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Division - Soil in Space and Time | Commission - Soil Genesis and Morphology • Rev. Bras. Ciênc. Solo 50 • 2026 • https://doi.org/10.36783/18069657rbcs20250167 linkcopy

Open-access Morphology and mineralogy of soils with petroplinthites in the Savannah biome central western Brazil

SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT

The Range of Maracaju (RM) in the central west of Brazil, formed by basic rocks of the São Bento Group (Upper Triassic), presents concretionary soils (Plinthosols – WRB / Plintossolos - SiBCS and Ferralsols – WRB / Latossolos - SiBCS with Petroplinthites) on the edges and low areas, a factor important for use and management; therefore, it is still little studied. The aim of this study was to understand the formation of petroplinthites (morphological facies) and their relationship to the soil formation (Ferralsols and Plinthosols) in a representative landscape of the RM, state of Mato Grosso do Sul, Brazil. For this purpose, five trenches were opened in different landscape positions: plateau (Ferralsol-P1), plateau edges (Plinthosol-P2 and Ferralsol-P3), backslope (Cambisol-P4), and footslope (Leptosol-profile P5). The petroplinthites occurred in all horizons of profile P2 and in horizons F1 and F2 of profile P3 with an abundance of 19 to 83 % and 75 to 86 %, respectively. The minerals kaolinite, gibbsite, goethite and hematite were observed in the clay fraction and hematite, goethite and gibbsite in the petroplinthites. In profile P4, quartz was present; the micromorphology of the soil was relatively enaulic and porphyric-enaulic, with a strongly marked microgranular pedality and the presence of amorphous nodules, magnetite, and fragments of petroplinthites. Various internal features were observed in the fragments of petroplinthites loose in the soil matrix, most with partially degraded structures and with reddish to yellowish color variations indicating dissolution and release of Fe oxides, manifesting as hydration halos. For the profiles at the edges of the plateau (P2 and P3), it is assumed that petroplinthites formed in situ in the past under more humid climatic conditions, followed by periods of drought. Under current climatic conditions, however, the petroplinthites exhibit degradation. Profiles P4 and P5, located on slopes, have high SiO2 contents, a predominance of kaolinite, and are associated with the degradation of basaltic rocks.

Keywords
micromorphology; basalt rocks; São Bento Formation; iron concretions; morphological facies

INTRODUCTION

Large areas of the central region of Brazil, of the Range Geral Formation and São Bento Group, including the Range of Maracaju plateau, consisting of basic rocks (Radambrasil, 1982; Nardy et al., 2008; Machado et al., 2009), present soils with an abundance of ferruginous concretions called petroplinthites (Campos et al., 2024). Petroplinthites are composed of iron, aluminum, and clay (Moreira and Oliveira, 2008; Campos et al., 2024), and in these areas, the soils are classified as Ferralsols or Plinthosols Petric, according to the World Reference Base for Soil Resources (IUSS Working Group, 2022), or as Latossolos or Plintossolos Pétricos, according to the Brazilian Soil Classification System (Santos et al., 2025).

Petroplinthites exhibit morphological diversity, and their description is commonly based on their structural components, such as nodular types and characteristics of the soil matrix, pores, and coatings (Alexandre, 2002). Based on morphological (macro- and micro-) and mineralogical, geochemical, and petrographic studies of petroplinthites (different morphological facies), it is possible to specify the sequence of environmental events associated with their formation (Boulangé et al., 1973; Bocquier et al., 1984). This methodology has been successfully used to study petroplinthites in regions such as Central and Western Africa (Boulangé et al., 1973; Bocquier et al., 1984) and Australia (Bourman et al., 1987). A recent study by Campos et al. (2024) in the southern Espinhaço Range demonstrated the value of this approach for interpreting the landscape genesis and evolutionary stages. This highlights the potential for similar research in other Brazilian regions with different geomorphological and climatic contexts.

In general, in the Range of Maracaju, soils formed by the weathering of basic rocks (with material petroplinthic) are deep, clayey, with high porosity, low natural fertility, and a predominance of iron and aluminum oxides. These soils are mainly observed on relief ranging from flat to undulating, typical of the Cerrado region (Theodorovisck, 2010). However, this environment has soil variations across the landscape, which can be better understood by considering topographic gradients (Motta et al., 2002). In addition, as result of paleoclimatic variations, especially in the Quaternary, the main hypothesis is that these concretions undergo a process of degradation that contributes to the formation of more weathered and deeper soil profiles, such as the Ferralsols, as well as release Fe and disaggregated materials on the edges of the plain, place of occurrence of Plinthosols (Horbe and Costa, 1997, 1999; Coelho and Vidal-Torrado, 2003; Cornell and Schwertmann, 2003).

The degradation processes of ferruginous concretions are complex, and the most important factor influencing the development of concretion degradation is palaeoclimatic change up to the present day; the alternation between wetter and drier climates at increasingly shorter intervals, especially in the Pliocene and Quaternary (Horbe and Costa, 1999). In periods of lower precipitation, the development of a more open landscape (savannah) predominates, which favors erosion by removing the upper part of the profile that accumulates on the slopes and the lower parts of the relief, thereby simultaneously exposing the petroplinthites. In wetter periods, the petroplinthite horizons are affected by the rising water table and the more closed vegetation, environmental changes that do not favor the preservation of the iron crust on the plateaus, leading to the leaching of iron by vertical and/or horizontal displacement across the landscape (Schwertmann and Taylor, 1989; Coelho et al., 2019).

The soils of the Range of Maracaju, an important grain production region in Brazil, are poorly studied. In this context, studying the morphology (macro- and micro-) and chemistry, as well as the mineralogy, of these soils is a good way to understand the processes of laterization and degradation to which they were subjected. This study aimed to investigate the genesis of petroplinthic material and its relationship with the formation of Ferralsols by characterizing basalt-derived soils across a representative landscape of the Maracaju Range, Mato Grosso do Sul State, Brazil.

MATERIALS AND METHODS

Localization and characteristics of the study area

The study was carried out between the edge and the plateau of the Maracaju Range (Radambrasil, 1982), in the municipality of Maracaju, in the southwest of the state of Mato Grosso do Sul, Brazil (Figure 1). The collected soils originated from basalt of the Serra Geral Formation, corresponding to the upper units of the São Bento Group (Upper Triassic), and resulted from the solidification of volcanic spillover material (Radambrasil, 1982). The basaltic rocks are superimposed on the sandstones of the Botucatu Formation and are composed of primary minerals plagioclase, clinopyroxenes (augite and pigeonite), opaque minerals, and olivines (Machado et al., 2009).

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Figure 1
Location and elevation map of the collected soil profiles of basaltic rocks from Serra Geral Group, São Bento Formation, in the Southwest (between the municipalities of Nioaque and Maracaju) in the state of Mato Grosso do Sul, Brazil.

The relief of this region varies from flat and undulating in the peripheral depression of the Miranda River, to undulating and strongly undulating (short slopes and strong slopes) near the contact zone with the Maracaju Range. The natural vegetation is dominated by tropical savannah phytophysignomies (Cerrado) and a current climate of Aw according Köppen (1936) and Alvares et al. (2013), which is tropical with dry winters and rainy summers, with an average annual minimum temperature of 15 °C and a maximum of 33 °C, and an average annual rainfall of 1,300 to 1,500 mm (Baradel and Romani, 2007).

In the Maracaju Range, soil studies have been carried out in areas of flat relief and associated with Ferralsols. However, on the edges of the dissected plateau and in extensive areas with undulating and convex/concave relief, there are Plinthosols. This fact motivated the choice of the location for the study. Some of these plateaus contain ferruginous petroplinthite remnants with varied morphologies. The persistence of these highly resistant petroplinthites allowed ancient landscapes to be preserved, exposing these soils to different climates and pedogenetic processes. This may explain the morphological variety of Plinthosols in the area. Additionally, the evolution of Ferralsols and Plinthosols in the landscape can be studied.

Description, sampling, and characterization of the soil profiles

Five soil pits were opened at different positions in the landscape, named plateau (profile P1), plateau edges (profiles P2 and P3), backslope (profile P4), and footslope (profile P5) (Table 1). Inside the pits, the profiles were morphologically described according to Anjos et al. (2025), and samples were collected from all horizons. Disturbed soil samples were air-dried, ground and sieved (soil <2 mm) to be used for physical and chemical analyses.

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Table 1
General information on developed soils of basaltic rocks from Serra Geral group São Bento Formation in the southwest of the state of Mato Grosso do Sul, Brazil

Morphological descriptions (morphological facies of the petroplinthites) were carried out according to Tardy (1993), Alexandre (2002), Eze et al. (2014), Campos et al. (2024) and Anjos et al. (2025). In the field, the nature, frequency, hardness and color of the petroplinthites were all described. In the laboratory, their size, shape and morphological facies were described.

The petroplinthites were classified according to their morphological facies, size (0.00-0.03, 0.03-0.05, 0.05-0.10 and >0.10 m), and quantity (common: 5-40 %, very frequent: 40-80 %, and dominant: more than 80 % of the volume) in relation to the soil matrix (Tardy, 1993; Eze et al., 2014; Campos et al., 2024).

Chemical and physical analyses were carried out on the air-dried fine earth (ADFE). Particle size analysis was performed using the pipette method with 1 mol L-1 NaOH as chemical dispersant and mechanical homogenization in a Wagner shaker for 16 h (Teixeira et al., 2017). The clay dispersed in water was also analyzed to assess the degree of flocculation. The pH was analyzed in water (soil solution ratio: 1:2.5) and the potential acidity was extracted with calcium acetate (0.5 mol L-1) at pH 7.0 and determined by titration as described in Teixeira et al. (2017).

Exchangeable Na and K, and available phosphorus were extracted with Mehlich-1 solution and determined by flame photometry and colorimetry, respectively; exchangeable Ca, Mg and Al were extracted with 1 mol L-1 KCl and determined by complexometric titration (Teixeira et al., 2017). Sum of bases (SB), cation exchange capacity (CEC) and base saturation (V%) were calculated (Teixeira et al., 2017). Total organic carbon (TOC) was determined by oxidizing organic matter with potassium dichromate in sulfuric acid at 100 °C, then titrating with ammonium ferrous sulfate (Yeomans and Bremner, 1988).

In the surface and subsurface diagnostic horizons and in the petroplinthite fractions, the contents of iron oxides (Fe2O3), silica (SiO2), alumina (Al2O3) and manganese (MnO) were determined after acid digestion (sulphuric attack method), from which the ki and kr indices were obtained (Camargo et al., 2009). The extracts were analyzed in an atomic absorption spectrophotometer to determine the oxides. Pedogenic Fe and Al contents were determined by atomic absorption spectrophotometry after extraction with sodium dithionite-citrate-bicarbonate solution (DCB) according to Mehra and Jackson (1960) or with 0.2 mol L-1 ammonium acid oxalate solution at pH 3 in the absence of light according to Schwertmann and Jackson (1964). The Fe and Al values extracted with the dithionite-citrate-bicarbonate (DCB) and oxalate-oxalic reagents were used in the equation developed by Shadfan et al. (1985) to determine the degree of isomorphic substitution of Fe for Al in iron oxides by chemical methods.

Mineralogical analysis was performed by X-ray diffraction (XRD) on the clay and petroplinthite fractions in the subsurface horizons. The petroplinthite samples, which were intended for XRD analysis, were ground and subsequently sieved through 0.053 mm mesh sieves. After processing, the clay and petroplinthite samples were mounted in an unoriented manner (powder sample) on a metal support. Analyses were performed using a Philips PW 2400 diffractometer with a copper anode (Cu Kα) and a Ni filter. The diffractometer was operated at 40 kV and 40 mA at a scanning speed of 1.2 degrees 2θ per minute. The samples were prepared according to the procedures described by Whittig and Allardice (1986).

The natural orientation samples taken for micromorphological analyses were collected from the surface AB and subsurfaces Bw and Bi horizons of the profiles and impregnated with a mixture of Polylyte polyester resin, styrene monomer, and fluorescent pigment, using Butanox as a catalyst (Teixeira et al., 2017; Castro and Cooper, 2019), to prepare 30-μm fine sections. The sections were observed using a polarizing optical microscopic (Carl Zeiss, Lab.A1 Axio, Germany) and a binocular magnifier (Carl Zeiss, 444036-9010, Germany), both under normal and polarized light. Photomicrographs were obtained using a photomicroscopic camera (Carl Zeiss, Axiocam 305 color, Germany). The micromorphological descriptions followed the criteria and terminologies proposed by Bullock et al. (1985), Stoops (2003) and Castro and Cooper (2019).

From the morphological descriptions and analytical data (morphological, physical, chemical and mineralogical), the profiles were classified according to the Brazilian Soil Classification System (SiBCS) (Santos et al., 2025) as follows: P1: Latossolo Vermelho Distroferrico tipíco; P2: Plintossolo Pétrico Concrecionário êndico; P3: Latossolo Vermelho Distróférrico petroplíntico; P4: Cambissolo Háplico Tb Distrófico léptico; and P5: Neossolo Litólico Eutrófico fragmentário. According to the World Reference Base for Soil Resources (IUSS Working Group, 2022), P1 was classified as: Ferritic Ferralsols (Clayic, Dystric); P2: Petric Plinthosols (Dystric); P3: Ferritic Ferralsol (Clayic, Dystric, Bathypetroplinthic); P4: Cambisols Eutric (Clayic); and P5: Leptosols Eutric (Loamic).

RESULTS

Morphological and physical properties

There was variation in the soil morphological properties in relation to position in the landscape (Table 2). Profiles P1, P2, P3, P4 and P5 presented a sequence of horizons A-Bw, Ac-Bwc-F, A-Bw-F, A-Bi and A-Cr, respectively. Except for profiles P4 and P5, which showed lithic contact within a depth of 0.60 m, the other soils were characterized as deep and with horizons of intense weathering. The transition between the horizons was plain and clear, except between the Bw2 and F1 horizons of profile P3 and between the Bwc and F1 of P2, which were plain and abrupt.

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Table 2
Morphological properties of developed soils of basaltic rocks of Serra Geral Group São Bento Formation in the southwest of the state of Mato Grosso do Sul, Brazil

In profiles P1, P2, P3 and P4 the hue 2.5YR predominated; and in profile P5 hue 7.5YR and 5YR, in the horizons A and Cr, respectively. In all profiles, granular structures (surface horizons) predominated, and subangular blocks (subsurface horizons) ranged in size from very small to medium. Profiles P3 and P5 differed from the other profiles, with granular structures in the subsurface horizons.

In profiles P2 and P3, in the form of petroplinthite, and in profile P5 in the basalt fragments, gravel and gutters were present (Table 3). Petroplinthites were classified into massive, nodular, and massive/nodular laterization facies, and pisolithic facies (Table 2). A massive facies was identified in profiles P2 and P3, associated with the F horizons. Petroplinthites with hardened massive facies have irregular shapes, channels, and non-oriented cavities, and in the case of profile P3, the petroplinthites with massive/nodular facies result from the coalescence of nodules. At the upper part of the F horizons, the concretions are slightly degraded, with hydration halos at the edges of channels and cavities, giving them colors ranging from yellow to red. Petroplinthites with a nodular facies were identified in the Apc, ABc, Bwc and F1 horizons of profile P2 and at the upper part of the F1 horizon of the profile P3, as well as loose nodules in the soil matrix; some of these nodules had a goethite cortex, a red-yellow to yellowish color, being pisolithic facies, indicative of rehydration and degradation.

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Table 3
Granulometric and physical properties of developed soils of basaltic rocks of Serra Geral Group São Bento formation in Southwest of the state of Mato Grosso do Sul, Brazil

Profiles P2 and P3 showed variations in the shape of the petroplinthites (Figure 2). In P2 petroplinthites are distributed through all the horizons (Figure 2a), with amounts varying from 19 to 24 % (frequent) in the surface horizons and 84 % (dominant) in the subsurface horizons, while in P3 petroplinthites were observed in horizons F1 and F2 (1.06 to 2.12 m) (Figure 2b), at 75 % (very frequent) and 86 % (dominant), respectively. Petroplinthite predominated at 0.00-0.03 m, ranging from 62 to 100 % in P2 and 53 to 73 % in profile P3. In P2, the petroplinthites were greater than 0.10 m, but in a smaller proportion, whereas in profile P3 (horizons F1 and F2) petroplinthites of 0.03–0.05 and 0.05–0.10 m were observed (Figure 2b).

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Figure 2
Occurrence of petroplinthites in profiles P2 (a) and P3 (b) of soils developed from basaltic rocks of the Serra Geral Group, São Bento Formation, in the southwest (between the municipalities of Nioaque and Maracaju) in the state of Mato Grosso do Sul, Brazil.

In profiles P1, P2, P3 and P4, the clay fraction predominated, with contents ranging from 508 to 730 g kg-1 and textural classes of clayey to very clayey (Table 3). In these profiles, the silt/clay ratio was less than 0.6, indicative of a high degree of weathering and advanced pedogenetic development (Santos et al., 2025). In profile P5, the sand fraction predominated, with a franco-clayey textured class and silt/clay ratio greater than 0.6. In the subsurface horizons of profiles P1, P2 and P3, the clay content that dispersed in water (CDW) was very low, and therefore, the values of the degree of flocculation were close to 100 %, and these may be related to the presence of iron oxides and aluminum hydroxide (Santos et al., 2025).

Bulk density (BD) values ranged from 1.10 to 1.69 Mg m-3, both encountered in profile P3. Particle density (PD) values ranged from 2.30 to 2.77 Mg m-3 in profiles P5 and P1, respectively. Total porosity ranged from 35 to 56 % in profiles P3 and P1, respectively, following the same tendency in the variations observed in BD.

Chemical and mineralogical properties

The pH(H2O) ranged from 4.5 to 6.2 and pH(KCl) from 3.8 to 5.3, characteristic of strongly to moderately acidic soils (Santos et al., 2025) (Table 4). Both pH in water and KCl showed little variation with depth, with values associated with Al3+ and H+Al contents that ranged from 0.08 to 1.93 cmolc kg-1 and from 5.98 to 13.49 cmolc kg-1, respectively. The ∆pH values were negative across all profiles, indicating the predominance of negative charges on the soil colloid surfaces.

In all five profiles, the highest levels of Ca2+, Mg2+, K+, sum of bases, base saturation and organic carbon (OC) occurred in the surface horizons. The highest cation exchange capacity (CEC) values in the surface horizons were associated with organic carbon content, ranging from 17.6 to 38 cmolc kg-1 in profiles P1 and P5, respectively. With the exception of profiles P4 and P5, the others showed subsurface horizons with clay activity (CA) values of less than 27 cmolc kg-1, indicating a predominance of type 1:1 clay and oxides and hydroxides. Labile P contents ranged from 0.5 to 25.6 mg kg-1 and decreased in depth proportional to the OC content in all profiles.

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Table 4
Chemical properties of developed soils of basaltic rocks of the Serra Geral Group, São Bento Formation, in the southwest of the state of Mato Grosso do Sul, Brazil

In the <2 mm fraction, SiO2 contents ranged from 123 to 223 g kg-1, Al2O3 from 105 to 242 g kg-1, Fe2O3 from 118 to 219 g kg-1 and TiO2 from 0.5 to 2 g kg-1 (Table 5). The high levels of these elements in the profiles are associated with high clay content, as well as the presence of Ki values below 2.2 (1.37 to 1.96) and Kr values between 0.8 and 1.3, indicating the advanced stages of pedogenetic weathering in these soils (Ghidin et al., 2006; Santos et al., 2025). In profile P5, this pattern was not verified, likely due to its location in the landscape, which reduced weathering. In the petroplinthites, SiO2 content showed a slight reduction compared to that of the <2 mm fraction, ranging from 102 to 135 g kg-1. The Al2O3 and Fe2O3 contents were considerably higher in petroplinthites, with Al2O3 contents ranging from 141 to 240 g kg-1; Fe2O3 from 226 to 383 g kg-1, consequently decreasing the values of Ki and Kr (Table 5).

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Table 5
Chemical composition of the fine earth (<2 mm) and petroplinthite fractions of soils developed from basaltic rocks of Serra Geral Group, São Bento Formation, in southwest of the state of Mato Grosso do Sul, Brazil

Iron contents extracted with ammonium dithionite and oxalate (Fed and Feo) showed a tendency to increase in the subsurface horizons, while those of Al (Ald and Alo) remained constant was higher than 0.64 in the profiles, in contrast to the Feo/Fed ratio, which was low (<0.05), (Table 6), suggesting that the iron is predominantly in the oxidic and crystalline form (Coelho and Vidal-Torrado, 2003; Anjos et al., 2007; Miguel et al., 2013), since the amorphous fraction (Feo) was not very expressive. The isomorphic substitution values were below 12 cmolc mol-1, indicating a low substitution of Fe by Al.

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Table 6
Iron and aluminum contents extracted with dithionite citrate bicarbonate and ammonium oxalate and their relationships in developed soils of basaltic rocks of Serra Geral group São Bento formation in Southwest of the State of Mato Grosso do Sul, MS, Brazil

In the X-ray diffractograms (XRD) of the clay fraction of the matrix, the soils are mineralogically similar, with the main minerals being kaolinite, gibbsite, goethite, and hematite (Figure 3). Profile P4 also showed quartz. The XRD of the petroplinthites indicated that they are mineralogically similar across profiles (Figure 4), differing only in the soil matrix, which showed a higher hematite intensity (P2 and P3).

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Figure 3
X-ray diffraction of the clay fraction of soils developed from basaltic rocks of the Serra Geral Group, São Bento Formation, in the southwest (between the municipalities of Nioaque and Maracaju) in the state of Mato Grosso do Sul, Brazil. Ct: kaolinite; Gb: gibbsite; Gt: goethite; Hm: hematite; Qz: quartz.
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Figure 4
X-ray diffraction of petroplinthite from soils developed from basaltic rocks of the Serra Geral Group, São Bento Formation, in the southwest (between the municipalities of Nioaque and Maracaju) from state of Mato Grosso do Sul, Brazil. Gb: gibbsite; Gt: goethite; Hm: hematite; Qz: quartz.

Micromorphology

All profiles presented a relative distribution between porphyric and enaulic, or both (Table 7), with a granular microstructure (Figure 5a), in some cases, with coalesced granules, and a degree of development ranging from strong to moderately developed. The profiles P1 and P3 also presented structures of the subangular blocks type in the superficial horizons (Figure 5b).

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Table 7
Micromorphological description of some soil horizons developed from basaltic rocks of the Serra Geral Formation, São Bento Group, in the southwest of the state of Mato Grosso do Sul, Brazil

The most common voids in the profiles were compound and complex packaging (present in profile P2), polyconcave vughs (inter-aggregate and intra-aggregate), elongated planars (intra- and inter-aggregated in subangular blocks) and channels and biological vughs (inter-aggregated). The coarse material is composed of magnetite with subhedral and anhedral grains, and anhedral quartz grains, moderately selected, in addition to ferruginous concretions of various sizes, with smoothed, rounded to rounded shapes, which were present in all profiles. The fine material consists of kaolinite mixed with iron oxides, isotropic material.

Profiles P1 and P2 had pedofeatures of mamillated and bacillo-cylinder excrement, filling voids of the biological channel and vugh type. Profile P3 and P4 presented textural coating pedofeatures, mainly located in vugh pores. In the Bw horizon, profiles P1 and P3 exhibit channel-type and trans-aggregate voids with smooth, subrounded walls and fillings of loose microgranules (continuous and discontinuous) (Figure 5c).

The external micromorphology of iron nodules ranged from subrounded in profile P3 to subrounded and rounded in P2 and was individualized from the soil matrix with clear and prominent limits (Figures 5d, 5e, 5f, and 5g). Internally, they presented the predominance of isotropic thin material with a reddish color and in some, yellowish nodules. In addition, they presented a few quartz grains, moderately selected and similar to those of the soil matrix. However, approximately 50 to 65 % of the rusting concretions of profiles P2 and P3, present internal degradation expressed by a more yellowish color or by worn internal structures (Figures 5e and 5f), hydration halos with yellowish color coating the concretions (Figure 5g), in addition to the occasional fragments of concretions encountered in profile P1. The Bi horizon of profile P4 presented aggregates coated with fine material (Figure 5h).

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Figure 5
Photomicrographs of profile P3 (a), horizon Bw2 - relative distribution of porphyric-enaulic (P/E) with compound packing voids (pv) and microgranular with coalesced granular aggregates (mc); (b) Profile P4, horizon Bi - relative distribution of porphyric type (P), with subangular blocks (bs); (c) Profile P3, horizon Bw2 - faunal void chamber with loose granular infillings (ic); (d, e and f) Profile P2, horizon Bwc - relative distribution of enaulic type (e), with nodules whole petroplinthics (n) and degrading (nd) and microgranular structure (m); (g) Profile P2, horizon Bwc - nodule coated with hydration halo (r); (h) Profile P4, horizon Bi - aggregate coated with fine material (r and fn).

DISCUSSION

In terms of distribution across the landscape, profiles P4 and P5 along the slopes exhibit chemical and mineralogical characteristics indicative of a lower stage of pedogenic weathering. In profiles P1, P2, and P3, the silica contents are close to those of Fe₂O₃ and Al₂O₃, and, in addition to the predominance of kaolinitic, hematite, and goethite in the X-ray diffraction, this confirms the oxidic kaolinitic nature of these soils. This fact is associated with the landscape position, a dissected plateau, moderate wavy relief, free drainage, and hot, humid weather (Motta et al., 2002; Lira‑Martins et al., 2022). These conditions contribute to the silica-loss process by desilicating primary minerals, leading to the formation of kaolinite. Subsequently, this process results in the formation of hematite, which predominates in soils developed from basalt in tropical climates. This process is called laterization (Melo et al., 2001; Coelho and Vidal-Torrado, 2003; Lima et al., 2021). The increase in humidity throughout the Quaternary period contributed to the formation and preservation of goethite along landscape edges (Coelho and Vidal-Torrado, 2003; Lima et al., 2021).

Petroplinthite was found in profile P2 located on the edge of the plateau, with smooth wavy relief (convex) and lower elevation than profiles P1 and P3; and in profile P3 located on the edge of the plateau with plain relief (concave). In these hardened materials there is evidence of accumulation of iron oxides, hematite and low isomorphic substitution of Al, likely originating from the reduction and translocation of iron to more aerated zones during past hydromorphic periods, followed by crystallization in drier periods (Nahon and Tardy, 1992; Tardy and Roquin, 1992; Coelho and Vidal-Torrado, 2003; Santos et al., 2022; Campos et al., 2024).

Between the Upper Cretaceous and the Paleogene, the studied region had flat terrain (Valeton, 1994), with springs and drainage networks on the geomorphological surface, which is related to wetter climatic conditions in the past that contributed to the oscillation of the water table, resulting in alternating conditions of reduction and oxidation, causing a mottled formation and weakly cemented basement (Eze et al., 2014). During this period, South America experienced cyclical paleoclimatic events that led to pediplanation, lateralization, and fluvial channel incision-induced dissection (Butt et al., 2000; Rosolen et al., 2017). The alternation of these climatic periods with drier seasons favored the crystallization of Fe oxides (Table 5) and greater aggregation, cementation and hardening of the plinthites, resulting in petroplinthites (Schellmann, 1994; Anjos et al., 1995; Widdowson, 2009; Eze et al., 2014; Laroca et al., 2018).

Given the past environmental conditions that favored the formation of petroplinthites, the morphological facies of the petroplinthites in profiles P2 and P3 (Table 2 and Figure 2) reflect distinct stages of iron mobilization, accumulation, and cementation in these soils (Coelho et al., 2001; Campos et al., 2024). The in situ petroplinthites from profiles P2 and P3 showed little variation in their internal fabric relative to the matrix background. They lacked primary minerals, such as plagioclase and pyroxene, and showed a predominance of nodular and massive facies. These characteristics result from iron enrichment through the remobilization of ferrous ions and the concentration of immobile elements in the soil, originating from basalt weathering (Coelho et al., 2001).

The nodular facies found in the Apc, ABc, Bwc, and F1 horizons of P2, as well as in the F1 horizon of P3, represent an intermediate phase of lateritization. This phase is characterized by rounded to subangular iron-rich nodules that are embedded in an uncemented matrix. The formation of nodular facies is characterized by the accumulation of iron oxyhydroxide within peds in concentric shapes. As this process progresses, the nodules become denser and more cohesive due to internal cementation (Campos et al., 2024). Nodules form under fluctuating redox conditions with periodic water saturation that induces Fe mobilization and subsequent precipitation during dry periods (Schwertmann and Taylor, 1989).

The massive morphological facies recognized in the F horizons of P2 and P3 reflect an advanced pedogenic stage. These facies are characterized by irregularly shaped hardened blocks with internal channels and unoriented cavities. This suggests that the soil matrix has undergone advanced cementation by iron oxides (Coelho et al., 2001; Campos et al., 2024). These facies form due to intense iron enrichment, nodule growth and coalescence, and consequent fusion with the matrix base, followed by hardening of the entire matrix (Tardy, 1993; Widdowson, 2009). The massive/nodular facies identified in P3 exemplify the transition from nodular organization to massive hardpan formation, likely driven by progressive cementation and the filling of internodular pore spaces.

The presence of small (0.00-0.03 m) typical and pisolithic nodules in the surface horizons, associated with massive facies petroplinthites between 0.05-0.10 m and greater than 0.10 m in size, in the subsurface horizons of profiles P2 and P3 (Figure 3), indicates a more recent process of degradation by hydration, leading to the Fe depletion and dismantling of the petroplinthites (Beauvais and Tardy, 1993; Tardy, 1993; Horbe and Costa, 1999, 2005; Albuquerque et al., 2020). The presence of pisolithic nodules with yellowish-red hydration halos (Figures 5e, 5f and 5g) observed in the morphology and micromorphology indicates the processes of hydration, dissolution, and removal of hematite (Coelho et al., 2001). This material may consist of more hydrated oxides such as goethite, which initially coats the nodules in the form of halos, goethitic cortex, and is later incorporated into the soil matrix (Tardy, 1993; Beauvais and Tardy, 1993; Cornell and Schwertmann, 2003; Beauvais, 2009; Horbe and Anand, 2011; Campos et al., 2024).

The presence of hematite, goethite, and gibbsite in the laterites and soils (Figure 4), as well as the higher Fe₂O₃ and Al₂O₃ content in the soil matrix of the subsurface horizons of profiles P2 and P3 (Table 5), support the hypothesis that laterite is a soil-forming material. As previously mentioned, in humid, well-drained environments, gibbsite and goethite dominate degraded lateritic crusts (Tardy, 1993; Tardy and Roquin, 1998; Cornell and Schwertmann, 2003). In contrast, profiles P4 and P5, located on slopes, have higher SiO₂ content and a mineralogy dominated by kaolinite, indicating a genesis associated with basalt degradation.

Ferruginous concretions have degraded primarily due to paleoclimatic changes involving alternating wet and dry periods since the Pliocene (Horbe and Costa, 1999). Dry phases promote surface erosion and petroplinthite exposure, while wet phases enhance iron leaching through rising water tables (Schwertmann and Taylor, 1989; Coelho et al., 2019). These dynamics vary with topography: In higher elevations with smooth to flat relief (e.g., P3), prolonged water contact accelerates crust dissolution, leading to fragmentation (e.g., P3) or disappearance (e.g., P1). Profile P1 contains fragments of nodules throughout its matrix, which are possibly remnants of older, thicker petroplinthite crusts that dissolved in wetter environments. Near slopes (e.g., P2), shorter saturation periods and erosive action preserve thicker crusts. The soils formed on petroplinthites reflect multiple weathering cycles.

Iron dissociates from concretions and is transported to positions farther along the plateau edge (profile P3). This material probably moves with sediments or through lateral translocation of ferrous ions to lower positions in the landscape. This results in high iron levels in profiles further down the slope (Tanner and Khalifa, 2010). Profile P4, located on the backslope, shows that its iron content originates from basalt degradation, an in situ accumulation, with a kaolinite nature, and though the contribution of iron from higher parts of the landscape cannot be discounted. Under open savannah vegetation, there is less surface protection, leading to the disintegration and transport of large volumes of material from edges and plateaus during prolonged rainfall events (Motta et al., 2002). However, the petroplinthites present at the edges of the area act as barriers to the free movement of water, reducing erosion and contributing to a longer water residence time in the soils, thereby accelerating weathering along the plateau (Motta et al., 2002; Tsozué et al., 2012).

CONCLUSIONS

The weathering of basalt, mainly the formation of petroplinthites and their subsequent degradation, has been strongly influenced by complex pedogenic and landscape-evolutionary processes in a dissected-plateau region characterized by moderately undulating topography, free drainage, and a hot, humid climate.

The petroplinthites in soils located at the edge of the plateau (profiles P2 and P3) show a genesis-in-situ associated with paleoclimatic changes. Hot and humid periods contributed to oxidative conditions and later, in drier periods, to greater aggregation and cementation. Profile P1 reveals scattered nodular concretions throughout the matrix, which may suggest the dissolution and degradation of petroplinthites in higher, flatter areas of the landscape.

The variations in the reddish-yellow color of the petroplinthites and the hydration halos, verified by micromorphology, indicate the reduction and dissolution of hematite, forming hydrated oxides of goethite and gibbsite, which separate from the ironstone and are incorporated into the soil matrix. When the Quaternary changed to a warm, humid climate, this favored the degradation of the petroplinthites, and Ferralsols formed on the plateau. The iron dissociated from the ironstone is transported to the lower parts of the landscape.

FUNDING

National Council for Scientific and Technological Development – CNPq, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES and Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado de Mato Grosso do Sul – FUNDECT, for their financial support (INCT PantaClima).

How to cite:

Faria TAC, Oliveira NS, Pereira MG, Moraes EMV, Souza AC, Farias PGS, Abreu GM, Schiavo JA. Morphology and mineralogy of soils with petroplinthites in the Savannah biome central western Brazil. Rev Bras Cienc Solo. 2026;50:e0250167. https://doi.org/10.36783/18069657rbcs20250167

DATA AVAILABILITY

The data will be provided upon request.

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*

Corresponding author: e-mail: schiavo@uems.br

Editor:

José Miguel Reichert https://orcid.org/0000-0001-9943-2898 and Alberto Vasconcellos Indá Júnior https://orcid.org/0000-0001-5252-0313

Publication Dates

  • Publication in this collection
    05 Oct 2026
  • Date of issue
    2026

History

  • Received
    19 Sept 2025
  • Accepted
    23 Feb 2026
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