Diversity and Terminology of Lateritic Profiles

t Dcvclopnients in Earth Surfacc Proccsscs 2

Edited by

I.P. MARTINI and W. CI-IESWORTH Department of Land Resoitrce Science, University of Gitelph.

i Giielph, Ont. NlG 2 WI, Canada

ORSTOM Fonds Bocumentaire

. II I f 5 /%R, 2993. No 6 q-,n-g/zp:;li 2: f -< i! ?! ‘.Ø,*.,“.:+tc.-+ cotte1 & L . ‘i , . ELS EVI ER .,. I Amsterdam - London - Ncw York -Tokyo I992 .. ,.. ._ , ,. I . ., . . .. u ." /.. . ..

319

Chprer 1.5

Diversity and terminology of lateritic profiles

Y. TARDY

Introduction: definition of laterites

This chaptcr will dcal with thc dcfinition and thc tcrminology of a related series of materials ranging from laterites, latcritic bauxitcs, lateritic soils to all kinds of intertropical weathering products, of dctailcd features characterizing the different horizons, of horizons constituting a large variety of profiles, and of profiles within a largc intertropical climatic arca. The tcrm laterite (from the Latin luter, brick), is commonly attributed to Bu- chanan (1807) who dcscribcd, in Malabar, surficial natural hard matcrials used as bricks. In some African dialects, surficial matcrials which arc gcnerally red are called brick earth (Maignien, 1964) and the term laterite refers to blocs used in con- struction (Prescott and Pendleton, 1952, in McFarlane, 1976). A controversy over the dcfinition of latcritc has bccn going on for 150 years (Maignien, 1958; 1966; McFdrlanc, 1976). 'Ibday, two positions cmcrgc. First, many scicnfists have used lhc lcrm lafcritc lo dcsign;ifc wcathcring producls gcncrally formcd undcr tropia1 conditions, rich in iron and aluminum, and cithcr hard or subjcct to hardening upon cxposurc lo altcrnatc wctting and drying (Pendleton, 1936; Kcllog, 1949). Laterites also include certain highly weathered material with sesquioxidc-rich, humus-poor nodules, that may be surroundcd by carthy matcrial that docs not harden (Sivara- jasingham ct al., 1962). The tcrm also includcs all kinds of plinthitcs (from the Grcekplinrhos, brick) which arc latcritcs in the rcsfrictcd scnsc (Mohr ct al., 1972, but cxcludcs soft kaolinitic lithomargcs, finc saprolitcs and non-indurated ferrallitic soils. Sccond, for Maignicn (1964) and Millot (1964) followcd by Schcllmann (1983, 1986) thc word latcritc is not rcstrictcd lo induratcd materials but largcly includes all kinds of tropical wcathcring products. Ø I rccommcnd usage of thc tcrm latcritc in its broadcst scnsc, that is as products of intense wcalhering madc up of mineral assemblages that may include iron or aluminum oxidcs, oxyhydroxidcs or hydroxidcs, kaolinitc and quartz, and charactcrizcd by 11 ralio Sioz : (A1203+Fcz03) which docs not cxcccd thc ViilUC rcquircd to 381 380 Y Tardy Diversity and terminology of laleritic profiles h

+..- . ,-c-, A DlSHANlLlNG GRlllV YELLOU (GOETIIITE) THE SOFT ZONE CLAY (KAOLINITE) SOFT HORIZON FERRICRETZ . DISMANTLED HEMATITIC NODULES

REO (HEMATITE) B THE GLAEEULAR ZONE CLAY (KAOLINITE) MOTTLE ZONE INDURATED HORIZON PEOOSIRUCTURES HEMATITIC NODULES IN FORMATION

OR FINE SAPROLITE

SAPROLITE C LITHOSTRUClURfS ( QUARTZ THE ALTERATION ZONE KAOLINITE,GOETHITE ------K- ANO HEMATITE ) ABSENCE OF NODULES Fig. 15.1. The zonc of rubefaction (Pedro, 1968) coincides with the intertropical area of laterite formation, characterized by the development of kaolinite and hydroxides, ox¡-hydroxides or oxides of iron and aluminum. Laterita are limited northwards or southwards, under semiarid or arid climatcs, by belts of silcreta, caluetcr, vcrtisols or other smcctitic weathering pronies. The two unbroken lines drawn by Schwcrtmann (1988) delineate the two cold climatic domains characterized by absence of hematitc from the hot intertropical zone characterized by presence of hematite in almost all soils and weathering Fig. 15.2. Schematic reprcscntations of a latcritic profile capped by a fenicrete and of the thrcc major pronia. zones of a nodular lateritic profilc: altcration zone (A), glaebular zone (O) and soft suficial zonc (C). In the alteration zonc, quartz, kaolinite, goethite and hematite form together, In the glacbular domain hematitic nodulcs form in a rcd kaolinitic clay matrix. In ihc soft zone, hcmatitic nodules are dismantled , characterize quartz and kaolinite. togethcrwith the development of a secondary goethite. Thus the term laterite includes bauxites, ferricretes, iron or aluminum duricrusts, mottlcd horizons, "carapaces", "cuirasses", plinthites, pisolite or nodule bearing materials and is extended to the formations or horizons which are parts of red In the zone of alteration (coarse saprolite or arene, fine saprolitc or lithomarge), or yellow ferrallitic soils, tropical ferruginous soils and other formations such as the parent rock volumcs and structurcs arc roughly conscwcd. This domain is es- kaolinitic lithomarges which are soft, cannot be hardened but therefore arc, for sentially characterized by thc incongruent dissolution of primary minerals and by example, commonly associated with indurated ferricretcs. I propose here that the the leaching of most of the soluble materials; the least mobile elements (Al,Fe), zone of the present-day laterite formation would correspond to the rubefaction zone in sim little I liberated by wcathcring, rcorganizc almost with or no transport. of Pcdro (1968) (Figure 151). The glacbular domain, gcncrally shows indurated accumulations of iron or aluminum, either continuous (fcrricrctcs or bauxitcs) or discontinuous (nodules or piso- The three domains of common lateritic profiles litcs) resulting in a rcorganization of the original matcrial and in an absolute accumulation of iron and aluminum crystallizcd in various oxides, hydroxidcs oxyhydrox- In typical latcritic profiles Bocquier et al. (1984) have distinguished three zona ides and also kaolinite. The soft zonc, non-indurated, is charactcrized by a rclative accumulation of or major horizons (Figure 15.2): I , ,/, I (a) zone of altcration, at the base; I primary mincrals such as quartz, or sccondary minerals such as kaolinite and oxyhy- - (b) a glaebular zone, located in the middle part; and droxides, cither resulting from dissolution, degradation and dismantling of glaebular I (c) a soft zone, non indurated, located in the higher part of the profile. matcrial or rcworking from bclow by termite activity. 382 Y 7àdy Divcrsiry and terminolog, of laleritic profiles 383

Most of the lateritic profiles clearly show three typical domains but in several as demonstrated by Tdrdy et al. (1973), kaolinite and especially gibbsite occuring situations one or wo of these zones are absent either by incomplete formation or by in open systems are good indicators of the drainage conditions. On the contrary, posterior erosion. smectites found at the base of the lateritic profiles (in the coarse saprolites or arenes) are controlled by the specificity of the primary mineral closed microsystems and do not show any climatic significance. Saprolites and the so called alteration domain The fine saprolite or lithomarge The so called alteration domain, including different kinds of saprolites, is common to all lateritic profiles. These horizons are normally located below the ground Above the coarse saprolite is the fine saprolite (or lithomarge) in which the water table in the saturated zone, that is in permanently wet conditions. Here, the structures of the parent rock and the original volumes are still preserved (Leneuf, mineralogy of the alteration domain is less sensitive to climatic conditions than 1959; Millot, 1%4; Tdrdy, 1969). The progress of weathering is petrographically that of horizons located above the ground water table. %o kinds of saprolites are expressed by an increase in porosity, a complete or partial transformation of most normally distinguished (Figure 15.2). of the parent rock minerals and a decrease in induration of the rock (’Rudy, 1969; Nahon 1986). Beside quartz, which dissolves slowly, and small pieces of resistant The coarse saprolile primary minerals remaining partly undissolved, the dominant species are secondary kaolinite and ferruginous hydroxides, oxyhydroxides and oxides (goethite, hematite At the bottom of lateritic profiles, is the unweathered parent rock. Immediately and amorphous phases). above, lia the coarse saprolite, where abundant fragments of unweathered rock and Finally, in the fine saprolites, located below the ground water Icvel, there is no primary minerals are conserved, with original structures intact. On granitic rocks, important loss or gain of aluminum or iron (able 15.1), nor important migration the coarse saprolite is traditionally called “arhe” (from Latin arena, sand; Leneuf, observable under the microscope (Eirdy and Nahon 1985). Compared to the glae- 195% Millot, 1W; ’hrdy, 1969). The limit between parent rock and coarse saprolite bular horizon located above, in which clement transfers arc important, iron and is not generally a horizontal plane and the weathering fronts progress irregularly, aluminum, in lithomarges, are almost immobile elcmcnts. penetrating deeper down cracks, fractures and so on, and leaving unaltered boul- ders, fragments of rocks and relicts of primary minerals sometimes far up the profile. Leaching or accumulation of iron and aluminum in lirtioniarges This gives a fairly difiuse weathering front. Under certain circumstances, the coarse saprolite may be thick [as frequently observed on granitic rocks (Leneuf, 1959)], or There are three types of lithomargcs, as related to movement of iron and alu- very thin [as generally observed on basic rocks (Delvigne, 1965)l. Furthermore, the minum: leaching, lixiviation or accumulation (Figure 15.3, Tdblc 15.1). thickness of the saprolite is reduced in the humid tropics and relatively enlarged in A 1ifhomarg.e sensu stricto, or an idealizcd lithomarge called a C horizon, the , arid and in temperate regions (Tardy, 1969; Boulet, 1978). definition of which corresponds to the dcscription given in the previous paragraph. In the early stages of weatherin?, the first steps of alteration of primary minerals A leached Iifhomarge or lithomarge Caz, in which a2 stands for an excess of leach- develop imperfectly closed systems (Tìrdy, 1969; nescases, 1973; Nahon et al., 1977; ing which corresponds to the definition of the pallid zone which includes: (a) the Nahon and Boquier, 1983), where the secondary minerals formed are site-specific. removal of iron around voids, canalicules and channels from a fine saprolite which Their nature depends on the rate of weathering of each primary mineral, and on the locally becomes white, (b) out of these previously iron-poor areas, the removal of rate at which the initial closed systems become progressively open to the circulating kaolinite which is cithcr dissolvcd or mcchanically Icachcd, and (c) the dissolution solutions. or thc mechanical transport of quartz which finally lcads to the formation of some Wcathcring products in the coarse saprolite may be either a single mineral phase large cavities which may look karstic in origin (Tdrdy, 1969; Ambrosi and Nahon, such as vermiculite, smectite, kaolinite, gibbsite, imogolite, amorphous and hydrated 1986). Often, at the top of the fine saprolite and below the ferricrete, exists a hori- I aluminosilicates, or assemblages such as vermiculite-kaolinite, smectite-kaolinite, zon, rich in coarse grained quartz, depletcd in fine grained quartz and kaolinite, and kaolinite-gibbsitc (Lencuf, 195% nrdy, 1969; Novikoff, 1974;.MovikoIT et al., 1972). showing somc convcrgcnce of charactcrs with stonc lines or with thc A2 horizon, /,/ In a normal succession of minerals the most soluble phases (calcite, smectites) described by Leprun (1979) as the result of dismantling a lower ferricrete horizon. I appear in closcd systems, while the less soluble ones (kaolinite, gibbsite) appear A lithomarge CU& in which the letter b stands for a secondaty, either ncoformcd 1 in-the most opcn systems, at thc contact with circulating solutions. Conscqucntly, or dc posited kilolin itc, accon1pu II icd by goct li itc, li Ili ng t hc s111:iII or la rgc lui rstic .. . . .-

MASSIV1 REO ~4TRlÆ

LlTHOPELICTUI~ 4MO Q'EOORCLICTUAL UOOULCS

CARAPACE LA119

Fig. 153. a. Alteration zone: marse saprolite or ar&ç fine saprolite or lithomarge (C horizon SCNU m*cfo),leached lithomarge (Caz) and illuviated lithomarge (Ca,). b. Mottle zone and nodular horizon: channels, macrwoids, bleached domains, yellow-red soil matrix, litho- and pedo-relictual mottles and ndulsc Femcrete zone and surliaal horizon: a progressive nodulation towards the top of the profile and a secondary development of pisolite dose io the soil surfaœ can be distinguished. Three rypa of nodules are distinguished lithorelictual (white), pedorelictual (striped) and undifferenciated (black). .Y

TABLE 15.1 9 3 Isovolumetric chemical balance (in g per 10 cm3 of rock) of lithomarges and fine saprolites (horizon C), showing that some samples appear leached and z-2 depleted in both Fe and Al (Caz) while others appear enriched in kaolinite, goethite, Fe and AI (Ca,,) S' % Nature of rock Density Quartz Sioz Al203 Fez03 Mn304 Ti02 Ca0 MgO Na20 KzO HzO .c Parent rock unaltered 2.60 70 188 35 6 0.08 0.5 4 1.5 10 12 1 .o 8 Lithomarge C 1.40 60 9328 5.5 0.03 0.8 0.2 0.2 o. 1 0.2 12.0 (leached) Caz 1.20 55 a25 2.0 0.01 0.2 0.1 0.1 o. 1 0.1 8.9 (illuviated) Cazb 1.80 65 223 40 7.5 0.17 3.0 0.3 0.3 o. 1 0.3 17.5 Lithomarge (calculated) 1.65 70 111 35 6 0.0 0.0 0.0 0.0 0.0 0.0 13.0 Mottle zone 1.49 54 93 33 8.0 0.03 0.9 0.3 0.3 0.2 0.1 13.5 Sourœ: Tardy (1%9).

aW Diversity and terininolagr of Iateri!ic profites 387

cavities charactcrizing the lithomarge (23%.This accumulation, brown in color, con- has bccn considered as sccondarily dcvclopcd, aftcr thc formation of the lateritic trasts clcarly with the pale colour of the void and channcl surroundings which often profilc, in pcculiar cascs, whcn thc climatc has bccn changing from humid (latcrite) appcar dccolorcd prdy, 1969). When most of the initial porosity is filled, a massive to arid (Vcrtisol). Thc mcchanism was obscrvcd in Senegal and in Burkina Faso by saprolite is produced (Ambrosi and Nahon, 1986). Leprun (1979) and also described by Boulet (1978), Pion (1979), Ambrosi (1984) Lithomargcs Ca2 and Ca3 both are located at the top of the fine saprolite zone, 1 ' and Ambrosi and Nahon (1986) (Figure 15.3). undcrlining the mottle zone. I In this case, the development of smectite, associated with kaolinite, invading a I part of the fine saprolite has a climatic significance according to Tardy et al. (1973). Problems Óf reminology This secondary development of underlying smectitic horizons is interpreted as one of the most probable mechanisms of the secondary degradation of the basal hori- In my opinion, fine saprolite, lithomarge, pallid zone and variegated clays refer zons of the ferricretes previously formed during humid periods and then subjected to thc same thing. Thcy all designate the same quartz-kaolinite-iron oxyhydroxide to semi-arid climates. formation, previously described. However, the two last terms produce possibilities In summary, arenc and lithomarge, equivalent to coarse and fine saprolites, are of confusion and interpretive error. basal weathering domains (C horizons) in which (a) volumes of the parent rocks are The use of the term variegated clays (argiles bariolbes) instead of lithomarge preserved, (b) amounts of iron and aluminum and amounts of quartz are, conserved or fine saprolite (Leprun, 1979; Ambrosi, 1984) introduce risks of confusion with within lo%, and (c) porosity is due to the lcaching of the alkaline, alkaline-earth ,

mottled clays (argiles tachettes) which are, genetically speaking, entirely different. elements, and silica in cxcess of that required to form kaolinitc and to remain as pri- II .. . Varicgation was used to dcscribs differences in colour due to the original distri- mary quartz. At the top of the lithomarge sensu szricro (C horizon) one may observe I 1 bution of iron-rich (biotites, amphiboles) and iron-poor (quartz, feldspars) primary additional leaching (Ca,) or secondary enrichment (Ca2b) of kaolinite and iron hy- mincrals. Mottles are differences in colour due to a pedogenetic or a secondary droxides. Howcver, there are ncither short scale movements nor reconcentration rcdistribution of iron, removed from some domains and reconcentrated in othcr of iron and conscqucntly no mottlc formation. Both lithomarge Ca;!and Cazb are domains. transitional with thc overlying mottlc zone, they arc part of the alteration zone and The distinction bctwcen mottle zone and pallid zone is due to Walther (1915). are not part of the glacbular zonc which includes mottlc and nodular horizons. Ncvcrthclcss, the term pallid zone, introduced by McLaren (1906) and Simpson Finc saprolitcs or kaolinitic lithomargcs can bc vcry thick. Sevcral tens of mcters (1912), according to Prcscott and Pendlcton (1952) and McFarlane (1976), also crc- arc not cxccptional (McFarlanc, 1976) and an ordcr of magnitudc of 100 m is not ates confusion. In fact, it has been understood by definition as a zone, pale-colored uncommon (Pham et al., 1988; Freyssinet, 1990). ., ' duc to leaching of iron, which is considered as the source of iron enrichment in

' _I . 1. the overlying mottle zone ferricrete. McFarlane (1976) has presented a series of argumcnts showing that this cannot be the case mostly because thcre are no rela- Glaebular domains and the ferricrete profile tions between the thickness of the pallid zone and the amounts of iron accumulated , Under contrastcd tropical climates (3" = 25-30°C,rainfall = 1500 mm per year, 1 abovc it, and also because some so called pallid zones do not show any iron release, 5 months of dry season, rclativc humidity of the air, HR, = 65%) and above fine . .. according lo the observations described below. Consequently both terms varicgatcd saprolitcs and lithomarges, iron is gcncrally rcdistributcd and concentrated to char- I clay or pallid zonc should be avoidcd. acterize a glaebular zonc, in which fcrricrcte may develop. A typical ferricrete profilc consists of threc major horizons: mottlc zonc, fcrricrctc itsclf (carapace and Sntectitic saprolite and pistachio horizons cuirassc) and, at thc soil surfacc, a gritty laycr and dismantling horizon. Under semi-humid or semi-arid conditions (rainfall < lo00 mm per year) within the coarse and the fine saprolites, a smectitic horizon called sometimes the pistacipo Mottle zones and nodular horizons i horizon because of its pale green color, may be developed. Thjs horizon is gcnerally The mottle zone is characterizcd by a contrast bctwecn bleached domains and overlain by a smectitic soil in a dynamic equilibrium reflecting the local climatc, Fc-mottlcs which can bc casily distinguishcd on a ccntimetcr scale, in outcrops and I rhat is, rainfall 800-900 mm per year, temperature 25-30°C, for a typical Vcrtisol in samplcs, and on a micromctcr scale, undcr thc microscope (Figure 15.3). 1 (Bocquicr, 1973). It may also be covered by a lateritic soil consisting of a lithomarge, Blenched doninins consist mainly of quartz and kaolinite and exhibit a white or 1 a-mottle zonc and in some cases a carapace and a ferricrete. The pistachio horizon gray colour duc to thc dc-fcrruginimtion of thc prcviously associated kaolinite and ! . .I .I a.

388 X Tardy Diversity and terminology of lateritic profiles 389

FORHlTlON OF NOOULES FORHATION OF HALROVOIDS hematite together with kaolinitc. Thus, domains of previous accumulation of kaoli- ruon noTiLEs FROH BLEACHED DOHAINS nite are natural hosts for secondary accumulation of iron in mottles and nodules ECONOARY PEDOSTRUCTURE (Figure 15.4). ACROVOID SECONDARILY FlLLEO In the mottle zone, there are two situations in which kaolinite accumulation is ,

the precursor of nodule formation: lithorelicts and pedorelicts. 1I PEOORELJCiUAL LITHORCLICTUAL @ Lifhorelicts and pedorelicts. Lithorelictual mottles or nodules are iron accumula- ‘1 tions in which the original structure of the parent rock can be seen (Figure 15.4). ‘I They are more accurately called alterorclicts by Faure (1985) and are mostly pri- 1 mary assemblages of aluminum and iron-rich minerals which, by alteration, give ’ sccondary stable associations of kaolinite and iron oxyhydroxides, in which porosity is small in size. This is the casc for schists, amphibolites and layers of migmatite which are particularly rich in biotite, for example. Granitoid rocks including aplitic

IRON LEACHED FROH lBOVE and quartzo-feldspathic vcins, initially poor in iron and consequently sensitive to ANO ACCUl4ULATED DELOU ,- leaching of kaolinitc, arc not idcal parent rocks for thc further accumulation of this clcmcn t. Pedorelictual features (orpedorelicts) arc duc to secondary accumulation of kaolin- *. * :. ...(‘.+ ite, filling voids previously crcatcd in the blcached domains. They are relicts of soil forming processes. Kaolinitc, derived from solution or translocation from overlying upslopc laycrs, followcd by precipitation or dcposition lower in the profile. These ’ secondary accumulations of kaolinite and associated quartz are generally accom- PRIMARY LlTHOSTRUCTUPt PEOOSTRUC TURl LITHOSTRUCTUA E panicd by a secondary accumulation of iron-gcncrating brown-red colored clays, RICH IN KAOLINITE RICH IN QUARI.2. providing a contrast with the whitc-gray dccolored arcas, located around the tubules Fig. 15.4. Formation of mottles and nodules, in originally kaolinile-rich domains, leading lo lithore- and channcls (Nahon, 1986). Highcr in thc fcrricrctc, thcsc mottlcs evolve into liaual features developcd in primary lithostructures and pedoreliclual features, dcvclopcd in seconday nodulcs as accumulation progrcsscs. Thcy are what Nahon (1976) has callcd the , -. .I -. pcdosiructure. Formation of bleached domains, macrovoids and channcls from originally quartz-rich do- argilomorphous nodules, which arc old voids, fillcd by kaolinite and iron oxyhy- mains. Secondary formalion of pcdorclictual mottles or argilomorphous nodules in previously formed macrovoids. droxide. Mottles and nodules of this kind arc traces of the secondary pcdogenetic I activity occuring in thc unsaturatcd zonc. In thc nodular zonc, lithorelicts are more abundant on schists and amphibolitcs whilc pcdorclicts dominate on sandstoncs and granitic rocks rich in quartz (Figurc 15.4). iron oxyhydroxidcs. Original kaolinite aggregates, free of iron, can bc disperscd and %)wards thc top of thc profilc, thc moctlc zonc cvolvcs towards cithcr a nodular kaolinitc particlcs can migratc and cvcn be leachcd out. These changcs arc accom- horizon (Ambrosi ct al., 1986) or a fcrricrcte (Figurc 15.3; Nahon, 1976).

I. panicd by a strong incrcasc of porosity, leading subsequently to thc formation of macrovoids, such as tubulcs and alvcolcs. At that stage quartz also can prccipitatc in The ferrìcreh?forniarion thc gcncralizcd cluvial migration (Figurc 15.4; Nahon, 1986). Quartz and kaolinitc rcmoval are probably duc to tcrmite activity (Eschenbrenner, 1987). ‘ Thc fcrricrcte profilc has bccn dcscribcd by Nahon (1976), Nahon et al. (1979) Thc dc-fcrruginiiration takes place in domains originally enriched in quartz and and Leprun (1979). Its mcchanism of dcvclopmcnt has bccn studicd and interprctcd poor in kaolinitc. It bcgins at thc top of the finc saprolite (lithomargc Caz) and by Didicr CI al. (1983a, b, 1985), Thdy and Nahon (1985), Nahon (1986), Muller and becomes progrcssivcly more important towards the top of the mottle zone, where Bocquicr (1986) and Ambrosi CI al. (1986). channcls and tubules arc abundant (the mottle clay zone: “$rgile tachetCe”, which Abovc thc mottlc zonc and the non-induratcd nodular horizon, one finds succes- was somclimcs callcd channel clay zone: “argile B canaux”). sivcly, from the bottom to the top, variously induratcd horizons (Figure 15.3). Fe-morrles, mostly of a brown red colour, are difiuse glaebules (Brewer, 1964) Carapace is intermcdiatc bctwccn mottle zonc locatcd below and cuirasse located and result in a concentration of iron which precipitates mainly as gocthitc and as abovc. It corresponds to a progrcssivc accumulation of iron and as a consequence, . ;! i

3!N X 'Ihrdy Diversity tim1 tcrtninolqy oJkitcrific pmJilev 39 1 to a progrcssive development of hematitic iron nodulcs, eithcr pcdorclictual or dissolution of thc fcrticrctc, and mixcd with (ti) pchhly lnycr which dcvelnps at thc llthorellctual. The bleachcd zone is progresslvely reduccd In she, so tlitlt the yellow- expense of the plsolltlc iron crust and which comes from the early fonned hematlllc white colored domains decrease in size while the purple-red indurated domains nodulcs. The size of the pebbles diminishes with time while goethite develops at enlarge and develop. the expense of hematite. Iron, aluminum and silica leached from the surface in the Cuirasse or jemkrere or ùon duncrust sensu stricto is indurated, purple-red in dismantling upper horizon precipitate in a deeper horizon and thus reconstitute the color, considerably enriched in iron, particularly in hematite. In ferricrete sensu ferricrete below. The formation of gritty layers at the surface is a component of stricto, the blephed zones are almost absent. Furthermore channels may be reduced ferricrete metabolism and is an essential phase of its reconstitution. in size but always survive in abundance. The edges of the channels often appear lined by iron accumulations in which goethite dominates. The channels are com- The A2 leached horizon and sub-suqace femkrete dismantling monly empty but may be filled by kaolinite and fine quartz. They are progressively transformed into argilomorphous, pedorelictual nodules in which iron accumulates Below the fcrricrete, the top of thc lithomarge and the lowest part of the mottle as hematite (Nahon, 1976). zone often appear decolorcd and leached, so that a stonc-line (labelled AZ) made nwards the soil surface, and from the carapace to the cuirasse, in the discontin- of large quartz grains, undcrlics the carapace and the fcrricrete and sometimes re- uous iron accumulation horizon, hematitic nodules grow and join each other in a places cntirely the mottle zone. The impoverishment of all of the fine fractions in a way recognized by McFarlane (1976) as the major process of ferricrete formation. coarse material characterized by a high permeability over a lithomarge richer in clay However, there are domains in which the clay matrix surrounding the early fordcd and less pcrmcablc, produces a pcrchcd water table. Here, water circulates laterally nodulcs is ilself invaded by iron (also as hematite) so that in certain circumstances , and creatcs a reduccd zonc whcrc iron is solubiliscd. This leads to the Ccstabi- the resulting ferricretes form an almost continuous accumulation of iron in a kaoli- lization of iron-kaolinite aggregatcs, which induces the leaching of the kaolinite nite plasma. This results in a massive structure which may or may not be crossed particles (Chauvel, 1977) and the dismantling of the overlying ferricrete. In humid by large voids and channels. This facies of ferricrete is called for that reason mas- regions, the A2 horizon may bc attributcd to termite activity inducing a selective sive and vacuolar, and generally is lhe most evolved, that is the richest in iron, in upward and downward motion of finc matcrial (Eschcnbrcnncr, 1987). In less hu- hematite, in kaolinite and the poorest in quartz, if present in the original parent mid regions, close to thc Sahclian fringcs, this phcnomcnon, dcscribed by Leprun rock. The massive ferricrete is equivalent to the jacib griseux simple defined by (1979) is gcncral. In dry climatc arcas, the pistachio smcctitic horizon, sometimes Nahon (1976) on quartz-rich sandstones. well developed, may also induce a lateral circulation of ground water high in the The jemcrete dismantling hoti" Towards the top, a new system of secondary profile, favorable to a sub-surface dismantling of ferricrete previously formed under voids is developed. The hematite-kaolinite nodules are rehydrated and corroded more humid climates (Leprun, 1979). at their edges. Kaolinite is dissolved and Al-hematite is transformed into Al- goethite. A goethitic cortex (concentric yellow brown) develops at the periphery Terminology of the purplc-rcd hcmaiitic nodules. The ferricrete facies becomes either pseudoconglomeratic if blocs are present, sometimes recemented by goethite, or pisolitic if Mottlc zoncs, nodular horizons, nodular fcrricrctcs (carapace or cuirasse) and single nodulcs are individualized (Nahon, 1976). pisolitic horizons arc'all part of the glacbular zonc. Glacbulcs are three dimensional In summary, a ferricrete is a glaebular domain made of several successive hori- units within the s-matrix, rccognizcd in this casc by a grcatcr concentration of iron, zons: a mottle zone, a nodular non indurated horizon, a carapace and a cuirasse, the edges of which are cithcr sharp (nodulcs) or dilTusc (mottlcs) (Brewer, 1964). massive, pseudonodular, nodular and pisolitic. All these horizons are parts of a Clcarly, thc mottlc zone is a part of thc glacbular zone. Chatclin (1972) has intro- 'single dcstruction-formation system. duccd a distinction bctwccn altcritc and allotcrite. Thc first slands for saprolites and was extended to mottlc zona in which the parcnt rock structures arc distinguishable The gn'try lqer of sutface dismantling and the original volume is conscrvcd. Thcy arc clcarly isovolumetric alterations in the sense of Millot and Bonifds (1955), Allotcrilc, on the contrary, stands for hori- A gritty horizon is developed at the top of the profile, madkof thc products of thc zons in which the original primary architccturc of parcnt rocks is dcstroyed. Thus, dimantling of lhc pseudoconglomeratic or the pisolitic-underlined ferricrete. At thc in most ases, carapaccs, cuirasscs and morc gcncrally bauxitcs and ferricrctcs can soil surface, over the induratcd horizon, two kinds of soft materials are accumulatcd bc considercd as allolerites (Boulangb, 1984). Hcrbillon and Nahon (1988) have dis- I~cally:(a) surficial sandy or silty layer made of corroded quartz liberated by the tinguishcd two major zoncs: a rclativc accumulation zonc including most saprolites I

.. Diversity and tertninobgy lateritic profiles 393 392 Y Tardy of

and in some cases, mottle clays, and an absolute accumulation horizon including all (1) a destructive or dismantling stage in which dissolution and degradation takes the ferricrete layers. Muller et al. (1981) have described a mottled alterite distinct place close to the soil surface, in porcs of large size tcmporarily hydrated, together from a nodular zone, so that a certain ambiguity is introduced, concerning a pos- with (2) a formative or a reconstitutive stage in which precipitation concretion and sible difference between mechanism of formation of mottles in mottle zones and nodule formation takes place in clay-rich and small sized pore domains of deeper : nodules in nodular horizons. Mottles and nodules form in response to an identichl horizons temporarily dehydrated. In the first case (1) of the process, goethite, a filiation process. Both diffuse mottles and sharp edged nodules are glaebules, so that hydrated mineral, prevails, while in the second case (2), hematite, a dehydrated mottlcd zone and ferricrete must be included in the glaebular zone, Despite this, mineral, dominates. The first stage (1) takes place during the wet season and is the so-called mottled clay horizon is generally isalteritic, though the results of the dominated by circulation in pores of large diameter, leaching and reactions occuring beginning of the mobilization and the absolute concentration of iron can be seen. in saturated domains and high water activity. The second stage (2) takes place An essential distinction is proposed between (a) coarse and fine saprolites, per- during the dry season and is dominated by imbibition and impregnation of pores of manently located in wet conditions, below the ground water table and in which little small size, occuring in unsaturatcd domains and low water activity. if any movement of iron is detected, and (b) mottle zones, nodular horizons and ferricretes, located in the unsaturated domain, seasonally hydrated and dryed, in The soft zone which mass transfer of iron is currently observed. In tropical areas a large number of glaebular soils are capped by a soft horizon (Figure 15.2), the origin of which is diverse subject to different interpretations. Conclusion:femkrere metabolism or

An iron crust is generally built at the top of the lateritic mantle by a combination Three possible origins of successive small-scale migration of iron, leaching or dissolution of kaolinite and In some regions, such as Amazonia for cxample (Lucas, 1989), the soft hori- quartz grains, formation of voids, secondary accumulation of kaolinite together with zon is vcry thick (several mctcrs) and is rcgularly and widcly distributed. This so small quartz clystals, and ferruginization of these accumulations (Nahon, 1986). called "argilla de Belterra", 2-10 m thick, has bccn altcrnatcly considered either Several remarks can be added. as a sedimentary cover (Sombroek, 1966; Tricart, 1978; Putzer, 1984; Tkuckenbrodt (a) Despite some lateral and upward vertical movements, particularly observed and Kotschoubey, 1981) or as an in sifu alteration of the sedimentary parent rock downslopes, where the water table is close to the soil surface, the transfers of iron (Chauvel et al., 1983; is and aluminum are supposed to be essentially vertical and downwards, Irion, 1984). The qucstion not yet settled. In Africa, soft horizons covcr large tropical or subtropical areas, overlying in most (b) Kaolinite-rich domains are selective hosts for hematitic iron accumulations. cases, a glaebular zone the They evolve towards lithorelictual nodules if kaolinite accumulation comcs directly or a ferricrctc. In latter situation the soft horizon or the surficial sandy laycr, non induratcd and charactcrizcd by a rclative accumulation from the original parent rock (lenses of schists, mica-rich layers in migmatites, of primary minerals such as quartz, or sccondary minerals such as kaolinite and aluminous-rich domains in basic rocks) or towards argilomorphous pedorelictual goethite, generally shows somc gcochcmical rclationship to thc underlying ferricrcte nodules if kaolinite comes from a sa"ary accumulation either by neoformation or and has been considered as an situ by mechanical deposition in the macrovoids. They are Small sized porous domains, in dismantling product (Nahon, 1976; Leprun, 1979; Muller et al., 1981; Roscllo ct al., 1982; Chauvel et al., 1983; Fritsch, 1984; in which thermodynamic activity of water is commonly low (Didier et al., 1983a, b; Bocquicr et al., 1983). Howevcr, despite a surficial degradation of the underlying - 'l?drdy and Nahon, 1985). fcrricrctc, the soft horizon as a whole cannot cntirely dcrivc from the ferricrete. ". 1 (c) By contrast, primary quartz-rich domains appear as sclcctivc sitcs for decol- The soft horizon may be duc to upward transfer of material coming from the mottle ' oration, deferrugini7ation and destabiliiration of the iron-clay aggrega tes. Consc- zone, by tcrmitc activity. : i qucntly, thcy are also privileged sites for chcmical or mechanical lcaching of quartz ' grains and finally for creation of channels, glosses and zones of water circulation...... : Concretion and pisolite formation in Ultisols or Oxisols ..._.. .. ., They appcar generally to have large pores and are seasonally filled by water of high ... ' thermodynamic activity, close to the conditions of saturation (T'rdy and NovikofT, I Ir.-'. '.;' .,,. ,. 1988). Concretions and pisolites arc widcsprcad in non-induratcd latcritcs (Maignicn, 'I ,- :. 1 .. (d) Finally these two types of facies taking over from one to anolhcr in space, arc 1958; 1964; Martin, 1967; Stoops, 1968; McFarlanc, 1976) and nodular soils arc , thi: two privilcgcd sites for lhe two simultancous stagcs of ferricrete mctabolism: widcly distributed in South Amcriw and in Arria (d'Hoorc. 1963).

I ... . . 394 Y Tardy Diversifyund tertninobgy ojluteritic profiles 395

I In western Africa, thcy appear distributed in two major climatic zones (Maig- Muller (1987) dcscribcd in detail the soil-sequence of Goyoum in the Eastern nien, 1958). The first zone corresponds to the highly contrasted tropical climates part of Ccntral Cameroon. Above the fine saprolite, the glaebular domain com- . .’ under which Ultisols (sols ferrugineux tropicaux) show a B horizon cnrichcd in prises a red hematitic clay in which dark red nodules are individualized. The soft red kaolinite, in which mottles, ferruginous concretions and nodules are currently de- clay is, dilTusely impregnated by hcmatite and goethite (hcmatite > goethite), asso- vclopcd (Maignien, 1958; Buck, 1973; Leveque, 1975; Boulet, 1978; Leprun, 1979). ciated with kaolinite in microstructurcS. Within the soft red clay matrix two types of The second zone corresponds to humid tropical climates under which Oxisols (sols nodules are developed: (a) argilomorphous, with a structure similar to that of the ferrallitiques) also show a nodular or pisolitic horizon (Martin, 1967). clay matrix, or (b) lithorelictual, showing a structure similar to the petrographic or- Thcse kinds of soft but nodular Oxisols or ferralitic soils have been extensively ganization of the parent rock. Nodules are in general made up of an assemblage of dcscribcd by Mullcr et al. (1981) in Cameroon, Gabon and Congo, and by Eschen- kaolinite and hcmatite. lbwards thc top of thc profile, the red clay matrix becomes brcnncr (1987) and Eschcnbrcnner and Badarello (1987) in Ivory Coast, all regions progrcssivcly yellow and gocthitc dominates ovcr hematite. The hematitic nodules subjcctcd to humid tropical climate. arc surroundcd by a gocthitic corlcx thc thickncss of which also increases’rowards thc top of thc profilc. Closc to thc soil surfacc, the rcd nodular horizon is dismantled and rcplaccd by a soft claycy horizon, madc of an asscmblage of goethite, kaolinite KA HE GO GI and quartz. In Ultisols nodules or pisolitcs can bc of two diffcrent origins: (a) either (Figure 15.5) thcy are relatively rcccnt sccondary nodulcs growing within the clay-rich soil matrix, cn route to the dcvclopmcnt of a fcrricrcte, and following a similar pro- I.4.. . . cess of accumulation of iron as hematite; or (b) they are primary hematitic nodules

a generated by a secondary disagrcgation of older ferricretes, today covered by a soft goethite-kaolinite horizon and en route to bcing dismantled through the formation of a nodular stone-line. Thc first proccss is generally acccptcd and nodular oxisols arc often considcred as the prccursors of thc fcrricrctc (McFarlane, 1976, 1983). The second was rcccnlly discovcrcd with thc rcalizaiion that the equatorial rain forest was previously underlain by ferricrctes which are now being dismantled (Novikoff, 1974; nrdy et al., 1988; Martin and Volkoff, 1989; Nahon et al., 1989).

Oxisols with no glaebiilur development

Mohr ct al. (1972) statc: “typical oxisols without plinthitc arc dccp, friable soils. Horizon diflerentiation is indistinct and there is gcncrally- no clay movement. These soils are porous and arc rapidly draincd. The oxic horizon does not harden upon cxposurc to air; thcrc is no scgrcgation of iron; its distribution is vcry homogcncous I and concretions arc abscnt or ncar absent. If thc solum is vcry dccp, it is sometimes underlain by a motllc clay horizon. Othcrwisc thc solum lies dirccily on the weathered rock”, that is on thc lilhomargc or thc fine saprolite. Thcse rcd fcrralitic soils, I, also known in Brazil as Latosols, arc dcvclopcd on various parent materials under Fig. 15.5. Nodular Oxisol profile in Cameroon (rrom Muller, 1987). Distingyishablc arc thc alteration humid tropical or cquatorial climatcs in rcgions covcred by rain forest and particu- zone (A), the glaebular zone (B) and the soft horizon (C) and within them:(I) saprolitc with inhcritcd larly in the wcll draincd upslopes of landscapes (Chauvcl, 1977; Volkoff, 1985); thcy rock structure and Icxture; (2) red and compact matrix; (3) yellow and friable matrix; (I)ferruginous typically show a micronodular struclurc madc up of an asscmblagc of kaolinite and lithorclia; (5) argilomorphous nodules; (6) yellow and compact matrix; (7)organic mattcr accumulation (from Mullcr and Boquier, 1986). The mineralogical distribution of keolinitc (U),hcmatitc (HE), quartz ccmentcd by iron oxides (Figurc 15.6; Bcnncma ct al., 1970; Pcdro et al., go?thite (GO), gibbsitc (GI) and quam (QU) (from Mullcr, 1987). 1976; Chauvcl and Pcdro, 1978a, b). * ", \ 1. L,

3%. Y Tardy Diversity and terminology of lateriticprofiles 397 by relatively low thcrmodynamic activity watcr and rclativc humidity air 'E a of of the (HR = 60%, annual avcragc) and high tcmperaturc (T = 28°C) while bauxite and aluminum enrichment can stand lower temperature (T > 22°C) and are favored by ITE a higher thermodynamic activity of water and a higher relative humidity of the au : (HR > 80%; Tardy and Novikoff, 1988, nrdy et al., 1988a, b). Some of these lateritic bauxites are very old and have been evolving under various tropical climates, since the Jurassic, the Cretaceous, the Paleocene or the Eocene (Michel, 1973). Others arc younger and may have formed since the Miocene, thc Plioccnc and cvcn thc Quatcrnary (Bardossy, 1979). Lateritic bauxites are widesprcad all around the world and appear under different latitudes. Thc old bauxitic profilcs which havc bccn submittcd to climatic fluctuations show in gcncral a large varicty of pctrographical facics and a high complcxity of mineral

distribution. ~

Distribulion of gibbsite Scvcral latcritic bauxitc profilcs, rcprcscnting quite a large varicty of situations, were recently described by BoulangC (1984) and by Lucas (1989). Most profiles 1 Fig. 15.6. ?Lpical Oxisols do not generally show clay movements, colour gradients and subsequent hori- . zon diflcrentiations. Because of their micronodular structures made up of an assemblage of hcmatitc, show two diffcrcnt gibbsitic layers. Thc first one occurs in the coarse saprolite kaolinite and quanz, they are porous and characterized by rapid drainage (from Chauvel, 1977) at the bottom of thc profilc and closc to thc contact with the parcnt rocks. The sccond onc, bauxitic, sensu sfricfois locatcd at thc top or thc profilc closc to thc soil surface. In bctwccn, a kaolinite-rich horizon, from which gibbsitc may be absent, There are few studies devoted to the composition of iron minerals in these kinds appcars similar lo thc finc saprolitc or lithomarge alrcady dcscribcd. Howcvcr other 7 of soils. As suggestcd by the red colour they are mixtures of ferrihydrite, goethite situations are also dcscribcd. In some cases only kaolinite is found in the coarse and hcmatite (Schwcrtmann, 1988). Gibbsite also common and sometimes abun- is saprolite, while in others, gibbsite may occur all through the profile. In the latter dant in Oxisols or Latosols (Sieflermann, 1973; Bourgeat, 1972). The mineral dis- situation, kaolinite, always prescnt in the finc saprolitc,- disappcars in the bauxite tribution within thc profile is a function of depth: hematite increases towards the (Figurc 15.6). top of thc soil-surfacc whilc kaolinite or gibbsite may either increase or decrease Thc bauxitc zone itsclf occurs in dill?" major facics. Thc most classical (Lcncuf, 1959; Schcllmann, 1964, Sieffcrmann, 1973; Dclvignc, 1965; Mohr ct al., bauxilic profilcs show thc dcvclopmcnt, niorc than 10 to 15 ni dccp, of 11 massive * 1972). Furthcrmorc Volkoff(1985) pointcd out that the mincralogical composition ' gibbsitic accumulation. 'lbwards thc top, vacuolcs dcvclop within il gocthitc-gibbsitc may dcpcnd on thc naturc of thc parcnt rock kaolinite, hcmatite, gibbsitc on basic ycllow plasma while nodules rcd in color, madc of an asscmblage of hcmatite- rocks; quartz, kaolinite, goethite on sandstones. gibbsitc, arc individualized (BoulangC, 1984). Thc formation of pisolitcs is not obscrvcd.

' ' Lateritic bauxites Thc sccond type remarkably, shows a sccondary dcvclopmcnt of boehmite to- gethcr with a largc formation of pisolitcs (Tardy ct al., 19SSa, b). - A bauxitc is a thick accumulation of aluminum resulting from long term lateritic wcathcring undcr humid tropical or equatorial climates (Millot, 1964; Valeton, 1972; Secondary formation of pkolites and boehnrire 'i'. I A- Bardossy, 1982; Boulange, 1984; Lucas, 1989). Ø Thc rainfall rcquircd for thc separation bctwccn a fcrricrctc and a bauxitc is Bochmiic forms in some bauxitic profilcs, particularly those Itxatcd closc to thc .. cdgcs of a platcau and in an horizon closc to thc soil surracc. Whcn this occurs, $ probably about 1700 mm a ycar. Abovc this limit, fcrricrctc dismantling takcs placc whilc gibbsitc forms, develops and accumulates, The humidity of the air and tcm- thc gibbsitc-hcmatitc original plasma is rcorganizcd into pisolitcs and a strong .' pcfaiurc arc also dctcrminant factors. Fcrricrctc and iron accumulation arc favorcd scgrcgation of hcmatitc is obscrvcd. \ 9.. 1” .”, . . 4,-.

I; ’ 398- Y Tardy Diversity and terminology of lareririe profiles 399

LV -z

_, . a. Iron accumulation b, Aluminuln accumulation Conclusion Aluminum and iron are distributed from the bottom to the top of the profiles, O000 in (00001 Boehmite -+ Hematite two mincral sequences: 000000 gibbsite (1) - kaolinite - gibbsite (2) bochmite - gibbsite (3) 000000 - 000000 000000 Bauxite g g Gibbsite + Hematite + Goethite goethite (1) - hematite - goethite (2) i’ Gibbsite 1; ;;;E; ;;;E; ,” 000000 *. 000000 These are ideal sequences and not cvcry layer is always present. However, in both Hematite + Goethite + Iholinite cases, hematite and boehmite which are dehydratcd or relatively dehydrated minerals are gencrally locatcd between more hydratcd phases, (goethitc and gibbsite). Thc gibbsitic profilcs dcvclopcd undcr humid tropical or cquatorial climates arc often capped by an iron rich horizon (Grandin, 1976). By contrast, gibbsitic horizons which were later transformed into boehmitic horizons under seasonally contrasted Ipodzols. ‘ ’ (Figurc 15.7). (b) Yellow Oxisols, primarily rich in kaolinitc and gocthitc, with quartz and gibbsite as acccssorics, arc locatcd on quartz-poor rocks particularly on downslope Distribution orhemarite and goethite setting of the landscape. In both the coarse saprolite and the fine saprolite, goethite is more abundant than (c) Rcd Oxisols primarily rich in kaolinitc and hcmatite, with quartz and gibbsite - hcmatitc, whilc in thc bauxitic layer hematite dominates goethite. Furthermore, I as acccssorics, are found in the upper slopes of a landscape. (d) Thcy may or may not havc a glacbular horizon, associatcd in the latter case 1. I when bochmitc dcvclops, gocthite disappears and hcmatite is‘ the single iron-rich mincral prcscnt. This distribution, frequently observed in West African altcritic only with a mottlc zone. (e) Ferricretc profilcs may bc covcrcd by sandy surficial horizons originating * bauxitcs (Boulang6, 1984) is similar to that frequently described by Combes (1969) and by Bardossy (1982) in karst bauxites of the South of France, Hungary and from tcrmitc activity lcading to lhc subscqucnt dismantling of hematitic duricrusts, the product of which rcmains within thc soft horizon as a relictual nodular stone Grcccc. j. line. (0 Fcrricrctc profilcs may show thc dcvclopmcnt of sandy, quartz-rich, bleached horizons forming stonc lincs at the top of mottlc zoncs and lcading subscqucntly to

m. sub-surfacc dismantling of iron duricrusts,

i l. l...... X. -._I._..__...,_,.__ ...... Y 'Ibrdy i Diversity iind tertninology of koteritic profiles 401

m (II) I3cigc Ull¡sols sltow lltc tlcvclol)ltlcnt ol' siirf¡4:1I Ic;iclicd Itorizons (AZ) * ovcrlyitig horimis (U) in which kaoliniic accuiiiulalcs and in wtiiclr inoltlcs and hematitic nodules subsequently develop. /i

Ii c All these soils are laterites, characteristic of a large variety of present-day climate, . I. : paleoclimates, parent rocks, topographic positions and hydrologic regimes.

References

Ambrosi, J.P., 1984. Petrologie et gtochimie d'une sCquence de profils laleriiiqucs cuirasses fcrmgineux de la region de Diouga, Burkina Faso. Thhse, Univ. Poitiers, 223pp. Ambrosi, J.P. and Nahon, D., 19%. Petrological and geochemical diffcrenciation of lateritic iron crust .. I profiles. Chem. Geol., 57: 371-393. I. .L Ambrosi, J.P., Nahon, and Hcrbillon, AJ., The epigenetic rcplaccment of kaolinite by hematite .- D. 1986. in laterite. Petrographic cvidcnce and the mechanisms involved. Gcodcrma, 37: 283-294. I Bardossy, G., 1979. The role of tectonism in the formation of bauxite deposits. In: 'Ravaux dc I'ISCOBA, h . Zagreb, IS, pp. 15-34, Bardossy, G., 1982. Karst Bauxites. Developments in Economy Geology 14. Elsevier, Amsterdam, 441 PP. Bcaudet, G.,1978, Essai sur la zonation et la signification geomorphologiquedes cuirasses ferrugineuses RED NODULAR EQUATORIAL YELLOW en Afrique Occidentale. 'kav. Doc. CEGE7; Bordeaux, 33: 35-52 u ox I SOL OXISOL PODZOL oxIsoL Bcnncma, J., Jongcrius, A. and Lemos, R., 1970. Micromorphology of some oxic and argillic horizons in south Brazil in relation to weathering scqucnccs. Gcodcrma, 4: 335-355. Bocquicr, G., 1973. Gcnksc ci Cvoluiion de deux IoposCqucnccs dc sols iropicaux du Tchad. Inlcrprdia- F30:I ZON lion biogbodynamiquc. These Sci. Strasbourg, MCm. ORSTOM, 62,350pp. Bocquicr, G., BoulangC, B., Ildelonse, P., Nahon, D. and Mullcr, D., 1983. 'kansfcrs, accumulaiion I: ISTONE modcs, mineralogical transformations and complcxity of historical development in laieriiic profiles. In: A. J. Melfi and A. Carvalho (Editors), 2nd Int. Scminar on Latcritisation Proccsses, Sä0 Pauto, pp. 331-343. Bocquier, G., Mullcr, J.P. and BoulangC, 13., 1984. Lcs IatCriics. Connaissanccs CI pcrspcciivcs actuelles sur Ics mCciinismcs dclcur difrCrcnciaiion, Livre Jubilairc du Cinquanicnairc de I'AFES, Paris, pp, 123-1 38. DoulangC, B., 1984. Lcs formaiions bauxiiiqucs IaiCriiiqucs de CÔic d'lvoirc. 'kav. Doc. ORSTOM, I f Paris, 175, 341 pp. f Doulct, R., 1978. Exislcncc.. de systtmcs A forte diffCrenciation IatCralc cn milieu ferrallitiquc guyanais: un nouvcl cxcmplc de couvcrlurcs ptdologiqucs cn dCsCquilibrc. Sci. Sol, 2: 75-82. I' Dourgeat, E, 1972. Sols sur socle ancicn A Madagascar. Mdm. OIISTOM Paris, 57, 335 pp. 13rcwcr, li., 1964. Fabric and Mineral Analysis of Soils. Wilcy, New York, N,Y., 470 pp. z i

Pig. 15.8. Scqucncc of laicritic soil profiles from Oxisols (At n, C,D) io iron duricrusi (E, F,G) and Ultisols (II). In the oxisol group and abovc lhe liihomargc and finc saprolite, fcrralliiic soil is either palc and madc of quariz (A), ycllow and madc of quartz, kaoliniic, gibbsitc and goethite (B) or red and madc of quartz, kaolinite, gibbsiic and hcmaiitc (C and D). In the lcrricrctc group some of the profilcs NOOULAR FERRICRETE FERRICRCTE FERRUGINOUS arc covcrcd by a sandy horizon which may bc originiiicd lrom tcrmitc aciiviiy (E) and some othcn OXISOL WITH UNDERNEATH ULTISOL prcscni a lcachcd horizon bclow ihc lcrricrctc (G). Dccp saprolites locatcd bclow Ultisols may prcscnt o . LEACHED WITH SMECTlTE a pistachio colored smcciitic horizon (II). ., HORIZON HORIZON W

4, *.

.- ~

c 402 Y Tardy Diversify and terminology of lateritic profiles 403

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