An Introduction to Laterite

Products  and processes of intensive rock weathering

W. Schellmann

Historical development

Laterite is well  known in Asian countries as a building material for more than 1000 years. It was excavated from the soil and cut in form of large blocks; temples at Angkor are famous examples for this early use. At begin of the 19.century it obtained scientific interest when the English surgeon Francis Buchanan travelled along the western coast of southern India and published his manifold observations and results. He coined the term laterite when he wrote (1807): “What I have called indurated clay …is one of the most valuable materials for building. It is diffused in immense masses, without any appearance of stratification and is placed over the granite that forms the basis of Malayala. It is full of cavities and pores, and contains a very large quantity of iron in the form of yellow and red ochres. In the mass, while excluded from the air, it is so soft, that any iron instrument readily cuts it, and is dug up in square masses with a pick-axe, and immediately cut into the shape wanted with a trowel, or large knife. It very soon after becomes as hard as brick, and resists the air and water much better than any brick that I have seen in India. … The most proper English name would be laterite, from lateritis, the appellation that may be given to it in science”. (The Latin word later means brick)

These statements were the start of a tremendous number of studies of this strange material, hitherto resulting in more than 2000 publications. Above all  geologists dealt at first with laterite but in later epochs also soil scientists, mineralogists, geographers, geomorphologists, mining and construction engineers participated in laterite research. The great interest of so many scientists with their different viewpoints did not only result in a great increase in knowledge but created also a great confusion in the basic understanding and interpretation of laterite formation.

Already the early observations of Buchanan have shown that laterite is a weathering product of the underlying parent rock. In later studies it was found that intensive chemical decomposition of rocks is a widespread phenomenon in tropical regions and affects each kind of rock. Obviously tropical weathering causes an increase of iron indicated by the red-brown colour of laterites. But many lateritic weathering products do not show the properties described by Buchanan e.g. the pronounced hardening after drying making possible their use as brickstones. The question arose if soft and friable ferruginous weathering layers without any hardening properties can be equally defined as laterites. The publication of papers with the title “What is laterite ? ” demonstrated the growing confusion and dispute. With the progress in chemical analysis more and more samples were analysed and showed the typical increase of iron and frequently of aluminium and decrease of silica in relation to the underlying parent rock. Therefore it was tried to define laterites by the ratio Si : (Al + Fe) but a definite limit was not applicable for laterites on different parent rocks.

In the last fifty years many soil scientists devoted to laterite research and contributed their specific experience with soil forming processes. A major role was seen in the movement and precipitation of dissolved iron (absolute iron accumulation) in contrast to the predominant interpretation by residual (relative) enrichment due to removal of silica and other soluble elements. It was assumed that the iron

-  migrated upwards from deeper layers (early theory of capillary rise).

-  migrated downwards from the overlying soil and concentrated in deeper “illuvial horizons”.

-  migrated laterally in the ground water and precipitated within the zone of a fluctuating ground water table (“ground water laterite”). Other scientists stressed the origin of iron from higher areas in the landscape (plateaus, low hills) and precipitation in deeper parts above all at the foot of slopes. Geomorphologists even suggested that all laterites formed by such processes and their frequent positions on top of plateaus were caused by a subsequent inversion of the relief.

Other difficulties resulted by the introduction of soil names such as oxisol, latosol and ferralitic soil and their definition by specific soil properties. They are without doubt useful within the systems of international soil classification but not practicable for geologists dealing with thick laterite mantles and huge lateritic ore deposits as bauxites and nickel laterites. These occurrences cannot be considered as soils which should be restricted to loose surface covers and not extended to complete weathering mantles with thicknesses up to ten meters and locally more.

The different interpretations of laterite became occasionally rather exotic because of the conflicting laterite aspects and definitions. Even nowadays the differences are not wholly overcome but in the last decades many studies contributed to a better and undivided understanding. Thus the International Geological Cooperation Programme No. 129 “Lateritisation Processes” which was sponsored 1975-1983 by UNESCO and the Union of Geological Sciences accelerated laterite research in all parts of the world. “Eurolat” was started 1984 by French geoscientists as  “European Network on Tropical Laterites”. Moreover, an “International Interdisciplinary Laterite Reference Collection (CORLAT)” was established at the “International Soil Reference and Information Centre (ISRIC)” at Wageningen, the Netherlands.

Modern interpretation  of  laterite formation

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 are grouped within the sedimentary rocks.

A modern laterite definition should comprise all products of intensive tropical weathering independent of their  parent rocks, which strongly control the composition and the property of the weathering product. 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 on alkaline rocks (nepheline syenites, phonolites) often results in formation of highly aluminious laterites (bauxites) with light color. On ultramafic rocks (serpentinites etc.) forms very soft, yellow-brown Ni-bearing goethite (nickel limonite ore). The described weathering products are formed by the same fundamental weathering process and can therefore be interpreted  as different members of a laterite family. 

A scientific definition proposed in former papers of the author includes all laterite varieties but excludes weaker weathering products (saprolites). Laterites are here defined as advanced tropical weathering products with Si : (Al + Fe) ratios below definite limits which on their part depend on the parent rock composition. This definition is criticized by Australian geoscientists who prefer a definition allowing a clear identification in the field  possibly by a hand specimen. This request can indeed not be fulfilled considering the broad variety of the lateritic weathering products. Even the most widespread laterites on acidic rocks cannot be distinguished from many bog iron ores only by their appearance      without additional information allowing genetic conclusions.

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.

                                                SiO2           Al2O3          Fe2O3            Fe2O3 : Al2O3

Laterite                                 46,2              24.5             16.3                      0.67

Granite                                 73.3              16.3                3.1                      0.19

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Laterite                                 39.2               26,9            19.7                      0.73

Clay                                       56.5                24.4              5.3                      0.22

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Laterite                                 23.7               24.6            28.3                       1.15

Basalt                                     47.9               13.7           14.9                       1.09

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Laterite                                    3.0                  5.5             67.0                      12.2

Serpentinite                         38.8                  0.7              9.4                       14.1

 

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 drawn up. 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. 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 French authors (1) 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. Detailed microprobe analyses 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, carried out in laboratories at Berkeley (2). 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 British authors (3) 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. These factors cause an incongruent dissolution of kaolinite. 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.

Economic relevance

Lateritic weathering has a considerable economic significance above all for the mining of relevant metals predominantly nickel and aluminium.  

Nickel laterites  supply presently 44 % of the world nickel production and constitute 73 % of the continental resources. In the near future they will obtain the first position also in the production. The ores are bound on ultramafic rocks above all serpentinites which consist largely of the magnesium silicate serpentine containing approx. 0.3 % Ni. This mineral is nearly completely dissolved in the course of lateritization leaving behind a very iron-rich, soft residue in which nickel is concentrated up to 1 - 2 % Ni (for main element percentages see the proceeding table). The bulk of this so-called nickel limonite or nickel oxide ore consists of the iron oxide goethite in which nickel is incorporated. Therefore it cannot be concentrated physically by ore dressing methods.

Below the nickel limonite another type of nickel ore has formed in many deposits. This is called nickel silicate ore which consists predominantly of partially weathered serpentine. It is depleted in magnesium and forms with 1.5 – 2.5 % Ni the most relevant type of lateritic nickel ores. In contrary to the relatively enriched limonite ore, the nickel silicate ore owes its nickel content to a process of absolute nickel enrichment. The nickel is leached downwards from the overlying limonite zone since not all of the nickel, which is released from the serpentinite in the course of nickel limonite formation, can be incorporated in goethite and therefore cannot be fixed in the limonite zone. The downward migrated excess nickel is incorporated in the Mg-depleted serpentine and occasionally in neo-formed clay minerals predominantly smectite. Some deposits show admixtures and layers of secondary quartz which is precipitated from weathering solutions supersaturated in Si, due to a rapid dissolution of serpentine.

A third type of lateritic nickel ore is garnierite which is found in pockets and fissures of the weathered ultramafic rocks. The green garnierite ore containing mostly 20 – 40 % Ni consists of a mixture of the phyllosilicates serpentine, talc, chlorite and smectite in which a high percentage of magnesium is substituted by nickel. It is also formed by downward nickel leaching and precipitation in hollow spaces of the weathered rock. Today, nickel silicate ores with high portions of garnierite are largely exhausted. Important deposits of nickel laterite are located in many tropical countries above all in New Caledonia.

Bauxites are almost the only source for the production of aluminium. There is no principal difference between laterites and bauxites since bauxites are nothing else than highly aluminous varieties of laterite showing continuous transitions. Bauxites largely consist of the Al-minerals gibbsite Al (OH)3, boehmite and diaspore AlOOH together with the iron oxides goethite and hematite, the clay mineral kaolinite and small amounts of anatase. In commercial bauxites the kaolinite content must be low. Bauxite was named after the French village Les Baux-de-Provence where it was first discovered.

In geosciences lateritic bauxites (silicate bauxites) are distinguished from karst bauxites (carbonate bauxites). The early discovered karst bauxites occur predominantly in Europe and Jamaica on karst surfaces of limestone. They also formed by lateritic weathering of silicates either from intercalated clay layers or of clayey dissolution residues of the limestone. These bauxites frequently contain boehmite and diaspore in addition to gibbsite. The bauxites in Jamaica rest on tertiary limestone and are exposed at the surface whereas the European bauxites are bound on older carbonate rocks of jurassic and cretaceous age. If they are covered by younger sediments they have to be mined underground. Their contribution to the world bauxite production is today relatively small.

Most dominant are nowadays the tropical silicate bauxites which are formed at the surface of various silicate rocks such as granites, gneisses, basalts, syenites, clays and shales. As  already discussed, the formation of bauxites demands a stronger drainage as laterite formation to enable precipitation of gibbsite which is the prevailing aluminium hydroxide in lateritic bauxites. Zones with highest Al contents are frequently located below a ferruginous surface layer and are due to downward leaching of aluminium which is more soluble than iron under oxidizing conditions. Near the parent rock interface gibbsite frequently replaces primary minerals predominantly feldspars which results in a preservation of the primary rock structure.

Large deposits of lateritic bauxites with a high production are in Australia, Brazil, Guinea and India together with Guyana, Suriname and Venezuela.

High grade iron ores on top of tropical deposits of banded iron formations (BIF) are also attributed to lateritic weathering which causes dissolution and removal of siliceous constituents in the banded iron ore.

Lateritization processes are equally relevant for the alteration of gold deposits. In the tropics the laterite cover around outcropping gold-quartz-veins often shows a pronounced gold dispersion halo which can be traced by geochemical exploration. It is proved that the primary gold is partially moved in solution and re-precipitated in secondary gold particles with lower grain size and reduced Ag content. Locally also the secondary lateritic Au mineralization is workable.

The importance of laterite as building material has already been mentioned. More relevant as their local use for the construction of simple houses is their application as a road building material. The suitability of lateritic materials above all of lateritic gravel is tested by several methods of engineering geology.

The significance of lateritization for agriculture is not positive since strong leaching results in the impoverishment also of nutrients in lateritic soils. Moreover the hardening properties of many laterites complicate agriculture. Only the friable red-brown lateritic soils on basaltic rocks enable good crops.

This article is intended to present a general view on laterite. It considered  a great number of publications which can hardly be cited here. Some interpretations of the numerous scientific results in the laterite literature reflect the personel viewpoint of the author.

References

(1) Balan E. et al. (2007): Inheritance vs. neoformation of kaolinite during lateritic soil formation: A case study in the middle Amazon  basin. Clays and Clay Minerals 55,       253-259.

(2) Malengreau, N. and Sposito , G. (1997): Short-time dissolution  mechanisms of kaolinite tropical soils. Geochim. Cosmochim. Acta 61, 4297-4307. 

(3) McFarlane, M. J. and Bowden, D. J. (1992): Mobilization of aluminium in the weathering profiles of the African surface in Malawi. Earth Surface Processes and Landforms 17,   789-805.

Monographs

Aleva, G.J.J. (Ed.) (1994): Laterites. Concepts, Geology, Morphology and Chemistry.    153 p. ISRIC, Wageningen, ISBN 90-6672-053-0

Bardossy, G. and Aleva, G.J.J. (1980): Lateritic Bauxites. 624 p. In: Developments in Geology 27, Elsevier, Amsterdam, ISBN 0-444-98811-4.

Tardy, Y. (1997): Petrology of Laterites and Tropical Soils. 419 p. ISBN 978-9054- 106784     

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Author:  Dr. Werner Schellmann  studied  Mineralogy at the University of Marburg and was employed  1958 -1991 at the Bundesanstalt für Geowissenschaften und Rohstoffe (Federal Institute for Geosciences and Natural Resources) in Hannover, Germany. He was predominantly engaged in the research of laterites, bauxites and lateritic nickel ores which were studied in many countries in the tropical world. The results were published in numerous papers.

Laterite images: Illustration of laterites, bauxites and lateritization

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