Laterite is a soil type rich iniron andaluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of highiron oxide content. They develop by intensive and prolongedweathering of the underlyingparent rock, usually when there are conditions of high temperatures and heavy rainfall with alternate wet and dry periods.[1] The process of formation is calledlaterization.[2] Tropical weathering is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and oremineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics ofCancer andCapricorn.
Laterite has commonly been referred to as a soil type as well as being a rock type. This, and further variation in the modes of conceptualizing about laterite (e.g. also as a complete weathering profile or theory about weathering), has led to calls for the term to be abandoned altogether. At least a few researchers, including T. R. Paton and M. A. J. Williams,[3] specializing inregolith development have considered that hopeless confusion has evolved around the name. Material that looks highly similar to the Indian laterite occurs abundantly worldwide.
Historically, laterite was cut into brick-like shapes and used in monument-building. After 1000 CE, construction atAngkor Wat and other southeast Asian sites changed to rectangular temple enclosures made of laterite, brick, and stone. Since the mid-1970s, some trial sections ofbituminous-surfaced, low-volume roads have used laterite in place of stone as a base course. Thick laterite layers are porous and slightly permeable, so the layers can function asaquifers in rural areas. Locally available laterites have been used in an acid solution, followed by precipitation to removephosphorus and heavy metals at sewage-treatment facilities.
Laterites are a source of aluminumore; the ore exists largely inclay minerals and thehydroxides,gibbsite,boehmite, anddiaspore, which resembles the composition ofbauxite. InNorthern Ireland they once provided a major source of iron and aluminum ores. Laterite ores also were the early major source ofnickel.
Francis Buchanan-Hamilton first described and named a laterite formation in southernIndia in 1807.[4]: 65 He named it laterite from theLatin wordlater, which means a brick; this highly compacted andcemented soil can easily be cut into brick-shaped blocks for building.[4]: 65 The wordlaterite has been used for variably cemented,sesquioxide-richsoil horizons.[5] A sesquioxide is anoxide with three atoms of oxygen and two metal atoms. It has also been used for any reddish soil at or near the Earth's surface.[5]
Laterite covers are thick in the stable areas of theWestern Ethiopian Shield, oncratons of the South American Plate, and on theAustralian Shield.[6]: 1 InMadhya Pradesh, India, the laterite which caps the plateau is 30 m (100 ft) thick.[7]: 554 Laterites can be either soft and easily broken into smaller pieces, or firm and physically resistant.Basement rocks are buried under the thick weathered layer and rarely exposed.[6]: 1 Lateritic soils form the uppermost part of the laterite cover.
In some places laterites containpisolites andferricrete, and they may be found in elevated positions as result ofrelief inversion.[8]
Cliff Ollier has criticized the usefulness of the concept given that it is used to mean different things to different authors.[9] Reportedly some have used it for ferricrete, others for tropical red earth soil, and yet others for soil profiles made, from top to bottom, of acrust, a mottled zone and a pallid zone.[9] He cautions strongly against the concept of "lateritic deep weathering" since "it begs so many questions".[9]
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.[10]: 3 The initial products of weathering are essentially kaolinized rocks calledsaprolites.[11] A period of active laterization extended from about the mid-Tertiary to the mid-Quaternary periods (35 to 1.5 million years ago).[10]: 3 Statistical analyses show that the transition in the mean and variance levels of18O during the middle of the Pleistocene was abrupt.[12] It seems this abrupt change was global and mainly represents an increase in ice mass; at about the same time an abrupt decrease insea surface temperatures occurred; these two changes indicate a sudden global cooling.[12] The rate of laterization would have decreased with the abrupt cooling of the earth. Weathering in tropical climates continues to this day, at a reduced rate.[10]: 3
Laterites are formed from theleaching of parentsedimentary rocks (sandstones,clays,limestones);metamorphic rocks (schists,gneisses,migmatites);igneous rocks (granites,basalts,gabbros,peridotites); and mineralized proto-ores;[6]: 5 which leaves the moreinsoluble ions, predominantly iron and aluminum. The mechanism of leaching involves acid dissolving the hostminerallattice, followed by hydrolysis and precipitation of insoluble oxides and sulfates of iron, aluminum and silica under the high temperature conditions[13] of a humid sub-tropicalmonsoonclimate.[14]
An essential feature for the formation of laterite is the repetition ofwet anddry seasons.[15] Rocks are leached by percolating rain water during the wet season; the resulting solution containing the leached ions is brought to the surface bycapillary action during the dry season.[15] These ions form solublesalt compounds which dry on the surface; these salts are washed away during the next wet season.[15] Laterite formation is favored in lowtopographical reliefs of gentle crests andplateaus which prevents erosion of the surface cover.[10]: 4 The reaction zone where rocks are in contact with water—from the lowest to highestwater table levels—is progressively depleted of the easily leached ions ofsodium,potassium,calcium andmagnesium.[15] A solution of theseions can have the correctpH to preferentially dissolvesilicon oxide rather than thealuminum oxides andiron oxides.[15]Silcrete has been suggested to form in zones in relatively dry "precipitating zones" of laterites.[16] To the contrary, in the wetter parts of laterites subject to leachingferricretes have been suggested to form.[16]
The mineralogical and chemical compositions of laterites are dependent on their parent rocks.[6]: 6 Laterites consist mainly ofquartz,zircon, and oxides oftitanium, iron,tin, aluminum andmanganese, which remain during the course of weathering.[6]: 7 Quartz is the most abundant relic mineral from the parent rock.[6]: 7
Laterites vary significantly according to their location, climate and depth.[13] The main host minerals for nickel andcobalt can be eitheriron oxides,clay minerals ormanganese oxides.[13] Iron oxides are derived frommaficigneous rocks and other iron-rich rocks;bauxites are derived fromgranitic igneous rock and other iron-poor rocks.[15] Nickel laterites occur in zones of the earth which experienced prolonged tropical weathering ofultramafic rocks containing the ferro-magnesian mineralsolivine,pyroxene, andamphibole.[10]: 3
Yves Tardy, from theFrenchInstitut National Polytechnique de Toulouse and theCentre National de la Recherche Scientifique, calculated that laterites cover about one-third of the Earth's continental land area.[6]: 1 Lateritic soils are thesubsoils of the equatorial forests, of thesavannas of the humid tropical regions, and of theSaheliansteppes.[6]: 1 They cover most of the land area between the tropics of Cancer and Capricorn; areas not covered within these latitudes include the extreme western portion of South America, the southwestern portion of Africa, the desert regions of north-central Africa, the Arabian peninsula and the interior of Australia.[6]: 2
Some of the oldest and most highly deformed ultramafic rocks which underwent laterization are found as petrified fossil soils in the complexPrecambrian shields in Brazil and Australia.[10]: 3 Smaller highly deformedAlpine-type intrusives have formed laterite profiles in Guatemala, Colombia, Central Europe, India and Burma.[10]: 3 Large thrust sheets ofMesozoicisland arcs andcontinental collision zones underwent laterization in New Caledonia, Cuba, Indonesian and the Philippines.[10]: 3 Laterites reflect past weathering conditions;[5] laterites which are found in present-day non-tropical areas are products of formergeological epochs, when that area was near the equator. Present-day laterite occurring outside the humid tropics are considered to be indicators of climatic change, continental drift or a combination of both.[17] In India, laterite soils occupy an area of 240,000 square kilometres.[1]
Laterite soils have a high clay content, which means they have highercation exchange capacity, low permeability, high plasticity and high water-holding capacity than sandy soils. It is because the particles are so small, the water is trapped between them. After the rain, the water moves into the soil slowly. Due to intensive leaching, laterite soils lack in fertility in comparison to other soils, however they respond readily to manuring and irrigation.[1] Palms are less likely to suffer from drought because the rainwater is held in the soil. However, if the structure of lateritic soils becomes degraded, a hard crust can form on the surface, which hinders water infiltration, the emergence of seedlings, and leads to increased runoff. It is possible to rehabilitate such soils, using a system called the 'bio-reclamation of degraded lands'. This involves using indigenous water-harvesting methods (such as planting pits and trenches), applying animal and plant residues, and planting high-value fruit trees and indigenous vegetable crops that are tolerant of drought conditions. These soils are most suitable for plantation crops. They are good for oil palm, tea, coffee and cashew cultivation. TheInternational Crops Research Institute for the Semi-Arid Tropics (ICRISAT) has employed this system to rehabilitate degraded laterite soils inNiger and increasesmallholder farmers' incomes.[18] In some places, these soils support grazing grounds and scrub forests.[1]
When moist, laterites can easily be cut with a spade into regular-sized blocks.[6]: 1 Laterite is mined while it is below the water table, so it is wet and soft.[19] Upon exposure to air it gradually hardens as the moisture between the flat clay particles evaporates and the larger iron salts[15] lock into a rigidlattice structure[19]: 158 and become resistant to atmospheric conditions.[6]: 1 The art of quarrying laterite material intomasonry is suspected to have been introduced from the Indian subcontinent.[clarification needed][20] They harden like iron when they are exposed to air.[1]
After 1000 CE Angkorian construction changed from circular or irregular earthen walls to rectangular temple enclosures of laterite, brick and stone structures.[21]: 3 Geographic surveys show areas which have laterite stone alignments which may be foundations of temple sites that have not survived.[21]: 4 The Khmer people constructed the Angkor monuments—which are widely distributed in Cambodia and Thailand—between the 9th and 13th centuries.[22]: 209 The stone materials used were sandstone and laterite; brick had been used in monuments constructed in the 9th and 10th centuries.[22]: 210 Two types of laterite can be identified; both types consist of the minerals kaolinite, quartz, hematite and goethite.[22]: 211 Differences in the amounts of minor elements arsenic, antimony, vanadium and strontium were measured between the two laterites.[22]: 211
Angkor Wat—located in present-day Cambodia—is the largest religious structure built bySuryavarman II, who ruled theKhmer Empire from 1112 to 1152.[23]: 39 It is a World Heritage site.[23]: 39 The sandstone used for the building of Angkor Wat is Mesozoic sandstone quarried in the Phnom Kulen Mountains, about 40 km (25 mi) away from the temple.[24] The foundations and internal parts of the temple contain laterite blocks behind the sandstone surface.[24] The masonry was laid without joint mortar.[24]
It is used as a local building material in places such asBurkina Faso, where it is valued for being strong and for reducing heating and cooling costs.[25]
The French surfaced roads in the Cambodia, Thailand and Vietnam area with crushed laterite, stone or gravel.[26] Kenya, during the mid-1970s, and Malawi, during the mid-1980s, constructed trial sections of bituminous-surfaced low-volume roads using laterite in place of stone as a base course.[27] The laterite did not conform with any accepted specifications but performed equally well when compared with adjoining sections of road using stone or other stabilized material as a base.[27] In 1984 US$40,000 per 1 km (0.62 mi) was saved in Malawi by using laterite in this way.[27] It is also widely used inBrazil for road building.[28]
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.[6]: 2 One example is the Southwestern Laterite (Cabook) Aquifer in Sri Lanka.[29]: 1 This aquifer is on the southwest border of Sri Lanka, with the narrow Shallow Aquifers on Coastal Sands between it and the ocean.[29]: 4 It has the considerable water-holding capacity, depending on the depth of the formation.[29]: 1 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 themonsoon rains.[29]: 10 The water table recedes slowly and is recharged several times during the rest of the year.[29]: 13 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.[29]: 13 The Cabook Aquifer laterites support relatively shallow aquifers that are accessible to dug wells.[29]: 10
In Northern Ireland, phosphorus enrichment of lakes due to agriculture is a significant problem.[30] Locally available laterite—a low-grade bauxite rich in iron and aluminum—is used in acid solution, followed by precipitation to remove phosphorus and heavy metals at several sewage treatment facilities.[30] Calcium-, iron- and aluminum-rich solid media are recommended for phosphorus removal.[30] A study, using both laboratory tests and pilot-scale constructed wetlands, reports the effectiveness of granular laterite in removing phosphorus and heavy metals from landfillleachate.[30] Initial laboratory studies show that laterite is capable of 99% removal of phosphorus from solution.[30] A pilot-scale experimental facility containing laterite achieved 96% removal of phosphorus.[30] This removal is greater than reported in other systems.[30] Initial removals of aluminum and iron by pilot-scale facilities have been up to 85% and 98% respectively.[30] Percolating columns of laterite removed enoughcadmium,chromium andlead to undetectable concentrations.[30] There is a possible application of this low-cost, low-technology, visually unobtrusive, efficient system for rural areas with dispersed point sources of pollution.[30]
Ores are concentrated in metalliferous laterites; aluminum is found inbauxites, 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.[6]: 2
Bauxite ore is the main source of aluminum.[4]: 65 It is a variety of laterite (residual sedimentary rock), so it has no precise chemical formula.[31] It is composed mainly of hydrated alumina minerals such asgibbsite [Al(OH)3 or Al2O3 . 3H2O)] in newer tropical deposits; in older subtropical, temperate deposits the major minerals areboehmite [γ-AlO(OH) or Al2O3.H2O] and somediaspore [α-AlO(OH) or Al2O3.H2O].[31] The average chemical composition of bauxite, by weight, is 45 to 60% Al2O3 and 20 to 30% Fe2O3.[31] The remaining weight consists of silicas (quartz,chalcedony andkaolinite), carbonates (calcite,magnesite anddolomite), titanium dioxide and water.[31] Bauxites of economical interest must be low in kaolinite.[11] Formation of lateritic bauxites occurs worldwide in the 145- to 2-million-year-oldCretaceous and Tertiary coastal plains.[32] 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.[32] Many high-level bauxites are formed in coastal plains which were subsequently uplifted to their present altitude.[32]
The basaltic laterites ofNorthern Ireland were formed by extensive chemical weathering ofbasalts during a period of volcanic activity.[14] They reach a maximum thickness of 30 m (100 ft) and once provided a major source of iron and aluminum ore.[14] Percolating waters caused degradation of the parent basalt and preferential precipitation by acidic water through the lattice left the iron and aluminum ores.[14] Primaryolivine,plagioclasefeldspar andaugite were successively broken down and replaced by a mineral assemblage consisting ofhematite,gibbsite,goethite,anatase,halloysite andkaolinite.[14]
Laterite ores were the major source of early nickel.[10]: 1 Rich laterite deposits inNew Caledonia were mined starting the end of the 19th century to producewhite metal.[10]: 1 The discovery of sulfide deposits ofSudbury, Ontario, Canada, during the early part of the 20th century shifted the focus tosulfides for nickel extraction.[10]: 1 About 70% of the Earth's land-basednickel resources are contained in laterites; they currently account for about 40% of the world nickel production.[10]: 1 In 1950 laterite-source nickel was less than 10% of total production, in 2003 it accounted for 42%, and by 2012 the share of laterite-source nickel was expected to be 51%.[10]: 1 The four main areas in the world with the largest nickel laterite resources are New Caledonia, with 21%; Australia, with 20%; the Philippines, with 17%; and Indonesia, with 12%.[10]: 4
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