridotite containing about 0,2 - 0,3% nickel) often results in a considerable nickel concentration. Two kinds of lateritic nickel ore have to be distinguished: A very iron-rich nickel limonite or nickel oxide ore at the surface contains 1-2% Ni bound in goethite which is highly enriched due to very strong leaching of magnesium and silica. Beneath this zone nickel silicate ore can be formed, frequently containing > 2% Ni that is incorporated in silicate minerals primarily serpentine. In pockets and fissures of the serpentinite rock green garnierite can be present in minor quantities, but with high nickel contents - mostly 20-40%. It is bound in newly formed phyllosilicate minerals. All the nickel in the silicate zone is leached downwards (absolute nickel concentration) from the overlying goethite zone. Absence of this zone is due to erosion.

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[edit] References

  • Aleva,G.J.J.(Compiler) (1994): Laterites. Concepts, Geology, Morphology and Chemistry.169 pp. ISRIC, Wageningen, The Netherlands, ISBN 90-6672-053-0
  • Bardossy, G. and Aleva, G.J.J.(1990): Lateritic Bauxites. 624 pp. Developments in Economic Geology 27, ELSEVIER, ISBN 0-444-98811-4
  • Golightly, J.P. (1981): Nickeliferous Laterite Deposits. Economic Geology 75, 710-735
  • Schellmann, W. (1983): Geochemical principles of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes, Sao Paulo, 119-135

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