LATERITE

'Laterite' 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.
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 laterite which is derived from the Latin word later = brickstone is given because of this usage. History of laterite monuments dates back to 200 B.C. with megaliths of Kerala, South India. Most of the third generation Khmer temples at Angkor are built with laterite and have survived for over 1000 years. Later, world heritage sites such as Churches of Old Goa (India) and Walls in G5 monuments of My Son, Vietnam are also built in Laterite. Till today it is a comon vernacular building material and profoundly used in road construction. Nowadays solid lateritic gravel is readily put in aquaria where it favors the growth of tropical plants.
Hardened laterite varieties are also used to construct laterite roads (also known as laterite pistes), especially in Africa. Such roads range from local roads to major highways. If well-constructed with attention to compacting the roadway base and drainage, and well maintained by grading, reasonably high average speeds and smooth travel can be achieved on them when dry. In East Africa, a softer laterite locally called murram is used for road building.
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.

Contents

See also

References

External links

See also

★ Oxisol, the more recent name for some laterite soils.

★ Ore genesis

★ Lateritic nickel ore deposits

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

★ Buchanan, F. (1807): A Journey from Madras Through the Countries of Mysore, Canara and Malabar, T. Cadell and W. Davies, London. (reprint: Asian Educational Services,New Delhi, India), ISBN 8120603869

★ 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

★ Sutapa Das (2007): Laterite Monuments of India. Construction History Society Newletter May 2007, UK, 15-19

External links

★ http://commons.wikimedia.org/wiki/Laterite Laterite images

★ http://www.laterite.de An Introduction in Laterite