Outline
Comptes Rendus

Surface Geosciences (Pedology)
Variation of the kaolinite and gibbsite content at regional and local scale in Latosols of the Brazilian Central Plateau
Comptes Rendus. Géoscience, Volume 340 (2008) no. 11, pp. 741-748.

Abstracts

The mineralogy of the Latosols of the Brazilian Central Plateau remains under discussion in the absence of a clear relationship with their age according to their geomorphic location. The aim of this study was thus to clarify the origin of the kaolinite and gibbsite content variation by studying a regional toposequence and using data from the literature. Chemical composition and soil color were used to discuss mineralogy. The mineralogy of the clay fraction was also investigated using X-ray diagrams. Our results showed that the large variation of kaolinite and gibbsite content can be explained by taking into account both their local and regional location, the variation of the hematite and goethite content remaining limited. The model that is proposed to explain such variation combines a regional component, which is mainly associated to the age of the geomorphic surface and a local component which is mainly associated to the hydraulic conditions along the toposequence.

La minéralogie des latosols du Plateau Central brésilien est discutée en l’absence de relation clairement établie avec leur âge qui est en fonction de leur position géomorphologique. L’objectif de cette étude est par conséquent de clarifier l’origine de la variation de teneur en kaolinite et gibbsite en étudiant une toposéquence régionale et les données de la littérature. La minéralogie a été discutée à partir de la composition chimique et de la couleur du sol. Elle a aussi été discutée à l’aide des données de la diffraction des rayons X. Les résultats montrent que la variation élevée de la proportion de kaolinite et de gibbsite des latosols peut être expliquée en prenant en compte à la fois leur localisation régionale et locale. Le modèle proposé combine en effet une composante régionale qui est principalement liée à l’âge de la surface géomorphologique et une composante locale qui est principalement liée aux conditions hydriques le long de la toposéquence.

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Published online:
DOI: 10.1016/j.crte.2008.07.006
Keywords: Oxisol, Ferralsol, Mineralogy, Iron oxyhydroxide, Soil color, Biome Cerrado
Mot clés : Oxisol, Ferralsol, Oxy-hydroxyde de fer, Couleur du sol, Biome Cerrado

Adriana Reatto 1, 2; Ary Bruand 2; Eder de Souza Martins 1; Fabrice Muller 2; Euzebio Medrado da Silva 1; Osmar Abílio de Carvalho 3; Michel Brossard 4

1 Empresa Brasileira de Pesquisa Agropecuária (Embrapa Cerrados), BR 020, km 18, 73310-970, Planaltina, Distrito Federal, Brazil
2 CNRS/Insu, institut des sciences de la terre d’Orléans (ISTO), université d’Orléans–université de Tours, 1A, rue de la Férollerie, 45071 Orléans cedex 2, France
3 Universidade de Brasília (UnB), Departamento de Geografia, 70910-000, Brasília, Brazil
4 Unité Valpédo, institut de recherche pour le développement (IRD), BP 64501, 34394 Montpellier cedex 5, France
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     author = {Adriana Reatto and Ary Bruand and Eder de Souza Martins and Fabrice Muller and Euzebio Medrado da Silva and Osmar Ab{\'\i}lio de Carvalho and Michel Brossard},
     title = {Variation of the kaolinite and gibbsite content at regional and local scale in {Latosols} of the {Brazilian} {Central} {Plateau}},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {741--748},
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Adriana Reatto; Ary Bruand; Eder de Souza Martins; Fabrice Muller; Euzebio Medrado da Silva; Osmar Abílio de Carvalho; Michel Brossard. Variation of the kaolinite and gibbsite content at regional and local scale in Latosols of the Brazilian Central Plateau. Comptes Rendus. Géoscience, Volume 340 (2008) no. 11, pp. 741-748. doi : 10.1016/j.crte.2008.07.006. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2008.07.006/

Version originale du texte intégral

1 Introduction

The Latosols of the Brazilian Soil Taxonomy [8], which are Oxisols in the Soil Taxonomy [33] and Ferralsols in the World Reference Base [12], cover approximately 40% of the Brazilian Central Plateau [24]. This region, that corresponds to 24% of Brazilian territory, is composed of two main geomorphic surfaces developed during the Upper Cretaceous and Tertiary:

  • • the South American Surface (SAS), which is the older and mainly made up of tablelands called chapadas, with smoothly convex plane portions with an elevation ranging from 900 to 1200 m;
  • • the Velhas Surface (VS) characterized by moderate and convex slopes at an elevation below 900 m [23].

In the Central Plateau, the Latosols are Red Latosols (∼28%) where hematite is the main iron oxyhydroxide, Yellow Red Latosols (∼10%) where hematite and goethite are present in similar proportions, and Yellow Latosols (∼2%) where goethite is the main iron oxyhydroxide. Besides iron oxyhydroxides, gibbsite and kaolinite were shown to be the main associated minerals in the Latosols of the SAS and VS, respectively [39]. However, several studies showed high proportions of kaolinite in the Latosols of the SAS and high proportions of gibbsite in Latosols of the VS. Indeed, Resende [27] studied a topolithosequence 67 km-long across the SAS and VS and showed high proportion of kaolinite in Red Latosols and Yellow Red Latosols developed in clay sediments on the SAS. Curi and Franzmeier [6] studied a toposequence 200 m-long on the VS with Latosols developed in weathered basalts and found Red Latosols upslope with a high proportion of gibbsite. Macedo and Bryant [14] studied a hydrosequence 3 km-long on the SAS and found Yellow Red Latosols downslope with similar proportion of kaolinite and gibbsite. Several authors [10,11,15,16,20] studied Latosols located on the two geomorphic surfaces and recorded a variable proportion of gibbsite and kaolinite for Latosols developed on the same surface. Thus, the mineralogy of the Latosols of the Brazilian Central Plateau remains under discussion because it appears weakly related to age according to their location on the two main geomorphic surfaces. In this context, the aim of this study is:

  • • to analyze the mineralogy of these Latosols by studying them along a regional toposequence and using data from the literature;
  • • to show that a model consistent with our data and those from the literature can be proposed.

2 Material and methods

Ten Latosols (L) developed in different parent materials were selected for study along a 350 km-long toposequence across the SAS (L1 to L4) and VS (L5 to L10). Location and basic properties of these Latosols can be found in [26] and Table 1. The Latosols L5 and L6 were located on the upper VS, L7 and L8 on the intermediate VS, and L9 and L10 on the lower VS. The Latosols L7 and L8 are those also studied by Volland et al. [37,38] and similar to those studied by Balbino et al. [1–3]. A set of 25 samples was collected in the diagnostic horizons Bw1, Bw2 and when possible Bw3 of the Latosols selected. The SiO2, Al2O3, and Fe2O3 content was determined on the < 2-mm material after dissolution in 1:1 H2SO4 [5,7,15,30,35]. This acid attack enables dissolution of the clays, Fe oxyhydroxides and Al hydroxides [22,28,31].

Table 1

General characteristics of the Latosols studied

Caractéristiques générales des Latosols étudiés

Latosols Geomorphic Surface Altitude (m) Position along the toposequence Slope length (km) Declivity (%)
L1 South American 1050 Median 3 < 1
L2 South American 1200 Median 5 2
L3 South American 1190 Median 5 2
L4 South American 1180 Down 12 3
L5 Velhas, Superior Level 920 Median-up 12 < 1
L6 Velhas, Superior Level 880 Down 20 6
L7 Velhas, Intermediate Level 820 Median-up 20 2
L8 Velhas, Intermediate Level 805 Median-down 7 2
L9 Velhas, Inferior Level 785 Median-up 15 < 1
L10 Velhas, Inferior Level 760 Down 15 7

The SiO2 and Al2O3 extracted with sulfuric acid were used to compute the kaolinite (K) and gibbsite (Gb) content as follows [4,28]:

K=SSiO2/KSiO2(1)
where K is the kaolinite content (%) of the sample, SSiO2 the SiO2 content of the sample recorded with sulphuric acid extraction (%), KSiO2 the specific proportion of SiO2 of the kaolinite and set equal to 0.465.

The goethite (Gt) and hematite (Hm) contents were computed by combining two equations relating Gt and Hm as follows:

SFe2O3=GtFe2O3×Gt+HmFe2O3×Hm(2)
Hm(Hm+Gt)=(RI3.50)8.33(3)
where SFe2O3 is the Fe2O3 content (%) of the sample recorded with sulphuric acid extraction, GtFe2O3 is the specific proportion of Fe2O3 in the goethite and equal to 0.899 for a non Al-substituted goethite and to 0.675 for a 33% Al-substituted goethite [32], HmFe2O3 is the specific proportion of Fe2O3 in the hematite and equal to 1 for a non Al-substituted hematite and to 0.890 for a 16% Al-substituted hematite [32]: RI is the red index [9,21,30] and equal to:
RI=M+CV(4)
with M a parameter related to the hue (M was 10 for 10R, 7.5 for 2.5YR, 5 for 5YR, 2.5 for 7.5YR and 0 for 10YR), C the chroma and V the value of the Munsell notation [9,21,30].

The gibbsite content of the sample was computed as following:

Gb=[(SAl2O3(Gt×GtAl2O3)(Hm×HmAl2O3)(K×KAl2O3)]GbAl2O3(5)
where Gb is the gibbsite content (%) of the sample, SAl2O3 the Al2O3 content of the sample recorded with sulphuric acid extraction (%), KAl2O3 the specific proportion of Al2O3 of the kaolinite and equal to 0.395, GbAl2O3 the specific proportion of Al2O3 of the gibbsite and equal to 0.654. Eqs. (1) and (5) assumed kaolinite and gibbsite to be without any substitution.

The mineralogy of < 2 μm fraction of the Bw2 horizons was determined by using X-ray diffraction on oriented samples by using a Thermo Electron ARL‘XTRA diffractometer [29]. The SiO2, Al2O3, and Fe2O3 contents of < 2 mm material of 162 Bw horizons collected in Latosols of the Central Plateau and earlier published [25] were also used to discuss the mineralogy of Latosols.

3 Results and discussion

3.1 Composition and mineralogy of the Latosols along the regional sequence studied

In the Bw horizons studied, the Fe2O3 content ranged from 15 to 33%, the Al2O3 content from 43 to 68% and the SiO2 content from 11 to 36% (Fig. 1a). For those belonging to Latosols developed on the SAS, the Fe2O3 content ranged from 15 to 33%, the Al2O3 content from 54 to 68% and the SiO2 content from 11 to 24%. On the other hand, for those belonging to Latosols developed on the VS, the Fe2O3 content ranged from 18 to 24%, the Al2O3 content from 43 to 52% and the SiO2 content from 22 to 36%, (Fig. 1a). The range of Fe2O3 content is consistent with that recorded by Melfi et al. [19] for the Latosols of the Central Plateau.

Fig. 1

SiO2, Al2O3 and Fe2O3 relative contents in the Bw horizons of the Latosols of the regional toposequence studied (a) and those of Bw horizons from the literature (b): latosols located on the SAS (+) and VS (▴).

Contenus relatifs en SiO 2 , Al 2 O 3 et Fe 2 O 3 dans les horizons Bw des Latosols de la toposéquence régionale étudiée (a) et ceux des horizons Bw issus de la littérature (b) : Latosols situés sur la Surface Sud-Américaine (+) et sur la Surface Velhas (▴).

The results showed a relatively small variation of the iron oxyhydroxide content between the Latosols studied, whatever the Al-substitution rate since Gt + Hm ranged from 13 to 27% in the absence of Al-substitution and from 15 to 29% when the goethite and hematite were 33% and 16% Al-substituted, respectively (Fig. 2a and b). On the other hand, there was a large variation of the kaolinite and gibbsite content with K ranging from 17 to 67% and Gb from 15 to 65% with non Al-substituted goethite and hematite and K ranging from 18 to 69% and Gb from 13 to 62% when the goethite and hematite were 33% and 16% Al-substituted, respectively (Fig. 2a and b). Thus, the Latosols sampled along the regional toposequence studied were gibbsitic Latosols on the SAS (L1 to L4) and kaolinitic Latosols on the VS (L5 to L10) (Fig. 2a). The mineralogical composition obtained with data from sulfuric acid extraction was consistent with the X-ray diagrams recorded for < 2 μm fraction of the Bw2 horizons studied (Fig. 3). X-ray diagrams showed also a greater kaolinite content in L3 than in L10, and a close gibbsite content between these two, thus indicating again no sharp variation of mineralogy between the Latosols developed on the SAS and VS (Fig. 3).

Fig. 2

Kaolinite, gibbsite, and (goethite + hematite) relative contents in the Bw horizons of the Latosols of the regional toposequence studied (a) with non Al-substituted goethite and hematite and (b) with 33% Al-substituted-goethite and 16% Al-substituted hematite) and in Bw horizons from the literature (c) with non Al-substituted goethite and hematite, and (d) with 33% Al-substituted-goethite and 16% Al-substituted hematite): Latosols located on the SAS (+) and VS (▴).

Contenus relatifs en kaolinite, gibbsite, et (goethite + hématite) dans les horizons Bw des Latosols de la toposéquence régionale étudiée (a) calculés avec une goethite et une hématite sans substitution par Al et (b) calculés avec une goethite substituée par Al à 33 % et une hématite substituée par Al à 16 %) et dans les horizons Bw issus de la littérature (c), calculés avec une goethite et une hématite sans substitution par Al et (d), calculés avec une goethite substituée par Al à 33% et une hématite substituée par Al à 16%) : latosols situés sur la surface Sud-Américaine (+) et ceux situés sur la surface Velhas (▴).

Fig. 3

X-ray diagrams of the oriented < 2 μm fraction (powder) of horizons Bw of the Latosols studied.

Diagrammes de rayons X de la fraction < 2 μm (poudre) des horizons Bw des Latosols étudiés.

3.2 Mineralogy of Latosols located in the Brazilian Central Plateau

Results from sulphuric extractions published earlier [25] were used to describe the mineralogy of < 2 mm material of Latosols as performed above for the Latosols of the regional toposequence studied. The Fe2O3 contents ranged from 9 to 34%, the Al2O3 content from 36 to 78% and the SiO2 content from 9 to 42% (Fig. 1b). For the Bw horizons of Latosols developed on the SAS, the Fe2O3 content ranged from 9 to 34%, the Al2O3 content from 39 to 78% and the SiO2 content from 9 to 39%. On the other hand, for the Bw horizons of Latosols developed on the VS, the Fe2O3 content ranged from 18 to 33%, the Al2O3 content from 36 to 60% and the SiO2 content from 13 to 42% (Fig. 1b).

The Fe2O3, Al2O3 and SiO2 content was used to compute K, Gb and Gt + Hm as done for the Bw horizons of the regional toposequence studied. In the absence of Al-substitution in goethite and hematite, results showed that K and Gb ranged from 11 to 78% and from 1 to 77%, respectively (Fig. 2c). On the other hand, with 33% Al-substituted goethite and 16% Al-substituted hematite, results showed that K and Gb ranged from 12 to 79% and from 0 to 75%, respectively (Fig. 2c). Results showed also a large overlapping of the mineralogical composition range between Latosols developed on the SAS and those developed on the VS (Fig. 2c). Indeed, for the Bw horizons of Latosols developed on the SAS, K ranged from 11 to 75% and Gb from 3 to 77% with non Al-substituted goethite and hematite, and K ranged from 12 to 78% and Gb ranged from 0 to 75% with 33% Al-substituted goethite and 16% Al-substituted hematite. On the other hand, for the Bw horizons of Latosols developed on the VS, K ranged from 21 to 78% and Gb from 1 to 57% with non Al-substituted goethite and hematite, and K ranged from 22 to 79% and Gb ranged from 0 to 55% with 33% Al-substituted goethite and 16% Al-substituted hematite. Results showed also that Gt + Hm from 9 to 31% in the absence of Al-substitution and from 9 to 35% when the goethite and hematite were 33% and 16% Al-substituted respectively (Fig. 2c and d) without any relationship with the location of Latosols on the two main geomorphic surfaces.

3.3 Variation of the kaolinite and gibbsite content at the regional and local scale

Macedo and Bryant [14] and Motta et al. [20] showed that the Latosols distribution on the SAS was closely related to the soil hydraulic regime thus explaining the Red Latosol, Yellow Red Latosols and Yellow Latosol sequence according to local variation of the topography. As a consequence, the Latosols distribution appeared roughly independent of the underlying geological material [20]. Motta et al. [20] suggested that more attention should be devoted to geomorphology to explain the variation of the Latosols characteristics and particularly their mineralogy. Melfi and Pédro [17,18] showed that Latosols mineralogy should be related to their geochemical functioning that is characterized by an hydrolytic environment according to landscape history at both regional and geological scale. Tardy [34] discussed the kaolinite/gibbsite ratio in tropical soils and showed that the kaolinite–gibbsite equilibrium would be preferentially controlled by variation of the hydraulic conditions along of the toposequences. Finally, Lucas et al. [13] showed that the spatial distribution in equatorial areas of the secondary minerals such as kaolinite, gibbsite and goethite can be related to their stability in aqueous solutions and then to the amount of the water percolating the soils. Thus, as discussed by Lucas et al. [13], the higher the volume of water percolating the profile is, the lower the soil-solution concentrations are.

On the basis of these results, we plotted the altitude at which every Latosol was located on the SAS and VS according to the Gb/(Gb + K) ratio. Fig. 4 shows that Gb/(Gb + K) varies according to the local topographic location of every Latosol (Axe 1) and to the regional topographic location of every Latosol (Axe 2). Locally, Latosols located on the slope showed higher Gb/(Gb + K) ratio than those located on the plateau of the same portion of landscape (Fig. 4). At the regional scale, our results showed the Gb/(Gb + K) ratio increased with the altitude thus explaining the trend to an increase in the Gb/(Gb + K) ratio value with the altitude, the age of the surface increasing itself with the altitude. Thus, the Axe 2 shows a regional variability that is mainly related to time. The older the topographic surface, the older the Latosols, and the higher is the weathering and consequently the hydrolysis process intensity, resulting in a higher gibbsite content in the Bw studied, as discussed by Vitte [36] and Melfi and Pédro [17,18]. On the other hand, the Axe 1 shows a local variability that would be mainly related to the volume of water percolating the soil. Indeed, because of local topographic characteristics, water can percolate more or less easily, maintaining the Fe, Si and Al concentrations that result from mineral hydrolysis at values that are more or less favorable to hydrolysis process continuation. Thus according to the local topographic location, the higher the volume of water percolating the Latosol is, the higher hydrolysis process is, and the higher resulting gibbsite content is.

Fig. 4

Altitude of every Latosol (L) according to the gibbsite/(gibbsite + kaolinite) ratio computed with non Al-substituted goethite and hematite (♦) and both 33% Al-substituted goethite and 16% Al-substituted hematite (▴). Every Latosol was also located on its portion of landscape according to the local topography (Axe 1: local variation associated to the hydraulic condition along the toposequence and Axe 2: regional variability according to the age of the surface). SAS: South American Surface, VS: Velhas Surface (VS – I: upper level, VS – II: intermediate level, VS – III: lower level).

Altitude de chaque Latosol (L) en fonction du rapport gibbsite/(gibbsite + kaolinite) calculé avec une goethite et une hématite non substituée par Al (♦) et avec à la fois une goethite substituée par Al à 33% et une hématite substituée par Al à 16% (▴). Chaque Latosol est localisé sur sa portion de paysage (Axe 1 : variabilité locale liée aux conditions hydriques le long de la toposéquence et Axe 2 : variabilité régionale en fonction de l’âge de la surface. SAS : surface Sud Américaine, VS : surface Velhas (VS – I : niveau Supérieur, VS - II : niveau Intermédiaire, VS – III : niveau Inférieur).

4 Conclusion

Our results showed that the kaolinite and gibbsite content in the Latosols developed on the SAS and VS of the Brazilian Central Plateau can be explained by taken into account both their local and regional location. The model proposed combines:

  • • a regional variation which would be mainly associated to the age of the surface, the more the surface being old, the more SiO2 removal from the soil being developed and thus the gibbsite content being high compared to the kaolinite content;
  • • a local variation which would be mainly associated to the hydraulic conditions along the toposequence at the scale of several hectometers or a few kilometers, the gibbsite content being the highest where SiO2 removal is the easiest at upslope and on the plateau border.

Acknowledgements

We thank the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) for its financial support of A. Reatto’s work in France. This research is part of the project Embrapa Cerrados-IRD, No.0203205 (Mapping of the Biome Cerrado Landscape and Functioning of Representative Soils).


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