Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock. Tropical weathering (laterization) is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics of Cancer and Capricorn.
Laterites are a source of aluminium ore; the ore exists largely in clay minerals and the hydroxides, gibbsite, boehmite, and diaspore, which resembles the composition of bauxite. In Northern Ireland they once provided a major source of iron and aluminium ores. Laterite ores also were the early major source of nickel.
Laterite is a residual material. This is what is left of common silicate rocks if we remove much of silica, alkali, and alkaline earth metals. It is mostly composed of iron, aluminum, titanium, and manganese oxides because these are the least soluble components of the rocks undergoing a type of chemical weathering known as laterization
Composition and properties
Laterites consist mainly of the minerals kaolinite, goethite, hematite, and gibbsite, which form in the course of weathering. Moreover, many laterites contain quartz as a relatively stable, relic mineral from the parent rock. The iron oxides goethite and hematite cause the red-brown color of laterites.
Laterites can be soft and friable as well as firm and physically resistant. Laterite covers usually have a thickness of a few meters, but occasionally they can be much thicker. Their formation is favored by a slight relief that prevents erosion of the surface cover.
The industrial use of laterite is in the cement industry . It is used as an additive for lowering the clinkerisation temperature and supplementing aluminous and iron contents required in the manufacture of cement. It is also reported that laterite is capable of removal of phosphorus from solutions and percolating columns of laterite remove cadmium, chromium and lead to very low concentrations.
Limonite type
Limonite type laterites (or oxide type) are highly enriched in iron due to very strong leaching of magnesium and silica. They consist largely of goethite and contain 1-2% nickel incorporated in goethite. Absence of the limonite zone in the ore deposits is due to erosion.
Silicate type
Silicate type (or saprolite type) nickel ore formed beneath the limonite zone. It contains generally 1.5-2.5% nickel and consists largely of Mg-depleted serpentine in which nickel is incorporated. 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.
