Belterra Clay Research Articles

Synthesis of geopolymer with KOH by two kaolinitic clays from the Amazon: Influence of different synthesis parameters on the compressive strength

The term geopolymer refers to “inorganic polymers” formed by a chemical reaction between solid aluminosilicate compounds and a highly concentrated alkaline hydroxide solution. The properties of geopolymers mostly result from the chemical and mineralogical composition of the material used as a source of aluminosilicates. Considering the influence of the raw material for the synthesis of geopolymers, the present study evaluates the use of two different types of kaolin in the synthesis of geopolymers using KOH as an alkaline activator. For the present study, a sample of Belterra clay (KLN-1) from the bauxite deposits of Rondon do Pará, a sample of processed kaolin (KLN-2), and microsilica (MCR-1) (Ecopodium) and PA reagent from Sigma (KOH) were selected. The samples were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. For the synthesis of the geopolymers, the KLN-1 and KLN-2 samples were calcined at 750 °C for 2 h. The syntheses of the geopolymers were performed at different KOH molar concentrations (12, 13, 14, 15 and 16), at different curing temperatures (60, 70, 80, 90 and 100 °C) and for different curing times (24, 48, 72, 96, and 120 h). The geopolymers were characterized by mechanical strength testing and FTIR. The samples had different mineralogical compositions, which contributed to different compressive strength results. The samples showed greater resistance at concentrations of 12 and 16 M. The curing temperature of 80 °C exhibited better mechanical compressive strength. At 120 h, the best compressive strength value was obtained, indicating that a longer time favored the geopolymerization reaction. The greater mineral diversity affects negatively the geopolymer reaction contributing to lower mechanical resistance.

Clinkering design of sulfobelite cements using clay overburden residue from bauxite mining

Sulfobelite (belite-rich calcium sulfoaluminate (BCSA)) cements are promising alternatives to ordinary Portland cement (OPC) due to their lower carbon dioxide generation and lower energy consumption in the clinkering process. Additionally, they are cost effective because of their less-restrictive raw meal composition compared with that of calcium sulfoaluminate (CSA), in which expensive bauxites are used as the alumina source. Diverse mining residues have been proved to be reliable alumina sources for BCSA production, including the clay residues that cover bauxites (Belterra Clay (BTC)) in the Brazilian Amazon region. The clinkering settings of BCSA using a different (less alumina) BTC were examined in this work. The mineralogy of the BCSA clinkers was optimised by a design of experiments and studied using X-ray powder diffraction, scanning electron microscopy and thermal analysis. The raw meal composition, boron doping and firing temperature were investigated to obtain the optimal clinker composition. Hydration of the cement pastes was investigated by calorimetry and the strength development of mortars was tested and compared with that of a commercial OPC. The BCSA mortars produced with gypsum additions developed compressive strength comparable to that that OPC within 28 days. The main hydrated phase was ettringite, with minor amounts of kuzelite, straetlingite and gibbsite.

Use of the clayey cover of bauxite deposits of the Amazon region for geopolymer synthesis and its application in red ceramics

The exploitation of bauxite deposits in the Amazon region is well known. Mining begins with the temporary or even permanent removal of an extensive surface layer of clayey material (Belterra clay) that can reach a thickness of up to 20 m. The mineralogical composition of this material has already been described in the literature and consists mainly of kaolinite. The extensive surface distribution of Belterra clay in the Amazon region and its composition is of interest in the evaluation of its viability for the synthesis of geopolymers. For the present study, a sample of Belterra clay (BTC-1) from the bauxite deposits of Rondon do Pará was selected, in addition to microsilica (MCR-1) (Ecopowder) and Sigma P.A. reagent (NaOH). The Belterra clay sample was characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), and thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses. To determine the physical and mechanical properties of Belterra clay, geopolymers were synthesized using BTC-1, MCR-1, and NaOH with different Si/Al and Na/Al ratios according to the Box-Behnken design (BBD). Technological tests, including water absorption, apparent porosity, and compressive strength tests, were performed. Based on the BBD, the highest compressive strength was 47.78 MPa, and the lowest compressive strength was 7.05 MPa. BTC-1 showed potential for the synthesis of geopolymers with high mechanical strength and an acceptable percentage of water absorption.

Mineralogical Appraisal of Bauxite Overburdens from Brazil

Mineralogical appraisal is an important tool for both mining and industrial processes. X-ray powder diffraction analysis (XRPD) can deliver fast and reliable mineralogical quantification results to aid industrial processes and improve ore recoveries. Furthermore, X-ray fluorescence (XRF) chemical data, thermal analysis (TA), and Fourier-transformed infrared spectroscopy (FTIR) can be used to validate and refine XRPD results. Mineralogical assessment of non-traditional ores, such as mining wastes, is also an important step to consider them for near-future industries. In the Brazilian Amazon, alumina-rich clays cover the largest and most important bauxitic deposits of the region and have been considered as a possible raw material for the local cement and ceramic industry. In this work, a mineralogical evaluation of these clays (Belterra Clays) is performed using XRPD, XRF, TA, and FTIR. XRPD-Rietveld quantification confirmed that kaolinite is the main phase of the clay overburden, followed by variable contents of gibbsite and goethite and minor quantities of hematite, anatase, and quartz. The chemistry derived from Rietveld, based on stoichiometric phase compositions, presents a good correlation with the XRF data and is also supported by the TA and FTIR data. The initially assumed homogeneous composition of Belterra Clay is revealed to be variable by the present mineralogical study.

Production of low-CO2 cements using abundant bauxite overburden “Belterra Clay”

Eco-friendly binders are among the opportunities to reduce the approximately 8% global CO2 emissions generated by the cement industry, with calcium-sulphoaluminate (CSA) -based cements standing as a potential low-CO2 alternative to the world most used ordinary Portland cements (OPC). Belterra Clay (BTC), an abundant low-cost alumina-rich clay overburden on the huge bauxite reserves of the Brazilian Amazon was tested to produce CSA-based binders. Successive Designs of Experiments were used to prepare mixtures containing BTC, CaCO3, and gypsum, seeking the maximum BTC and minimal CaCO3 consumption under the lowest possible temperature to maximize CSA's main phase ye'elimite, Ca4Al6(SO4)O12, in the clinkers. X-ray powder diffraction accompanied by Rietveld phase quantifications, X-ray fluorescence, and scanning electron microscopy were used to characterize the obtained products and guide the experimental work. CSA-belite and CSA-ternesite clinkers were successfully sintered at 1250 °C using up to 42% of BTC. The hydration of selected clinkers was investigated by isoperibolic heat flow calorimetry after blending with gypsum at 95:5 and 90:10 mass ratios. The main hydrations of the clinkers were faster (within 24 h) when using 5% of gypsum. Ettringite was the main hydrated phase, followed by kuzelite, straetlingite, and hemicarboaluminate. Produced mortars reached up to 40 Mpa after 28 days of curing, a strength development comparable to that of an OPC (46 Mpa) tested under the same conditions. BTC, currently a drawback during bauxite mining and rehabilitation of mined areas, was successfully used to produce clinkers that generate ca. 30% less CO2 during calcination when compared to OPC, with further CO2 and energy savings expected because of the required lower sintering temperature.

The Belterra Clay on the bauxite deposits of Rondon do Pará, Eastern Amazon

ABSTRACT: Bauxite deposits in the Amazon region are commonly covered by yellowish clays which can reach up to 25m thick, known as Belterra Clay (BTC). In Rondon do Para, Eastern Amazon, BTC is 13m thick and covers world-class bauxite reserves. Three pilot bauxite mines were investigated in Rondon do Para for an initial characterization of the local BTC. In discordant contact with the lateritic profile, the BTC has reddish brown colors at its base to ocher tones towards the top. It has a massive structure with silt-clayey texture and nodular bauxitic fragments at its base. Rietveld mineral quantification of the material attests that it is dominated by kaolinite, with goethite, gibbsite, hematite, anatase and residual quartz. The thermal behavior of the material also confirms its mineralogical composition. Kaolinite is of low structural order, which was considered the main difficulty in the application of the Rietveld method. Goethite has up to 33 mol% of Al. As observed by scanning electron microscopy (SEM), the minerals represent pseudo-hexagonal crystals measuring from 150 to 700 nm. The BTC in the studied area is correlated to BTC on others bauxitic deposits of the Amazon region, suggesting this material experienced the same genesis and geological evolution, probably during the Pliocene.

Viability of Belterra clay, a widespread bauxite cover in the Amazon, as a low-cost raw material for the production of red ceramics

Abstract The Amazon region has the largest bauxite reserves in Brazil, where their deposits are capped by a thick layer of clay material known as Belterra clay (BTC). Its extensive distribution and surface occurrence (and therefore accessibility) and the clayey nature of BTC were of interest for this study, which evaluated its technical viability for the production of red ceramics. BTC from the large bauxite deposits of Rondon do Para in the southeast region of Para state was selected for the study. BTC, yellow soil and clayey siltstone, these two last used as additives, were characterised by X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM) and laser particle analysis (LPA). To determine the physical and mechanical properties, 12 different mixtures of specimens were produced with the samples of Belterra clay and 40% yellow soil or clayey siltstone. All mixtures were calcined at three different temperatures (900, 1000 and 1200 °C). Next, the following were measured: linear shrinkage, water absorption, apparent porosity, apparent density and flexural strength. The BTC essentially consists of kaolinite, which contains quartz, goethite, anatase and gibbsite as accessory minerals. The pure and simple BTC samples do not present favourable technological aspects for the manufacture of ceramic products; however, with the addition of yellow soil or clayey siltstone from the same region, the technological characteristics of BTC are significantly improved.

On the geology, mineralogy and geochemistry of the bauxite-bearing regolith in the lower Amazon basin: Evidence of genetic relationships

The wide distribution of Cenozoic world-class bauxite deposits in the Amazon shows that this region was the scene of intense lateritization. The Juruti bauxites, currently being mined in the lower Amazon, are one of these deposits, where the bauxite-bearing profile is covered by thick yellow clay (Belterra Clay). The laterite profile sequence consists of a mottled horizon at the base, a massive to cavernous bauxite, an aluminous iron crust with ferruginous nodules and bauxitic nodules in contact with yellow clay on the top. Cryptocrystalline to microcrystalline gibbsite is the main mineral of bauxite; hematite, goethite, kaolinite and anatase are accessories. The same minerals constitute the upper horizons, but in distinct concentrations. The relict structures, minerals (zircon) and chemical composition suggest an affiliation to sedimentary rocks of the Paleo-Mesozoic Amazon basin (Alter do Chão formation). The mineralogy, geochemical associations and Ti mass balance show a continuous evolution from the mottled horizon to the aluminous iron crust and even to the ferruginous nodules. Kaolinite, hematite, goethite and anatase are the early minerals and gibbsite is the late mineral. Hematite, zircon and anatase are the potential carriers of trace elements. The dissolution of hematite and goethite and the reprecipitation of iron as Al-free goethite reached the bauxite horizon and strongly fixed the mobile elements (Ba, Ag, As, Bi, Hg, Mo, Se, V, Pb, Sb, Se and Zn). The increasing upward content of zircon, anatase and residual trace elements (HFSE, Sc, Ga, Sn, U, W and Y and REE) throughout the regolith, an inheritance of the bauxite-bearing laterites, enhances the affinities between the two units. The laterite was developed during the Eocene, and its cover may be a product of deep plant root activity (tropical soil forming) on the top of the profile, which formed kaolinite and Al-goethite that was transported by wash flooding during the Miocene. The bauxitic nodules may represent an incomplete second bauxitic event, as erosion had already established the actual plateau landscape during the Pliocene.

Geochemical and isotopic characterization of the Bodélé Depression dust source and implications for transatlantic dust transport to the Amazon Basin

The Bodélé Depression (Chad) in the central Sahara/Sahel region of Northern Africa is the most important source of mineral dust to the atmosphere globally. The Bodélé Depression is purportedly the largest source of Saharan dust reaching the Amazon Basin by transatlantic transport. Here, we have undertaken a comprehensive study of surface sediments from the Bodélé Depression and dust deposits (Chad, Niger) in order to characterize geochemically and isotopically (Sr, Nd and Pb isotopes) this dust source, and evaluate its importance in present and past African dust records. We similarly analyzed sedimentary deposits from the Amazonian lowlands in order to assess postulated accumulation of African mineral dust in the Amazon Basin, as well as its possible impact in fertilizing the Amazon rainforest.Our results identify distinct sources of different ages and provenance in the Bodélé Depression versus the Amazon Basin, effectively ruling out an origin for the Amazonian deposits, such as the Belterra Clay Layer, by long-term deposition of Bodélé Depression material. Similarly, no evidence for contributions from other potential source areas is provided by existing isotope data (Sr, Nd) on Saharan dusts. Instead, the composition of these Amazonian deposits is entirely consistent with derivation from in-situ weathering and erosion of the Precambrian Amazonian craton, with little, if any, Andean contribution. In the Amazon Basin, the mass accumulation rate of eolian dust is only around one-third of the vertical erosion rate in shield areas, suggesting that Saharan dust is “consumed” by tropical weathering, contributing nutrients and stimulating plant growth, but never accumulates as such in the Amazon Basin.The chemical and isotope compositions found in the Bodélé Depression are varied at the local scale, and have contrasting signatures in the “silica-rich” dry lake-bed sediments and in the “calcium-rich” mixed diatomites and surrounding sand material. This unexpected finding implies that the Bodélé Depression material is not “pre-mixed” at the source to provide a homogeneous source of dust. Rather, different isotope signatures can be emitted depending on subtle vagaries of dust-producing events. Our characterization of the Bodélé Depression components indicate that the Bodélé “calcium-rich” component, identified here, is most likely released via eolian processes of sand grain saltation and abrasion and may be significant in the overall global budget of dusts carried out by the Harmattan low-level jet during the winter.

EVOLUÇÃO GEOLÓGICA DA PORÇÃO MERIDIONAL DA PROVÍNCIA BAUXITÍFERA DE PARAGOMINAS DURANTE O NEÓGENO/PLEISTOCENO (NOROESTE DA BACIA DO GRAJAÚ, NORDESTE DO PARÁ E EXTREMO OESTE DO MARANHÃO)

The surface geology of the southern Paragominas Bauxite Province, which lies in the northwestern Grajaú basin, essentially consists of two sedimentary-lateritic sequences. The oldest one is composed of siliciclastic deposits of the Cretaceous Itapecuru Group overlain by a lateritic/bauxitic mantle, which sustains the Paleogene Sul-Americana surface, and a clayey overburden – the Belterra Clay, whose top is marked by a fairly plane regional surface. The youngest sequence unconformably overlies the Itapecuru deposits in the low areas, which separate the plateaus of the Sul-Americana surface. The lower unit of this sequence consists of a reddish, poorly sorted and friable clayey sandstone up to 30 m thick, which contains dispersed laterite fragments and quartz pebbles, besides lateritic stone-lines. Although it is normally massive, the sandstone occasionally shows an incipient stratification. The reddish sandstone is overlain, in sharp contact, by a ferruginous duricrust up to 3 m thick, that sustains the Late Velhas surface of Neomiocene age. According to its degradation degree, the duricrust displays a columnar, blocky or nodular structure or a pisolitc facies. When completely degraded, the duricrust is transformed into a ferruginous stone-layer. Conglomeratic accumulations, composed of laterite blocks, pelitic fragments and strongly subordinate quartz pebbles locally occur, corresponding to colluvial deposits resulting from the dismantling of the oldest residual cover. The ferruginous duricrust is overlain in sharp contact by a homogeneous, yellowish sandy clay, up to 5 m thick, that contains minute laterite fragments. The surface which marks the top of this overburden is situated 60 to 80 m below the surface sustained by the Belterra Clay. The reddish sandstone represents a continental deposit originated by successive debris-flows and is correlated with the Pirabas and Barreiras Formations of Neo-Oligocene to Mesomiocene age. The ferruginous horizon, interpreted as a water-table laterite, was probably formed during the Neomiocene, while the clayey sandy overburden, comparable with the continental Pós-Barreiras sediments described to the north, in the Bragantina Platform, should be of Pliocene age.

EVOLUÇÃO GEOLÓGICA DA PORÇÃO MERIDIONAL DA PROVÍNCIA BAUXITÍFERA DE PARAGOMINAS DURANTE O NEÓGENO/PLEISTOCENO (NOROESTE DA BACIA DO GRAJAÚ, NORDESTE DO PARÁ E EXTREMO OESTE DO MARANHÃO)

The surface geology of the southern Paragominas Bauxite Province, which lies in the northwestern Grajau basin, essentially consists of two sedimentary-lateritic sequences. The oldest one is composed of siliciclastic deposits of the Cretaceous Itapecuru Group overlain by a lateritic/bauxitic mantle, which sustains the Paleogene Sul-Americana surface, and a clayey overburden – the Belterra Clay, whose top is marked by a fairly plane regional surface. The youngest sequence unconformably overlies the Itapecuru deposits in the low areas, which separate the plateaus of the Sul-Americana surface. The lower unit of this sequence consists of a reddish, poorly sorted and friable clayey sandstone up to 30 m thick, which contains dispersed laterite fragments and quartz pebbles, besides lateritic stone-lines. Although it is normally massive, the sandstone occasionally shows an incipient stratification. The reddish sandstone is overlain, in sharp contact, by a ferruginous duricrust up to 3 m thick, that sustains the Late Velhas surface of Neomiocene age. According to its degradation degree, the duricrust displays a columnar, blocky or nodular structure or a pisolitc facies. When completely degraded, the duricrust is transformed into a ferruginous stone-layer. Conglomeratic accumulations, composed of laterite blocks, pelitic fragments and strongly subordinate quartz pebbles locally occur, corresponding to colluvial deposits resulting from the dismantling of the oldest residual cover. The ferruginous duricrust is overlain in sharp contact by a homogeneous, yellowish sandy clay, up to 5 m thick, that contains minute laterite fragments. The surface which marks the top of this overburden is situated 60 to 80 m below the surface sustained by the Belterra Clay. The reddish sandstone represents a continental deposit originated by successive debris-flows and is correlated with the Pirabas and Barreiras Formations of Neo- Oligocene to Mesomiocene age. The ferruginous horizon, interpreted as a water-table laterite, was probably formed during the Neomiocene, while the clayey sandy overburden, comparable with the continental Pos-Barreiras sediments described to the north, in the Bragantina Platform, should be of Pliocene age.

Paleosurfaces from northeastern Amazonia as a key for reconstructing paleolandscapes and understanding weathering products

The characterization and mapping of paleoweathering surfaces, as well as their associated deposits, throughout the northeastern Amazonia can be a useful tool for inter-basin stratigraphic correlations and for establishing the evolution of landscapes and climates through time. This type of investigation has led to the recognition of five weathering surfaces developed from the Upper Cretaceous through the Quaternary, designated here as S1 to S5. Surface S1 occurs at the top of Late Cretaceous deposits, represented by the Ipixuna/Cujupe succession, as well as on older Precambrian and Paleozoic rocks. Surface S2 marks the top of an indurated, massive, semi-flint 1 1 The term semi-flint is applied here for a flint-like fire clay consisting of indurated kaolinite, which develops no plasticity when grounds up. unit. For cases in which surfaces S1 and S2 are amalgamated, a thick, lateritic paleosol displaying hexagonal, branched concretions is present. Surface S3 occurs at the top of the Pirabas and lower portion of the Barreiras Formations of Late Oligocene/Early Miocene age, and it is also associated with paleosol, but has no lateritization. When surface S3 is amalgamated with surfaces S1 and S2, bauxite is present in association with the underlying laterites. Surface S4 bounds the top of the middle to upper Barreiras Formation of Middle Miocene age, as well as correlatable deposits known as the Belterra Clay. This surface is marked by a lateritic paleosol having columnar concretions. Surface S5 occurs between a lower unit consisting of massive, light red sands of the Plio-Pleistocene Post-Barreiras 1 sediments and an upper unit of yellow, eolian sands of the Holocene Post-Barreiras 2 Sediments. The paleoweathering surfaces present in the northeastern Amazonia enable the reconstruction of several depositional episodes represented by thin (only few meters thick) sedimentary units from the Late Cretaceous to the Quaternary. The creation of new accommodation space on subsiding areas gave rise to sediment accumulation during transgressions. Intervening weathering surfaces formed during significant falls in relative sea-level, associated with tectonically stable episodes. The overprinting of weathering surfaces formed during different times, and thus under different climatic conditions, seems to have been crucial for the development of the various weathering products observed throughout the region, which include economically important laterites, bauxites, semi-flint beds and kaolin deposits. This paper shows that the genesis of these deposits was controlled by the complex interplay of tectonics, climate, sediment deposition and erosion, resulting in a complex evolution of paleolandscapes.

Genesis, Mineralogy and Geochemistry of Kaolin Deposits of the Jari River, Amapá State, Brazil

AbstractKaolin samples from the Jari deposit (Amazon region) were studied using various techniques to characterize its structural and crystallochemical aspects, and to establish its origin and evolution. A profile 60 m thick was selected in a kaolin mine (Morro do Felipe) located at the banks of the Jari river. Despite the great thickness of the deposit and the variety of kaolin types, the mineralogical composition is rather homogeneous and is mainly kaolinite associated with gibbsite and small amounts of quartz, anatase, goethite and hematite. The field observations and the morphological analysis indicate the existence of sedimentary features throughout the whole profile except for the upper aluminous clayey layer (Belterra Clay). This is evidence that the Rio Jari kaolin deposit originated from sedimentary material, the Alter do Chão Formation. The presence of alternating clay and sandy layers is explained by sedimentation processes with great depositional energy variation. Thus, the accumulation of thick clay layers was related to a low-energy phase, and during the high-energy phases, the deposition process led to the accumulation of sandy materials, constituted essentially of quartz and showing strong textural and structural variation. Later on, periods of hydromorphy were responsible for iron removal and consequently for the bleaching of the sedimentary formation. The crystallinity data show an increase of the structural disorder toward the surface associated with an increase in the amount of structural Fe in the kaolinite. The Rio Jari kaolin deposits should be considered as having originated from kaolinitic clay sediments of the Alter do Chão formation (protore) that was submitted to intensive lateritic weathering processes.

COBERTURA BAUXÍTICA E ORIGEM DO CAULIM DO MORRO DO FELIPE, BAIXO RIO JARI, ESTADO DO AMAPÁ

The Morro do Felipe kaolin deposit, occurring on the left margin of the Jari river (Northern Brazil), constitutes the lower part of an ancient weathering profile which is bounded to a planation surface about 170 m high. The kaolin layer, up to 35 m thick, consists mainly of poorly ordered kaolinite with deferrification features and thin sand intercalations. In the lower part of the deposit, the kaolin is only slightly porous and disintegrates easily in water whereas in the upper part, it is more porous, firm, displays conchoidal fracture and resists disintegration in water. In both cases, the kaolin (mineral) is very similar to fire-clay type kaolinite. The kaolin deposit grades down into altered sandstones and mudstones, with conglomerate intercalations, of the Alter do Chao Formation and is overlain by an iron crust, lower nodular bauxite, upper nodular bauxite and a clayey overburden (Belterra Clay). The evolution of this alteration profile likely involved the following steps: 1) formation of an iron crust by podzolization or ground-water lateritization of Alter do Chao deposits; 2) regional bauxitization; 3) degradation of the bauxite horizon and formation of colluvial deposits; 4) deposition of the clayey overburden upon the colluvial deposits or directly on Alter do Chao sediments; 5) lowering of the base level and reactivation of weathering giving rise to the transformation of the overburden into a latosol (Belterra Clay) and the thickening and deferrification of the saprolite horizon which resulted in the present day kaolin deposit. As shows the evolution of the alteration profile, the Morro do Felipe kaolin represents a product of deep weathering which affected the Amazon region since the Lower Tertiary. The kaolinization, related firstly to bauxitization (Lower Tertiary) and triggered, at a later stage, by reactivation of tropical weathering (Upper Tertiary/Quaternary) and epirogenetic movements, resulted from the decomposition of clay minerals and possibly some feldspar, partial dissolution of quartz, neoformation of poorly ordered kaolinite and intense deferrification. It seems very likely that the process of kaolinization continues even today.

Composition and origin of the clay cover on North Brazilian laterites

The Belterra Clay in Northern Brazil is a uniform, yellowish, unstratified kaolinitic clay with variable contents of gibbsite. It is generally 5–10 m thick and covers a lateritic weathering crust on remnants of a dissected table-land. Complete profiles with Belterra Clay, the underlying bauxite/laterite and saprolite/bed rock were sampled in three areas of the Amazon region. The chemical and mineralogical data show a distinct affinity between the Belterra Clay and the underlying layers above all with the saprolite. Variations in the composition of the underlaying layers are reflected by the Belterra Clay. Among the structural and textural features important for the discussion of the origin of the Belterra Clay are the sharp contact between the clay cover and the underlying laterite as well as conglomeritic/brecciate textures and occasional bedding structure in the upper part of the lateritic crust.

Andean icefields to Amazonian jungles: a radical new view of the late pleistocene of South America

Late Pleistocene glaciation in the Altlplano of the Central Andes was far more extensive than previously recognized, with all of the northern Altiplano being covered by a glacial icecap. Large paleolakes existed in the Altiplano, but their relationship with glacial episodes is unclear. One Altiplano paleolake filled the basin to the 4,100±m contour drained catastrophically through the Achocalla Valley of Bolivia approximately 45,000-40,000 years B.P. The catastrophic floodwaters flowed north and south upon reaching the lowlands of eastern Bolivia, scouring much of lowland Amazônia and depositing the Belterra Clay. Asymmetric ripplemarks on the Amazon Cone record the passing of the floodwaters into the Atlantic Ocean. The Belterra Clay dammed the eastern outlet of the Amazon drainage to an elevation of 260±m, resulting in the formation of a great freshwater lake, Lake Amazonas Lake Amazonas accumulated vast quantities of fine-grained sediments through deposition in basin-wide "birds's foot" deltas before it began draining eastward about 13,500 years B.P. Except for minor drainage system refinements, the Amazon Basin had assumed its current physical parameters by 6,000 years B.P. Paleolakes may have covered the Altiplano up to the 3,950±m contour in the early Holocene, desiccating to form the modern lakes and salares only in the late Holocene. The geological, biological and climatological ramifications of the Altiplano glaciation and paleolakes, the catastrophic flood and the formation, filling and draining of Lake Amazonas are enormous.

ASPECTOS GEOLÓGICOS DOS LATERITOS DA AMAZÔNIA

Lateritic rocks are widely found all over the Amazon region. They include mature and immature autochthonous laterites and allochthonous laterites, the latter usually reffered to as stone lines. The mature laterites were formed in the Tertiary, while the immature in the Quaternary. The mature laterites are characterized by well-defined and varied horizon when compared to immature laterite profiles. The parent rocks are of different ages and their petrochemical compositions have given rise to different kinds of laterites. among which the most prominent are the bauxitic, the phosphatic, the ferruginous, the caolinic, the manganesiferous and the nickelliferous. The outtermost cover is called Belterra Clay, a yellow clay material, originated from the lateritic profile weathering. Non-lateritic ironstones are common in the region, sometimes associated with the lateritic profiles. Besides their significant ore potentiality for iron, aluminium, nickel, gold, manganese, phosphates, kaolin, titanium, niobium and REE, laterites are important tools for geochemical exploration, geological mapping and also for geological and geomorphological studies of the evolution of the Amazon region.

Argila de belterra-cobertura terciaria das bauxitas amazonicas

Belterra Clay is a uniform, yellowish kaolinitic clay lacking visihle stratification. It overlies laterite plateaus with a maximum thickness of 20 m. The clay is derived from a lateritic terrane occuring outside and within the Amazonian bauxite belt. Redistribution of clay material from the basal layers of the laterite zone by mud flow or sheet flood in a semiarid climate is suggested for the origin of Belterra Clay. Considering the history of Velhas surface a pliocene if not a mio-pliocene age for this clay cover cannot be ruled out.

ARGILA DE BELTERRA - COBERTURA TERCIÁRIA DAS BAUXITAS AMAZÔNICAS

Belterra Clay is a uniform, yellowish kaolinitic clay lacking visihle stratification. It overlies laterite plateaus with a maximum thickness of 20 m. The clay is derived from a lateritic terrane occuring outside and within the Amazonian bauxite belt. Redistribution of clay material from the basal layers of the laterite zone by mud flow or sheet flood in a semiarid climate is suggested for the origin of Belterra Clay. Considering the history of Velhas surface a pliocene if not a mio-pliocene age for this clay cover cannot be ruled out.