<P> Bedrock in tropical zones is often granite, gneiss, schist or sandstone; the thick laterite layer is porous and slightly permeable so the layer can function as an aquifer in rural areas . One example is the Southwestern Laterite (Cabook) Aquifer in Sri Lanka . This aquifer is on the southwest border of Sri Lanka, with the narrow Shallow Aquifers on Coastal Sands between it and the ocean . It has considerable water - holding capacity, depending on the depth of the formation . The aquifer in this laterite recharges rapidly with the rains of April--May which follow the dry season of February--March, and continues to fill with the monsoon rains . The water table recedes slowly and is recharged several times during the rest of the year . In some high - density suburban areas the water table could recede to 15 m (50 ft) below ground level during a prolonged dry period of more than 65 days . The Cabook Aquifer laterites support relatively shallow aquifers that are accessible to dug wells . </P> <P> In Northern Ireland, phosphorus enrichment of lakes due to agriculture is a significant problem . Locally available laterite--a low - grade bauxite rich in iron and aluminium--is used in acid solution, followed by precipitation to remove phosphorus and heavy metals at several sewage treatment facilities . Calcium -, iron - and aluminium - rich solid media are recommended for phosphorus removal . A study, using both laboratory tests and pilot - scale constructed wetlands, reports the effectiveness of granular laterite in removing phosphorus and heavy metals from landfill leachate . Initial laboratory studies show that laterite is capable of 99% removal of phosphorus from solution . A pilot - scale experimental facility containing laterite achieved 96% removal of phosphorus . This removal is greater than reported in other systems . Initial removals of aluminium and iron by pilot - scale facilities have been up to 85% and 98% respectively . Percolating columns of laterite removed enough cadmium, chromium and lead to undetectable concentrations . There is a possible application of this low - cost, low - technology, visually unobtrusive, efficient system for rural areas with dispersed point sources of pollution . </P> <P> Ores are concentrated in metalliferous laterites; aluminium is found in bauxites, iron and manganese are found in iron - rich hard crusts, nickel and copper are found in disintegrated rocks, and gold is found in mottled clays . </P> <P> Bauxite ore is the main source for aluminium . Bauxite is a variety of laterite (residual sedimentary rock), so it has no precise chemical formula . It is composed mainly of hydrated alumina minerals such as gibbsite (Al (OH) or Al O . 3H O)) in newer tropical deposits; in older subtropical, temperate deposits the major minerals are boehmite (γ - AlO (OH) or Al O. H O) and some diaspore (α - AlO (OH) or Al O. H O). The average chemical composition of bauxite, by weight, is 45 to 60% Al O and 20 to 30% Fe O . The remaining weight consists of silicas (quartz, chalcedony and kaolinite), carbonates (calcite, magnesite and dolomite), titanium dioxide and water . Bauxites of economical interest must be low in kaolinite . Formation of lateritic bauxites occurs worldwide in the 145 - to 2 - million - year - old Cretaceous and Tertiary coastal plains . The bauxites form elongate belts, sometimes hundreds of kilometers long, parallel to Lower Tertiary shorelines in India and South America; their distribution is not related to a particular mineralogical composition of the parent rock . Many high - level bauxites are formed in coastal plains which were subsequently uplifted to their present altitude . </P>

How is laterite soil formed give the main characteristics of laterite soil