An Introduction to Laterite

Laterites are the products of intensive and long lasting tropical rock weathering which is intensified by high rainfall and elevated temperatures. Formation of most of the laterites started in the Tertiary. For a proper understanding of laterite formation we must focus on the chemical reactions between the rocks exposed at the surface and the infiltrated rain water. These reactions are above all controlled by the mineral composition of the rocks and their physical properties (cleavage, porosity) which favour the access of water. The second relevant factor for the formation of laterites are the properties of the reacting water (dissolved constituents, temperature, acidity pH, redox potential Eh) which are themselves controlled by the climate, vegetation and the morphology of the landscape.

Tropical and subtropical areas show generally a rather high annual precipitation but its temporal distribution varies strongly from countries with pronounced and long lasting dry seasons to equatorial areas with a more continuous precipitation. Chemical weathering slows down in dry seasons at least above the fluctuating water table. Aqueous dissolution of minerals proceeds when a chemical equilibrium is not arrived i.e. when the dissolved constituents are removed in the water. The chemical reactions are further controlled by the activity of water which is equal to one in freely moving water but lowered within small pores in the soil. Stability and reaction rate vary from mineral to mineral; e.g. quartz is more stable than feldspar. Minerals of the same species e.g. kaolinite can show different crystallinity which equally controls their stability. Strongest alteration proceeds at the surface of the parent rock whereas it is lower in the regolith above the rock.

The principal effects of the various factors on laterite formation are well known but it is difficult to determine them in space and time in the field. In the practise of laterite research most valuable informations are obtained by detailed studies of complete weathering sections (laterite profiles) reaching from the unweathered parent rock to the strongly altered surface layer. Sections showing physical disturbances as erosion or importation of transported material should be omitted to exclude effects other than weathering. An adequate number of laterite profiles on different parent rocks has been analysed which enable a clear understanding of the basic processes of lateritization.

The chemical and mineralogical results have shown that the primary minerals are generally not fully dissolved but partially transformed in secondary minerals which are more stable under the intensive weathering conditions. The elements in the primary rock minerals are released and show different reactions in the aqueous solution. The elements Na, K, Mg and Ca do not react with other elements and are removed in the percolating water. The initial dissolution is predominantly promoted by a higher acidity (lower pH) of the water. A high percentage of the dissolved Si is equally removed but another part reacts with dissolved Al and forms the clay mineral kaolinite. The aluminium hydroxide gibbsite is formed if the concentration of dissolved Si is extremely low due to a very strong drainage. Dissolved Fe is very reactive with hydroxyl ions and forms after oxidation goethite and hematite which cause the red-brown colour of laterites. Thus the dominant process of laterite formation is the residual (or relative) enrichment of iron and frequently of aluminium by removal of silica, alkalis and alkaline earths. This chemical alteration corresponds mineralogically with the formation of goethite, hematite, kaolinite and gibbsite. These minerals together with relicts of partially dissolved quartz form the bulk of laterites.

The transformation of rock into laterite proceeds in general gradually as indicated by the steady increase of iron and decrease of silica in laterite profiles above the parent rock. It goes without saying that the initial products of weathering can not be called laterites. They also form in moderate climates and are essentially kaolinized rocks still showing the structure of the rock. They are called saprolites in which iron is not as strongly concentrated as in laterites. Some saprolites show due to finely disseminated hematite a deep-red colour and are sometimes erroneously considered as laterite. Saprolites as well as laterites are presently classified as residual rocks which in their part can be seen as an individual rock group.

A modern laterite definition should comprise all products of intensive tropical weathering independent of their properties and parent rocks, but exclude products of weaker weathering which also occur in moderate climate. Redbrown laterites on granites, granitic gneisses, clays and shales are generally hard or harden after drying, whereas laterites an basalts are commonly friable and show an intensive reddish color. Lateritization of alkaline rocks (nepheline syenites, phonolites) often results in formation of highly aluminious laterites (bauxites) with lighter color. On ultramafic rocks (serpentinites etc.) forms very soft, yellow-brown Ni-bearing goethite (nickel limonite ore). All these weathering products are formed by the same fundamental weathering process and are therefore different members of a laterite family. 

Properties and definition of laterites are controversially discussed since Buchanan (1807) introduced this term in geosciences. Publications with the title "What is Laterite?" are evidence of a widespread confusion .Even presently the controversy is not overcome, allthough intensive research has greally increased the knowledge on laterite. A disputed definition is commonly, however, not due to a lacking knowledge but to a missing agreement among the interested scientists: moreover a good definiion is qute another matter as a comprehensive description.

Already Aristoteles (384-322 AC) required that a scientific definiton must specify the catagory of the defined matter (here: residual rocks) and the difference to other subjects in the same catagory (here: saprolites) . Following this instruction the author of this article suggested in cooperation with the IGCP-Projects 129 "Laterisation Processes" a new laterite definion (Schellmann 1986) on the basis of defining ratios Si : (Al+Fe) which differs with the composition of the parent rock This proposal was critisized by Bourman and Ollier (2002) as too complicated without giving another clear proposal. A quite other suggestion was contributed by Tardy (1997) who favors to designate each red soil as laterite. In consequence of this proposal he calculated that about one third of the continental land mass is covered by laterites.

Absolute iron accumulation which was frequently discussed in laterite papers is presently no longer regarded as a fundamental process in the formation of extended laterite layers. Iron accumulation after a lateral transport should cause iron depletion in neighbouring areas which is generally not observed. Stronger downward leaching in the profile or even from the top of a platau to slope position is posulated in several papers. This could happen in an acicic and reducing environment , but low pH- and Eh- values are normally not realized under conditions of strong precipitation and good drainage. Swampy environments do not correspond with the requirements for laterite formation. On the  other hand migration and precipitation of dissolved iron is indicated by the presence of iron mottles, nodules and concretions in laterite horizons. Therefore the author takes absolute iron accumulation definitely into account, but only in shorter ranges and on a minor scale. 

Lateritic weathering is only one relevant process wich is active in the superficial zone of tropical regions. Erosion or denudation, respectively, contribute equally to an alteration at the surface together with deposition of material by water and wind. Not each variation in lateritic profiles can be attributed to chemical weathering. There are ironstone formations in the world which can hardly be interpreted by normal lateritization processes. If they show signs of reworking, transport and deposition they should not be defined as laterites but as lateritic sediments. Lateritic sediments of older epochs can be overprinted by younger lateritic weathering. Complex lateritic occurrences are grouped as exolaterites, false laterites and laterite derivative facies. They are relevant in regional studies but not for a general understanding of the lateritization process. This is equally true for loose surface layers above autochthonous laterites, locally separated by a stone line. They commonly show a saprolitic composition with higher SiO2 contents and are deposited on the laterite surface. Very often termites carried this material upwards from deeper horizons. In other instances zirkonium contents in the surface horizons of laterite (nickel limonite) above ultramafic rocks indicate an admixture from areas with other parent rocks.

Chemical alterations and differences in lateritization

All laterites are marked by an enrichment of iron and a decrease of silica together with the highly soluble alkalis and alkaline earths. But apart of these characteristics the composition and properties of laterites can be quite different and are strongly controlled by the chemical and physical features of the parent rock. Above all the behaviour of aluminium is not uniform. In essence two principal groups can be distinguished:

• Laterites on mafic (basalt, gabbro) and on ultramafic rocks (serpentinite, peridotite, dunite). These rocks are free of quartz and show lower silica and higher iron contents.
• Laterites on acidic rocks. In this group not only granites and granitic gneisses but also many sediments as clays, shales and sandstone shall be included. These rocks contain quartz and have higher silica and lower iron contents.

The following table shows main element percentages of rocks from these two groups and their corresponding laterites. The cited percentages are typical average values of numerous laterite samples and their parent rocks which were collected by the author in many tropical countries.

The laterites formed above these rocks do not only show divergent chemical compositions but also contrasting physical properties. The very ferruginous yellowish brown laterite (nickel limonite) on serpentinite is generally soft and displays a very fine porosity, indicated by a low bulk density. In few occurences a hard cuirass has formed at the surface by dissolution and reprecipitation of iron. Also the laterites on basalts are commonly soft and friable. They are intensively redbrown coloured and form relatively fertile soils. The laterites on acidic rocks behave quite differently. They frequently show a typical sequence with a pallid zone (saprolite), a mottled zone and a darkbrown laterite on top which is often described. These laterites harden after drying which often allows their application as brickstones.

Genetically most relevant, however, are the differences in the Fe2O3 : Al2O3 ratios. Laterites on mafic and ultramafic rocks show generally similar ratios as the underlying parent rock. This can be easily interpreted by the loss of soluble elements causing an equivalent accumulation of the residual elements iron and aluminium which corresponds with the classic interpretation of laterite formation. On the other hand, laterites on acidic rocks show generally strongly increased Fe2O3 : Al2O3 ratios. There are only two explanations for this difference:

• Iron is introduced into the laterite from outer sources (absolute iron accumulation). The difficulties of absolute iron accumulation on a larger scale were already discussed. Moreover, it is inexplicable that an introduction of iron is only active in lateritization of acidic and not of mafic and ultramafic rocks.
• If absolute iron accumulation can be ruled out as a dominant factor, only a residual concentration can be quoted. This is particular strong if also aluminium together with silica is lost in solution resulting in highly increased Fe2O3 : Al2O3 ratios.

If aluminium would not be removed in the course of lateritization of acidic rocks, bauxites should commonly form on these widespread rocks because of their high Al2O3 : Fe2O3 ratios. But actually bauxites are rare compared with laterites. They are restricted to areas with exceptionally good leaching conditions which allow partial (incongruent) kaolinite dissolution with subsequent formation of gibbsite due to a very low concentration of dissolved silica. Under weaker leaching conditions gibbsite does not form or forms only in minor quantities. This is favoured by the presence of quartz and the lower permeability of weathered acidic rocks compared with the more porous and friable laterites on mafic and ultramafic rocks.

What is the reason for a stronger (residual) iron accumulation under comparatively weaker leaching conditions which might be paradox at the first sight? The author suggests the cause in the dissolution and neoformation of kaolinite , of wich two generations were observed in many laterite samples. In a first generation kaolinite replaces the silicate minerals of the parent rock which already occurs in the saprolite stage. In a second generation very fine-grained and iron-stained kaolinite with lower crystallinity fills small voids and fissures in the lateritic matrix.The two laterite generations can clearly be observed in laterite thin sections (see the last photo in the link with attached images). This secondary kaolinite increases with proceeding lateritization. Its occurence in open spaces shows precipitation from soil solution. Dissolution and neoformation of kaolinite proceed at different times and at different places in the lateritic profile, depending on the specific chemical and physical conditions in the respective microenvironment.The same kaolinite sequence was observed by Balan et al (2007) in a recent study of a lateritic profile above kaolinitic clays in Brazil. The well ordered primary kaolinite of sedimentary origin decreased in the lateritic profile with decreasing depth, accompanied by a rising fraction of poorly ordered, neoformed kaolinite. An impregnation with finely divided, gel-like secondary kaolinite might be the cause for the hardening properties of many laterites on acidic rocks which enable their use as brickstones and are not encountered in laterites on mafic (basaltic) rocks. Detailed microprobe analyses by the author of various laterite samples only proved the well-known laterite minerals and no amorphous compounds with variable composition which might equally cause hardening after drying. 

The author concludes that substantial amounts of dissolved Al and Si, originating from the congruent dissolution of primary kaolinite, are not re-precipitated in the secondary kaolinite but are removed with the percolating water. Waters originating from laterite areas have normally a neutral pH which excludes substantial Al amounts in true solution. Moreover, dissolved Al could not be proved by conventional analyses of filtered waters. Therefore it can be presumed that removal of Al takes place in form of larger Al-Si-compounds probably of colloidal size. This is promoted by kinetic factors especially by the extremely slow reaction rate of kaolinite formation which allows removal of Al and Si even in the stability field of kaolinite. Furthermore, colloidal mobilization of Al and Si is proved by informative short-time dissolution experiments with kaolinitic oxisols (Malengreau and Sposito (1997). Colloidal particles of kaolinite and amorphous silica were released in solution after dissolution periods of 1-12 h at pH 2-6.

Mobilization of aluminium could be clearly demonstrated by McFarlane and Bowden (1992) who analysed in detail soil and water samples from a toposequence in Malawi. Al and Si are released by congruent kaolinite dissolution in lateritic interfluve areas and re-precipitated in the neighboured bottomland in form of various alumo-silicates. Waters from wells and boreholes were analyzed after different treatments and show substantial amounts of Al if the samples were not filtered and acidified prior to analysis, whereas filtration diminished the Al content strongly. The authors conclude that dissolution of kaolinite is promoted by micro-organisms and the released Al is mobilized together with Si in organically bound form. They stress that mobility is not synonymous with solubility.

The presented analyses and observations led the author suggest a three-step-model of tropical weathering, depending on the intensity of the weathering processes:

• Weaker tropical weathering gives rise to formation of saprolites which are the prevailing weathering products in the tropics and are frequently misinterpreted as laterites.
• Advanced tropical weathering results in the formation of most of the laterites showing a much stronger residual enrichment of Fe as against Al. A higher tropical rainfall and a moderate drainage together with the presence of quartz are generally not sufficient for a pronounced incongruent kaolinite dissolution and a pronounced formation of gibbsite. As already discussed in this article, Al- and Si-bearing compounds of probably colloidal size are thus removed from the weathering mantle in high quantities. Laterites formed in this way are frequently indurated and predominate in the tropics above clays, shales, granites and granitic gneisses. Friable laterites with high contents of iron oxides and kaolinite form on basaltic rocks.
• Strong tropical weathering is promoted by a very pronounced rainfall, a deep ground water level and a high permeability of the weathered rock, allowing an excellent drainage which is essential for the formation of bauxites (Bardossy and Aleva, 1990) These factors cause an incongruent dissolution of kaolinite with formation of gibbsite. The composition of the laterite is determined by the composition of the parent rock. The most widespread acidic rocks with their high Al- and Si- and their low Fe-content give rise, in favourable cases, to formation of high grade-bauxites. Ferruginous bauxites of a relatively poor quality form on basaltic rocks. Ultramafic rocks are transformed in thick deposits of a very ferruginous laterite (nickel limonite ore) which frequently covers nickel silicate ore.

Author: W. Schellmann