Genesis and constitution of Sri Lanka laterites

An arcount is given of the genesis and constitution of Sri Lanka laterites. The warm climate and abundant rainfall alternating with dry periods, favours the development of laterites and lateritic soils in the Island. Lateritic material was examined from coastal areas and inland regions. Majority of the laterites are formed on gneisses of various types, charnockites and granites. It is, however, not possible to identify the parent rock of an individual deposit due to the heterogenity of rocks composing the basement complex. The predominant aluminous mineral in the lateritic materials is the trihydrate, gibbsite (Al₂O₃ 3H₂O) or hydragillite and the most common crystalline ferric oxide hydrate is geothite (Fe₂O₃ H₂O). Aluminous varieties approaching the composition of bauxites are rare. Massive highly ferruginous laterites rich enough in iron hydroxide, mainly goethite (poorly crystallised) to constitute iron ore are common in certain areas of the south-west sector of the Island. It is observed that the laterites of Sri Lanka including the ferruginous varieties are not of any worthwhile commercial importance. They are however, used as a building material (bricks), as clay ochers (specially yellow stains locally named 'samara') and the porous nature of laterites is a highly favourable factor for re-charge of groundwater. The lithomargic clay below the laterite acts as impervious strata and helps in building up the groundwater storage within the laterites.