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Laterite


Laterite is a pedogenic and highly weathered natural material or a red residual soil formed by the concentration of hydrated oxides of iron and aluminium. Laterite occurs mostly in the tropical and sub-tropical regions with lowest maximum temperatures above 18°C and the average temperature over 24°C. It is a rock product that is a response to a set of physiochemical conditions, which include an iron-containing parent rock, a well-drained terrain, abundant moisture for hydrolysis during weathering, relatively high oxidation potential, and persistence of these conditions over thousands of years. 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. It contains the iron oxide minerals goethite, HFeO2; lepidocrocite, FeO(OH); and hematite, Fe2O3. It also contains titanium oxides and hydrated oxides of aluminum, the most common and abundant of which is gibbsite, Al2O3.3H2O. The aluminum-rich representative of laterite is bauxite. The iron oxides goethite and hematite cause the red-brown color of laterites. The term 'laterite' was coined by Francis Buchanan-Hamilton in 1807 from southern India.In geosciences only those weathering products are defined as laterite, which are geochemically'mineralogically most strongly altered. 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. Laterite is frequently pisolitic (pealike). Exposed surfaces are blackish-brown to reddish and commonly have a slaggy, or scoriaceous, lavalike appearance. Commonly lighter in colour (red, yellow, and brown) where freshly broken, it is generally soft when freshly quarried but hardens on exposure. Indurated varieties are sometimes cut into blocks and used as brickstones for house-building'the term derives from Latin word later meaning brick, or tile. 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. Nowadays solid lateritic gravel is readily put in aquaria where it favors the growth of tropical plants. Lateritisation of ultramafic igneous rocks (serpentinite, dunite, or peridotite containing about 0.2-0.3% nickel) often results in a considerable nickel concentration. Laterite has been used, in Cuba, as a source of nickel. [Sanzida Murshed]