Laterite

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Monument of laterite brickstones at Angadipuram, Kerala, India, which commemorates where laterite was first described and discussed by Buchanan-Hamilton in 1807. Laterite monument. C 002.jpg
Monument of laterite brickstones at Angadipuram, Kerala, India, which commemorates where laterite was first described and discussed by Buchanan-Hamilton in 1807.

Laterite is both a soil and a rock type rich in iron and aluminum and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and prolonged 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.

Contents

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 [ who? ] specializing in regolith 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 at Angkor Wat and other southeast Asian sites changed to rectangular temple enclosures made of laterite, brick, and stone. Since the mid-1970s, some trial sections of bituminous-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 as aquifers in rural areas. Locally available laterites have been used in an acid solution, followed by precipitation to remove phosphorus and heavy metals at sewage-treatment facilities.

Laterites are a source of aluminum 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 aluminum ores. Laterite ores also were the early major source of nickel.

Definition and physical description

Laterite in Son Tay, Hanoi, Vietnam. Da ong Laterite.JPG
Laterite in Sơn Tây, Hanoi, Vietnam.

Francis Buchanan-Hamilton first described and named a laterite formation in southern India in 1807. [1] :65 He named it laterite from the Latin word later , which means a brick; this highly compacted and cemented soil can easily be cut into brick-shaped blocks for building. [1] :65 The word laterite has been used for variably cemented, sesquioxide-rich soil horizons. [2] A sesquioxide is an oxide 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. [2]

Laterite covers are thick in the stable areas of the Western Ethiopian Shield, on cratons of the South American Plate, and on the Australian Shield. [3] :1 In Madhya Pradesh, India, the laterite which caps the plateau is 30 m (100 ft) thick. [4] :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. [3] :1 Lateritic soils form the uppermost part of the laterite cover.

Formation

Laterite is often located under residual soils. Laterite-saprolite cross section.PNG
Laterite is often located under residual soils.
Soil layers, from soil down to bedrock: A represents soil; B represents laterite, a regolith; C represents saprolite, a less-weathered regolith; below C is bedrock Estructura-suelo.jpg
Soil layers, from soil down to bedrock: A represents soil; B represents laterite, a regolith; C represents saprolite, a less-weathered regolith; below C is bedrock

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. [5] :3 The initial products of weathering are essentially kaolinized rocks called saprolites. [6] A period of active laterization extended from about the mid-Tertiary to the mid-Quaternary periods (35 to 1.5 million years ago). [5] :3 Statistical analyses show that the transition in the mean and variance levels of 18O during the middle of the Pleistocene was abrupt. [7] It seems this abrupt change was global and mainly represents an increase in ice mass; at about the same time an abrupt decrease in sea surface temperatures occurred; these two changes indicate a sudden global cooling. [7] 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. [5] :3

Laterites are formed from the leaching of parent sedimentary rocks (sandstones, clays, limestones); metamorphic rocks (schists, gneisses, migmatites); igneous rocks (granites, basalts, gabbros, peridotites); and mineralized proto-ores; [3] :5 which leaves the more insoluble ions, predominantly iron and aluminum. The mechanism of leaching involves acid dissolving the host mineral lattice, followed by hydrolysis and precipitation of insoluble oxides and sulfates of iron, aluminum and silica under the high temperature conditions [8] of a humid sub-tropical monsoon climate. [9]

An essential feature for the formation of laterite is the repetition of wet and dry seasons. [10] Rocks are leached by percolating rain water during the wet season; the resulting solution containing the leached ions is brought to the surface by capillary action during the dry season. [10] These ions form soluble salt compounds which dry on the surface; these salts are washed away during the next wet season. [10] Laterite formation is favored in low topographical reliefs of gentle crests and plateaus which prevents erosion of the surface cover. [5] :4 The reaction zone where rocks are in contact with water—from the lowest to highest water table levels—is progressively depleted of the easily leached ions of sodium, potassium, calcium and magnesium. [10] A solution of these ions can have the correct pH to preferentially dissolve silicon oxide rather than the aluminum oxides and iron oxides. [10]

The mineralogical and chemical compositions of laterites are dependent on their parent rocks. [3] :6 Laterites consist mainly of quartz, zircon, and oxides of titanium, iron, tin, aluminum and manganese, which remain during the course of weathering. [3] :7 Quartz is the most abundant relic mineral from the parent rock. [3] :7

Laterites vary significantly according to their location, climate and depth. [8] The main host minerals for nickel and cobalt can be either iron oxides, clay minerals or manganese oxides. [8] Iron oxides are derived from mafic igneous rocks and other iron-rich rocks; bauxites are derived from granitic igneous rock and other iron-poor rocks. [10] Nickel laterites occur in zones of the earth which experienced prolonged tropical weathering of ultramafic rocks containing the ferro-magnesian minerals olivine, pyroxene, and amphibole. [5] :3

Locations

Yves Tardy, from the French Institut National Polytechnique de Toulouse and the Centre National de la Recherche Scientifique, calculated that laterites cover about one-third of the Earth's continental land area. [3] :1 Lateritic soils are the subsoils of the equatorial forests, of the savannas of the humid tropical regions, and of the Sahelian steppes. [3] :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. [3] :2

Some of the oldest and most highly deformed ultramafic rocks which underwent laterization are found in the complex Precambrian shields in Brazil and Australia. [5] :3 Smaller highly deformed Alpine-type intrusives have formed laterite profiles in Guatemala, Colombia, Central Europe, India and Burma. [5] :3 Large thrust sheets of Mesozoic island arcs and continental collision zones underwent laterization in New Caledonia, Cuba, Indonesian and the Philippines. [5] :3 Laterites reflect past weathering conditions; [2] laterites which are found in present-day non-tropical areas are products of former geological 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. [11]

Uses

Agriculture

Laterite soils have a high clay content, which mean they have higher cation exchange capacity and water-holding capacity than sandy soils. It is because the particles are so small, the water is trapped between them. After rain, the water moves into the soil slowly. Palms are less likely to suffer from drought because the rain water 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. They are good for oil palm, tea, coffee and cashew cultivation. The International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) has employed this system to rehabilitate degraded laterite soils in Niger and increase smallholder farmers' incomes. [12] it is use in Karnataka

Building blocks

Cutting laterite bricks in Angadipuram, India Laterite quarry, Angadipuram, India. C 004.jpg
Cutting laterite bricks in Angadipuram, India
Example of construction with laterite in Pre Rup, Angkor, Cambodia. Pre Rup, Angkor, Camboya, 2013-08-16, DD 04.JPG
Example of construction with laterite in Pre Rup, Angkor, Cambodia.

When moist, laterites can easily be cut with a spade into regular-sized blocks. [3] :1 Laterite is mined while it is below the water table, so it is wet and soft. [13] Upon exposure to air it gradually hardens as the moisture between the flat clay particles evaporates and the larger iron salts [10] lock into a rigid lattice structure [13] :158 and become resistant to atmospheric conditions. [3] :1 The art of quarrying laterite material into masonry is suspected to have been introduced from the Indian subcontinent.[ clarification needed ] [14]

After 1000 CE Angkorian construction changed from circular or irregular earthen walls to rectangular temple enclosures of laterite, brick and stone structures. [15] :3 Geographic surveys show areas which have laterite stone alignments which may be foundations of temple sites that have not survived. [15] :4 The Khmer people constructed the Angkor monuments—which are widely distributed in Cambodia and Thailand—between the 9th and 13th centuries. [16] :209 The stone materials used were sandstone and laterite; brick had been used in monuments constructed in the 9th and 10th centuries. [16] :210 Two types of laterite can be identified; both types consist of the minerals kaolinite, quartz, hematite and goethite. [16] :211 Differences in the amounts of minor elements arsenic, antimony, vanadium and strontium were measured between the two laterites. [16] :211

Angkor Wat—located in present-day Cambodia—is the largest religious structure built by Suryavarman II, who ruled the Khmer Empire from 1112 to 1152. [17] :39 It is a World Heritage site. [17] :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. [18] The foundations and internal parts of the temple contain laterite blocks behind the sandstone surface. [18] The masonry was laid without joint mortar. [18]

Road building

Laterite road near Kounkane, Upper Casamance, Senegal Laterite-Casamance.jpg
Laterite road near Kounkane, Upper Casamance, Senegal

The French surfaced roads in the Cambodia, Thailand and Vietnam area with crushed laterite, stone or gravel. [19] 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. [20] 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. [20] In 1984 US$40,000 per 1 km (0.62 mi) was saved in Malawi by using laterite in this way. [20]

Water supply

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. [3] :2 One example is the Southwestern Laterite (Cabook) Aquifer in Sri Lanka. [21] :1 This aquifer is on the southwest border of Sri Lanka, with the narrow Shallow Aquifers on Coastal Sands between it and the ocean. [21] :4 It has considerable water-holding capacity, depending on the depth of the formation. [21] :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 the monsoon rains. [21] :10 The water table recedes slowly and is recharged several times during the rest of the year. [21] :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. [21] :13 The Cabook Aquifer laterites support relatively shallow aquifers that are accessible to dug wells. [21] :10

Waste water treatment

In Northern Ireland, phosphorus enrichment of lakes due to agriculture is a significant problem. [22] 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. [22] Calcium-, iron- and aluminum-rich solid media are recommended for phosphorus removal. [22] 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. [22] Initial laboratory studies show that laterite is capable of 99% removal of phosphorus from solution. [22] A pilot-scale experimental facility containing laterite achieved 96% removal of phosphorus. [22] This removal is greater than reported in other systems. [22] Initial removals of aluminum and iron by pilot-scale facilities have been up to 85% and 98% respectively. [22] Percolating columns of laterite removed enough cadmium, chromium and lead to undetectable concentrations. [22] There is a possible application of this low-cost, low-technology, visually unobtrusive, efficient system for rural areas with dispersed point sources of pollution. [22]

Ores

Cretaceous iron-rich laterite (the dark unit) in Hamakhtesh Hagadol, southern Israel. K Laterite.JPG
Cretaceous iron-rich laterite (the dark unit) in Hamakhtesh Hagadol, southern Israel.

Ores are concentrated in metalliferous laterites; aluminum 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. [3] :2

Bauxite

Bauxite on white kaolinitic sandstone at Pera Head, Weipa, Australia Bauxite section on kaolinitic sandstone C 007.jpg
Bauxite on white kaolinitic sandstone at Pera Head, Weipa, Australia
The dark veins are precipitated iron within kaolinized basalt near Hungen, Vogelsberg, Germany. Absolute iron accumulation in kaolinized basalt. C 015.jpg
The dark veins are precipitated iron within kaolinized basalt near Hungen, Vogelsberg, Germany.

Bauxite ore is the main source for aluminum. [1] :65 Bauxite is a variety of laterite (residual sedimentary rock), so it has no precise chemical formula. [23] It is composed mainly of hydrated alumina minerals such as gibbsite [Al(OH)3 or Al2O3 . 3H2O)] in newer tropical deposits; in older subtropical, temperate deposits the major minerals are boehmite [γ-AlO(OH) or Al2O3.H2O] and some diaspore [α-AlO(OH) or Al2O3.H2O]. [23] The average chemical composition of bauxite, by weight, is 45 to 60% Al2O3 and 20 to 30% Fe2O3. [23] The remaining weight consists of silicas (quartz, chalcedony and kaolinite), carbonates (calcite, magnesite and dolomite), titanium dioxide and water. [23] Bauxites of economical interest must be low in kaolinite. [6] Formation of lateritic bauxites occurs worldwide in the 145- to 2-million-year-old Cretaceous and Tertiary coastal plains. [24] 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. [24] Many high-level bauxites are formed in coastal plains which were subsequently uplifted to their present altitude. [24]

Iron

Irregular weathering of grey serpentinite to greyish-brown nickel-containing laterite with a high iron percentage (nickel limonite), near Mayaguez, Puerto Rico. Laterite formation on serpentinite. C 008.jpg
Irregular weathering of grey serpentinite to greyish-brown nickel-containing laterite with a high iron percentage (nickel limonite), near Mayagüez, Puerto Rico.

The basaltic laterites of Northern Ireland were formed by extensive chemical weathering of basalts during a period of volcanic activity. [9] They reach a maximum thickness of 30 m (100 ft) and once provided a major source of iron and aluminum ore. [9] Percolating waters caused degradation of the parent basalt and preferential precipitation by acidic water through the lattice left the iron and aluminum ores. [9] Primary olivine, plagioclase feldspar and augite were successively broken down and replaced by a mineral assemblage consisting of hematite, gibbsite, goethite, anatase, halloysite and kaolinite. [9]

Nickel

Laterite ores were the major source of early nickel. [5] :1 Rich laterite deposits in New Caledonia were mined starting the end of the 19th century to produce white metal. [5] :1 The discovery of sulfide deposits of Sudbury, Ontario, Canada, during the early part of the 20th century shifted the focus to sulfides for nickel extraction. [5] :1 About 70% of the Earth's land-based nickel resources are contained in laterites; they currently account for about 40% of the world nickel production. [5] :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%. [5] :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%. [5] :4

See also

Related Research Articles

Bauxite Sedimentary rock rich in aluminium

Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), mixed with the two iron oxides goethite (FeO(OH)) and haematite (Fe2O3), the aluminium clay mineral kaolinite (Al2Si2O5(OH)4) and small amounts of anatase (TiO2) and ilmenite (FeTiO3 or FeO.TiO2). Bauxite appears dull in luster and is reddish-brown, white, or tan in color.

Weathering Breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earths atmosphere, water, and biota

Weathering is the breaking down of rocks, soils, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the transport of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.

Goethite

Goethite is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient times for its use as a pigment. Evidence has been found of its use in paint pigment samples taken from the caves of Lascaux in France. It was first described in 1806 based on samples found in the Hollertszug Mine in Herdorf, Germany. The mineral was named after the German polymath and poet Johann Wolfgang von Goethe (1749–1832).

Oxisol A soil type known for occurring in tropical rain forests

Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest within 25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB), they belong mainly to the ferralsols, but some are plinthosols or nitisols. Some oxisols have been previously classified as laterite soils.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere, biosphere, lithosphere and the hydrosphere. The pedosphere is the foundation of terrestrial life on Earth.

Ultramafic rock

Ultramafic rocks are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

Ore genesis How the various types of mineral deposits form within the Earths crust.

Various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.

Phosphorus cycle Biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs in only specialized, local conditions. Therefore, the phosphorus cycle should be viewed from whole Earth system and then specifically focused on the cycle in terrestrial and aquatic systems.

Lateritic nickel ore deposits are surficial, weathered rinds formed on ultramafic rocks. They account for 73% of the continental world nickel resources and will be in the future the dominant source for the mining of nickel.

Leaching (agriculture)

In agriculture, leaching is the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss. Leaching may also refer to the practice of applying a small amount of excess irrigation where the water has a high salt content to avoid salts from building up in the soil. Where this is practiced, drainage must also usually be employed, to carry away the excess water.

Angadipuram Laterite city in Kerala, India

Angadipuram Laterite is a notified National Geo-heritage Monument in Angadipuram town in Malappuram district in the southern Indian state of Kerala, India. The special significance of Angadipuram to laterites is that it was here that Dr. Francis Buchanan-Hamilton, a professional surgeon, gave the first account of this rock type, in his report of 1807, as "indurated clay", ideally suited for building construction. This formation falls outside the general classification of rocks namely, the igneous, metamorphic, or sedimentary rocks but is an exclusively "sedimentary residual product". It has a generally pitted and porous appearance. The name laterite was first coined in India, by Buchanan and its etymology is traced to the Latin word "letritis" that means bricks. This exceptional formation is found above parent rock types of various composition namely, charnockite, leptynite, anorthosite and gabbro in Kerala. It is found over basalt in the states of Goa, Maharashtra and in some regions of Karnataka. In Gujarat in western India, impressive formations of laterite are found over granite, shale and sandstone..

Saprolite Chemically weathered rock

Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds. Deeply weathered profiles are widespread on the continental landmasses between latitudes 35°N and 35°S.

Cobalt extraction

Cobalt extraction refers to the techniques used to extract cobalt from its ores and other compound ores. Several methods exist for the separation of cobalt from copper and nickel. They depend on the concentration of cobalt and the exact composition of the used ore.

Nickel mine

A nickel mine is a mine that produces nickel. Some mines produce nickel primarily, while some mines produce nickel as a side-product of some other metal that has a higher concentration in the ore.

Stone laterite Borneo

Stone laterite Borneo is a stone resembling hardened soil from the deposition of substances such as nickel and iron. Laterite itself naturally formed therein contained many elements and substances that make up the nutrients of the soil layer hardens like a rock. Long ago laterite stone often used as materials for laterite bricks because when moist easily cut, but once exposed to air for a long time will harden like stone.

The geology of Liberia is largely extremely ancient rock formed between 3.5 billion and 541 million years ago in the Archean and the Neoproterozoic, with some rocks from the past 145 million years near the coast. The country has rich iron resources as well as some diamonds, gold and other minerals in ancient sediment formations weathered to higher concentrations by tropical rainfall.

Geology of Ivory Coast

The geology of Ivory Coast is almost entirely extremely ancient metamorphic and igneous crystalline basement rock between 2.1 and more than 3.5 billion years old, comprising part of the stable continental crust of the West African Craton. Near the surface, these ancient rocks have weathered into sediments and soils 20 to 45 meters thick on average, which holds much of Ivory Coast's groundwater. More recent sedimentary rocks are found along the coast. The country has extensive mineral resources such as gold, diamonds, nickel and bauxite as well as offshore oil and gas.

Geology of South Sudan

The geology of South Sudan is founded on Precambrian igneous and metamorphic rocks, that cover 40 percent of the country's surface and underlie other rock units. The region was affected by the Pan-African orogeny in the Neoproterozoic and extensional tectonics in the Mesozoic that deposited very thick oil-bearing sedimentary sequences in rift basins. Younger basalts, sandstones and sediments formed in the last 66 million years of the Cenozoic. The discovery of oil in 1975 was a major factor in the Second Sudanese Civil War, leading up to independence in 2011. The country also has gold, copper, cobalt, zinc, iron, marble, limestone and dolomite.

The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.

The soil matrix is the solid phase of soils, and comprise the solid particles that make up soils. Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential, but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.

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