Asegurar Shell

The laterite (of Latin later , brick) is a Roche red or brown. It constitutes a common product of the deterioration of the rocks under the tropical climates. An old direction indicates a laterite like an hardened material or of indurant with the air, used to manufacture houses in the tropical areas. The broad direction indicates the whole of materials, pieces of furniture or hardened, rich in Hydroxyde S of iron or aluminum, consituant grounds, surface horizons, major horizons of Profil of deterioration.

One finds laterites in intertropical field especially. They recover 33% of the continents.

The lateritic grounds are thin, washed grounds and apprauvris in fertilizing nutritive elements (Ca, Mg, K, Na). The vegetation remains however abundant on these grounds, although fragile, for example the large Equatorial forests. During deterioration, the mobile cations not very remain on the spot (Fe3+, Al3+) and concentrate in exploitable metalliferous lodgings. It is also about an important reserve of aquifers, the lateritic grounds filtering approximately 50% of the total flow.

Materials at the origin of the laterites

A laterite can be formed starting from any type of rock, but only if the climate is arid over one prolonged period. However, it is formed as many types of laterites than there are rocks of origin. During deterioration, the basic minerals most unstable and disappear, like the Feldspath S, and the most soluble ions escape in solution. The others remain on the spot form new rocks of them.

Faded minerals

The principal faded minerals are:

Nésosilicate S and Inosilicates ferromagnésiens: Olivine, Peridot S, Pyroxene S, Amphibole S. They release from the ions Fe, MG, Ca and contribute to the neo-formation of Oxyhydroxyde S.
Mica S: the ions K and Fe release. Biotites, Muscovite S gives and Glauconite S, being transformed into Illite S then in Montmorillonite S of degradation.
Tectosilicates : Feldspar S, giving place to clay neo-formation in wet hot climate, or releasing just from the soluble and amorphous products in cold climate with moderate
Quartz: known for its very strong weather resistance, it can be found with a rate of scrubbing going up to 20% pennies the tropical climates.
Carbonate S: Calcite S, Dolomite S, which releases from the HCO₃- ions and deposits clays

Complexes of deterioration

The deterioration of the rocks at the origin of the lateritic grounds gives place to the creation of complexes of deterioration of two forms:

Clays: formed clays depend on the rate of scrubbing undergone by the rock
Oxyhydroxydes of iron and aluminum

Characteristics of the laterites

Typical lateritic profile

A typical profile of deterioration of the lateritic solid masses contains the great following units (top to the bottom of the profile of deterioration):

Armor and carapace: massive formation with iron oxides and aluminum, quartz, kaolinite
mottled Formation: nodular formation with iron oxides and aluminum, quartz, fine kaolinite
Saprolithe or lithomarge: zone saturated with water with quartz, marked by the dominence of secondary minerals of coarse deterioration
Saprolithe or arena: formation dominated by the nature of the bed rock, having fragements of rock and primary education minerals in separate grains
silico-aluminous Bed rock

The more one is high in the profile, the more the rate of chemical weathering is raised and the more the presence of clays is marked. The thicknesses have varaibles sizes, and can as well be a few meters as higher than 100 meters.

Minerals of the laterites

One finds minerals secondary newly formed following:

Of iron: Limonite, Ferrihydrite, Goethite, Hematite
Of aluminum: Gibbsite, Cliachite, Boehmite, Corundum, Cliaspore
Of titanium: Anatase
of manganese: Pyrolusite, Manganite
of silicon: Allophane, Imogolite, Halloysite, Kaolinite, Ferrikaolinite

One should not forget to quote the solid solutions obtained by mixture of poles: goethite aluminous, aluminous hematite… Certain primary education minerals far from liable to deterioration can be inherited: quartz, Rutile, Zircon, native Gold…

Clay neo-formation

According to Tardy, there exist three differential sequences of deterioration of primary education minerals, controlling the formation of various clays. The more fragile one primary education mineral is, the more the stage of deterioration reached is thorough. The factors controlling the nature of newly formed clays are the rate of scrubbing, the containment of the medium, the climate, topography. For example, at the top Kaolinite S, Gibbsite are formed, because the elssivage is very strong there, and thus the hydrolyzes are effective. On the contrary, at the base scrubbing is weak, and of the Illite S and Chlorite S are formed, if the medium is acid. In acid medium Smectite S and palysépioles are formed.

Process of deterioration and pedogenesis

Theories on the development of the laterites

Various theories can explain the development of lateritic grounds:

Residues : The laterites of would develop on a healthy bed rock after very a long period of deterioration and exposure to an arid climate. Such a development necessitarait a very great quantity of rock to produce enough residual iron, in the form of oxides, like hematite or the goethite. This theory is most usually recognized.
Horizon of ground : this theory consists of direct precipitation with the top of the zone of fluctuation of the sheet of water. However, this theory does not exist the very thick laterites in the case of.
Deposit : a deposit of iron and aluminum, starting from ions in solution. This would be valid for the laterites brecciated or made up of pisolitic aggregates , but the case of the massive laterites
Nappes influenced by the conditions of surface would not explain: the laterites would be formed by deterioration of the bed rock, because of acid water resulting from marshes, or enriched in organic acids by the action by the plants.

The truth would be a conjunction of these theories, playing each one with more or less large impotance.

Formation process of the complexes of deterioration

Various theories run on the development badly included/understood of the complexes of deterioration:

Heritage : it would be about a simple microdivision of the elements, without chemical conversion
minor chemical Conversion : the minerals would lose certain mobile ions, while preserving their structure, for example a transition Mica - Argile
Néoformation : ions would be lost by minerals, like their structure. The remaining elements would recristallize in situ.

Geochemical deterioration

The chemical mechanism concerned in the deterioration of solid rocks giving of the laterites is the total Hydrolyze. A reaction generates the destruction of all primary education minerals and the release of their components, elimination of the essential cations and part of If, as well as a insolubilization and a relative accumulation of oxyhydroxydes of Al and Fe. This reaction the local value of the pH and the local drainage influence, as well as the duration.

Various lateritic profiles

Types of grounds

Three great types of grounds constitute the lateritic horizons: ferrigineux grounds, grounds ferralitic, ferrisols.

Carapaces

The carapace is the part of the profile of deterioration located just in lower part of the armor, constitutes of it a zone of prelude to armouring. The carapace is a mottled formation. The clear zones are richer in quartz, and the spots of rubéfaction are due to kaolinite. The matric bottom can be yellow, pink or red. As one goes up in the profile of deterioration, the spots nodulisent and form ferruginous concretions.

Armours

The top of the profile is very enriched out of iron (up to 75% of Fe₂O₃), and very hardened. The transition between carapace and armor is done by increase in the number and the size of the nodules, of the iron incrustation on the walls, as well as the reduction in empty volumes, and the argillaceous beaches with goethite. The matric fill color transfers with the red of the iron of the iron concentration.

The armours, directements subjected to erosion, can be degraded. This degradation is marked by an increase in the size of the vacuums and a individualization of the nodules marked. The degradation of the nodules gives:

Of the granules: by selective dissolution of hematite
Of pisolites: degradation by hydration
Of the fine gravels: separation of the matric bottom

internal bonds

lateritic Armor
Ocher Bauxite
of Rustrel

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