# Morphological, mineralogical and geochemical characterization of Ngaoundal soils (Adamawa-Cameroon): Implications for weathering mechanisms and trajectories in tropical zones

Abdoul Aboubakar, Ilalie Bertille Kentsa Manefouet, Loique Rodolph

Mbounou Azefack, Yannick Langollo Tchedele, Dalil Adoulko, Demonstel

Yvan Ntomb, Abakar Mahamat, Jacques Wadjou Wassouo, Quentin Marc

Anaba Fotze

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Abdoul Aboubakar, Ilalie Bertille Kentsa Manefouet, Loique Rodolph Mbounou Azefack, Yannick Langollo Tchedele, Dalil Adoulko, et al. Morphological, mineralogical and geochemical characterization of Ngaoundal soils (Adamawa-Cameroon): Implications for weathering mechanisms and trajectories in tropical zones. Geosystems and Geoenvironment, 2025, 5, 10.1016/j.geogeo.2025.100450. hal-05434608

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# Morphological, mineralogical and geochemical characterization of Ngaoundal soils (Adamawa-Cameroon): Implications for weathering mechanisms and trajectories in tropical zones

Abdoul Aboubakar<sup>a,b,*</sup>, Ilalie Bertille Kentsa Manefouet<sup>b</sup>,

Loique Rodolph Mbounou Azefack<sup>b</sup>, Yannick Langollo Tchedele<sup>c,d</sup>, Dalil Adoulko<sup>f</sup>,

Demonstel Yvan Ntomb<sup>a,e</sup>, Abakar Mahamat<sup>a,g</sup>, Jacques Wadjou Wassouo<sup>a,g</sup>,

Quentin marc Anaba Fotze<sup>a,g</sup>

## ARTICLE INFO

**Article history:**

Received 7 May 2025

Revised 21 July 2025

Accepted 21 July 2025

Handling Editor: Sanzhong Li

**Keywords:**

Ngaoundal

Alteration

Hydrolysis

Morpho-structural

X-ray diffractometry

## ABSTRACT

Ngaoundal, located in the Adamaoua region of Cameroon, has a tropical climate and is known for its characteristic soils. The aim of the present study was to evaluate the action of weathering on Ngaoundal's lateritic soils. Morpho-structural, mineralogical and geochemical analyses enabled us to carry out this study. The morphological study carried out on 15 soil samples shows a variation in colour from red-dish (2.5YR 5/8) to dark brown (7.5YR 6/8), varying from reddish yellow (7.5YR 6/8), orange brown (10YR 5/6) and yellowish brown (10YR 5/4), and have clayey to sandy-clayey matrices with nodules of the order of millimetres to centimetres. X-ray diffractometry and x-ray fluorescence show the following mineral phases in Ngaoundal's lateritic soils: quartz, goethite, gibbsite, kaolinite, hematite, magnetite, anatase and boehmite. Geochemical analysis of these soils shows high average levels of SiO₂ (36.41 %), Fe₂O₃ (31.48 %) and Al₂O₃ (17.72 %), and low average levels of TiO₂ (1.02 %) and CaO (0.36 %), which characterise characteristic soils rich in ferruginous oxide and alumina oxide. The weathering indices of Ngaoundal soils (sesquioxide ratio) vary between 0.22 and 2.98 with an average of 1.09. This average value qualifies these soils as lateritic soils rich in gibbsite and kaolinite (average CIA = 96.38 %) and resulting from chemical alteration of the hydrolysis type (alloying and monosiallitisation).

© 2025 The Authors. Published by Elsevier Ltd on behalf of Ocean University of China. This is an open access article under the CC BY license ([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/))

## 1. Introduction

Supergene or meteoric weathering is one of the most important processes modifying the Earth's surface. This weathering is the result of interaction between the geosphere, hydrosphere, biosphere and atmosphere. Rocks on continental surfaces undergo weathering processes that are chemical "factories" with slow but continuous reactions and gigantic mass production over time (Ricordel-Prognon et al., 2009). Alterology studies the evolution of the rock

fraction towards the pedogenetic fraction (soil). Laterites are a natural resource used extensively in civil construction in the intertropics. These materials occupy 33 % of the surface area of the African continent and are found throughout the tropical zone between isohyets 750 and 1200 mm (Millogo et al., 2008). In Cameroon, studies have estimated the potential for laterite deposits at over 70 % of the country's surface area (Sikali and Djalal, 1987). These materials are the result of specific weathering processes (Bohi, 2008). Their geotechnical properties are highly variable, influenced by climate, geology and the degree of weathering or lateritisation (Duchaufour, 2001). These properties can therefore differ from one region to another, or within the same geographical area, depending on the nature of the parent rock from which they are derived.

<footnote><sup>a</sup> Center for Geological and Mining Research (CRGM), P.O. Box: 333, Garoua, Cameroon</footnote>

<footnote><sup>b</sup> Department of Earth Sciences, Faculty of Sciences, University of Dschang, and P.O. Box: 67, Dschang, Cameroon</footnote>

<footnote><sup>c</sup> Research Department, Local materials Promotion Authority (MIPROMALO), Yaounde, Cameroon</footnote>

<footnote><sup>d</sup> Department of Mining Engineering, School of Geology and Mining Engineering, University of Ngaoundere, Meiganga, Cameroon</footnote>

<footnote><sup>e</sup> Department of Earth Sciences, Faculty of Sciences, University of Yaounde I, Yaounde, Cameroon</footnote>

<footnote><sup>f</sup> Department of Earth Sciences, Faculty of Sciences, University of Ngaoundere, P.O. Box: 454, Ngaoundere, Cameroon</footnote>

<footnote><sup>g</sup> Department of Earth Sciences, Faculty of Sciences, University of Maroua, P.O. Box: 814, Maroua, Cameroon</footnote>

<footnote>* Corresponding author.
E-mail address: abdoulcraft8@gmail.com (A. Aboubakar).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Location map of the study area.</figcaption></figure>

The use of laterites requires a minimum of knowledge of their ge-
netic and geotechnical properties to avoid early deterioration and
deformation of pavement layers. Furthermore, the systematic use
of laterites leads to the depletion of deposits of good quality ma-
terials (Kassogue et al., 2002).

In recent years, there has been renewed interest in the
study of lateritic soils, with the development of road infras-
tructure concomitant with the economic development of sub-
Saharan African countries (Mahamadou et al., 2015). In Central
Africa, several studies have been carried out on the geotechni-
cal aspects (Ekodeck, 1984; Millogo et al., 2008; Onana, 2010;
Kamtchueng et al., 2015; Onana et al., 2015; Manefouet, 2016;
Nzabakurikiza et al., 2016; Mvindi et al., 2017; Hyoumbi et al.,
2018; Aboubakar et al., 2021) and genesis (Ekodeck, 1984) of la-
teritic soils, mainly in Cameroon. However, very little work has
been done on the weatherability of lateritic soils (Manefouet et al.,
2024). Yet in tropical zones, depending on the geology, topogra-
phy and climate, lateritic soils are formed from tropical weath-
ering. The lateritic soils formed have varied morphological char-
acteristics and mineralogical and geochemical compositions. This
raises the question of how the morphological characteristics of la-
teritic soils influence chemical weathering processes, and also of
the links between the distribution of oxides and neo-formed and
residual minerals during the formation and evolution of lateritic
soils in tropical zones. Our aim is therefore to show the link be-
tween the morphological, geochemical and mineralogical charac-
teristics of Ngaoundal lateritic soils. We therefore hypothesise that
the mineralogical and geochemical signatures of Ngaoundal's lat-

eritic soils are strongly influenced by the granitic nature of the
bedrock and local climatic conditions. Three research questions
guide this study: (1) What are the predominant weathering indices
in these soils? (2) What mineral phases are indicative of advanced
weathering? (3) How do these characteristics compare with tropi-
cal laterites from other regions of the world?

## 2. Geological settings

2.1. *Natural setting of the study area*

Ngaoundal, which is the subject of this study, is located in the
Adamawa region of Cameroon, more specifically in the Djerem de-
partment. It lies between latitudes N 6°24′30″ to N 6°30′59″ N
and longitudes N 13°14′40″ to 13°14′30″ E, with an average alti-
tude of 1020 m and covers an area of 29.95 km² (Fig. 1).

The climate in Ngaoundal is Sudano-Guinean, characterised by
two contrasting seasons (Suchel, 1987) of unequal length. A dry
season and a rainy season (Tchotsoua et al., 1998). The rainy sea-
son generally begins with a stormy period characterised by violent
downpours (Sieffermann, 1973). According to rainfall data collected
at the ASECNA meteorological station (2000-2019) at Ngaoundéré
airport, the rainy season varies between seven and eight months
(April-October) and the dry season between four and five months
(November-March). As a general rule, average rainfall in the region
varies between 900 mm and 1,500 mm per year, decreasing fur-
ther north. Rainfall peaks in August, and may and June are usually
wet. Temperatures drop from November to January, reflecting win-
<table><caption>**Table 1**
Rainfall and temperature data for the Ngaoundal Borough for the period 2000 to 2019 (source: meteorological data from the ASECNA station at Ngaoundéré Airport).</caption><thead><tr><th>Month</th><th>January</th><th>February</th><th>March</th><th>April</th><th>May</th><th>June</th><th>July</th><th>August</th><th>September</th><th>October</th><th>November</th><th>December</th></tr></thead><tbody><tr><th>P (mm)</th><td>2</td><td>7</td><td>57</td><td>133</td><td>184</td><td>217</td><td>262</td><td>279</td><td>266</td><td>175</td><td>22</td><td>1</td></tr><tr><th>T (°C)</th><td>21.9</td><td>23</td><td>24</td><td>23.8</td><td>23</td><td>22.2</td><td>21.4</td><td>21.9</td><td>21.86</td><td>22.4</td><td>22,4</td><td>21.8</td></tr></tbody></table>

P = pressure; T = temperature.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Umbrothermal diagram of Ngaoundal for the period 2000-2019.</figcaption></figure>

ter conditions. Temperatures rise from January onwards and reach a peak in April. Rainfall in may and June brings temperatures down again. The average temperature in the district is 22.5°C. An average of 133.75 mm of rain falls per year (Table 1).

With an average temperature of 24.0°C, march is the hottest month of the year and 21.4°C makes July the coldest month. The variation in rainfall between the driest and wettest months is 278 mm. There is a difference of 2.6°C between the lowest and highest temperatures throughout the year (Fig. 2).

### 2.1.1. Regional geology: The central African Pan-African chain in Cameroon (CPAC)

In Cameroon, the CPAC at the northern end of the Congo Craton (Deasso et al., 2022; Soh Tamehe et al., 2022) has been the subject of several petro-structural studies (Achu Megnemo et al., 2021; Ngassam-Mbinya et al., 2021; Blandine et al., 2022; Azefack et al., 2023; Sobze Ymedji et al., 2023) and several geochemical and isotopic studies (Hamdja Ngoniri et al., 2021; Kamguia Kamani et al., 2021; Ayonta et al., 2023). On the basis of geodynamic characteristics, Nzenti et al. (1988), Ngnotue et al. (2000) and Toteu et al. (2004) highlight three domains in the CPAC from the north to the south of Cameroon.

1. The Northern Domain (West and North Cameroon): This is located west of the Tcholliré-Banyo Shear Zone (Fig. 3) and extends along the western edge of Cameroon. It continues in the Poli (Toteu et al., 2006) and mayo Kebbi regions in SW Chad (Penaye et al., 2006). It consists of Neoproterozoic gneisses and schists of volcanic and volcano-sedimentary origin considered to represent magmatic arcs accreted to the Adamaoua domain (Penaye et al., 2006; Pouclet et al., 2006). Pan-African pre-, syn- and late-tectonic granitoids (diorites, granodiorites and granites) are abundant and mainly of calc-alkaline composition. They were emplaced between 660 and 580 Ma in the Poli region (Toteu et al., 1987, 2001) or between 737 and 723 Ma in the mAyo Kebbi region (Penaye et al., 2006).

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Location of the study area on the geological map of Cameroon (Toteu et al., 2004). CCSZ: Central Cameroon Shear Zone; KCSZ: Kribi Campo Shear Zone; SSZ: Sanaga Shear Zone; TBSZ: Tcholliré-Banyo Shear Zone.</figcaption></figure>

2. The central domain or Adamaoua-Yadé domain: This extends north of the Sanaga fault (Dumont, 1986) as far as the Tcholliré-Banyo shear (Fig. 3). It comprises large relics of orthogneiss and meta-sediments of Palaeo-Proterozoic age, meta-sediments and acid volcaniclastic rocks and syn- to late-tectonic Pan-
African granitoids (Soba et al., 1991; Toteu et al., 1994, 2001; Ganwa, 1998; Tagne Kamga, 2003; Tchameni et al., 2006). It should be noted that the Lom series (c. 700 mA) and the Bafia series (Tchakounté et al., 2007; Ganwa et al., 2008a) are part of this assemblage, which is interpreted as a bedrock of Archean to Palaeoproterozoic age dismantled during the Pan-African orogeny and intruded by Pan-African batholiths (Toteu et al., 2004; Ganwa et al., 2008b). The study area lies within this domain.

(3) The southern domain comprises the Yaoundé Group (Nédélec et al., 1986; Nzenti et al., 1988), which is bounded to the west by the Kribi-Campo fault (Fig. 3) but continues eastwards into the CAR in the Bolé and Gbaya series (Pin and Poidevin, 1987; Poidevin, 1991). Essentially metasedimentary in nature, it also contains a variety of plutonic rocks such as mafic rocks (pyriclasites) to intermediates at Yaoundé, serpenținised ultramafic rocks (Cr and Ni) associated with gabbros, diorites and mafic dykes in the Lomié region (Sémé Mouangué, 1998), pyroxenites, amphibolites and talcschists derived from hydrothermal alteration of ultrabasic and ultramafic rocks in the Boumnyebel region (Nkoumbou et al., 2006, 2008, 2009).

## 2.2. Local geology

The Adamaoua Plateau, located in the central area of CPAC (Nkouandou et al., 2017), is home to major plutonic massifs. These plutonic massifs have been the subject of several petrological studies (Tchameni et al., 2006). Pan-African syn- to post-tectonic granitoids are intrusive in the Palaeoproterozoic basement and locally covered by Cretaceous deposits (Djérem Basin) (Nkouandou et al., 2008). They form a wide variety of rocks (biotite and amphibole granites, muscovite and biotite granites, biotite-only granites, diorites, gabbros and syenites; Daouda, 2014). In general, these granitoids belong to the very potassic to shoshonitic calc-alkaline series; they are metaluminous to slightly peraluminous and type I (Lassere, 1961; Soba et al., 1991; Toteu et al., 2001; Njanko et al., 2006; Tchameni et al., 2006; Djouka-Fonkwe et al., 2008; Ganwa et al., 2011a, 2011b) and locally of type S (Soba et al., 1991; Tchameni et al., 2006; Djouka-Fonkwé et al., 2008).

The Ngaoundal sector hosts syn to late-tectonic granitoids (Dawai et al., 2017), partially overlain by basaltic, trachytic and phonolitic flows (Nkouandou et al., 2010). The granitoids encountered in the study area have crystallisation ages of between 560 Ma and 620 Ma (Nomo Negue et al., 2017; Hamdja Ngoniri et al., 2021). These granitoids include calc-alkaline diorites, granodiorites, monzogranites, syenites, syenogranites and granites (Hamdja Ngoniri et al., 2021; Fossi et al., 2022; Fig. 4). They are post-tectonic to late-tectonic and, together with their host rocks, have undergone polyphase deformation in a transgressive regime (Ganno et al., 2010; Kankeu et al., 2012; Nomo Negue et al., 2017; Bella Nké et al., 2018). In this region, bauxite ore deposits are developed exclusively from altered basalts (Eno Belinga, 1972). Calc-alkaline granitoids with a high quartz and feldspar content, which are dominant in the Ngaoundal region, favour the formation of kaolinite and gibbsite via hydrolysis in a humid climate. Their low base cation content explains the leaching of Ca, Na and K, a process confirmed by the high CIA and MIA values observed (Aliou et al., 2022).

# 3. Methodology

## 3.1. Field work

In soil science, as in geotechnics, one of the major problems lies in the representativeness of the samples. The precautions to be taken when taking samples in the field depend primarily on the

analysis problem (Millogo, 2008). Several field surveys were necessary to complete this study. The fieldwork consisted of a general exploration and selection of materials, which were then studied in detail. This phase involved manual test pits, pits and soil profiles to determine the nature, type and physical characteristics of the lateritic materials. Sufficient quantities of material were taken from the surveyed areas for the laboratory work.

Lateritic gravel samples were taken throughout the study area at 2 km intervals. These sampling points were selected randomly, but at precise intervals, depending on the areas that were accessible (as the area was militarised, some sectors were not accessible). The lateritic soil samples were coded Ng. Approximately 10 to 20 kg of material per site were taken for morphological observations and around 100 to 200 g for mineralogical and chemical analyses. A total of fifteen samples of lateritic soil were taken for laboratory analysis (Fig. 5).

As for the macroscopic analysis of the sampled soils, the size of the lateritic gravels was assessed in two stages: first, visually in the field and then by granulometric analysis (sieving) in the laboratory.

## 3.2. Laboratory work

### 3.2.1. Mineralogical study: X-ray diffraction (XRD)

X-ray diffraction (XRD) allows the identification and semi-quantitative determination of the main minerals present in a sample. The analysis was carried out on the powders of the lateritic soil samples in order to determine their mineralogy. The materials were oven-dried at a temperature of around 50°C. They were then ground at the Garoua Geological and Mining Research Centre using a Fritsch electric pulveriser.

The XRD analysis was carried out on the total sample without separating the clay fraction (<2 µm), as the main objective was to assess the overall weathering processes in the lateritic soil as a whole and not to study the neoformed clays specifically.

The samples were prepared in the form of a 200 µm powder packed and flattened in a steel dish, so as to orientate the minerals. The analysis consisted of applying radiation of X-ray wavelength (0.1 < λ > 10 nm) to the sample. The apparatus then measured a refraction signal, which was converted into basal distance or spacing between reticular planes. Mineralogical analysis was carried out using a PAN analytical X' PERT PRO using cobalt Kα radiation (λ = 1.7890 Å) on the total sample (gravel and fine earth), at the University of Bejaia laboratory in South Africa. The spectra were recorded at a speed of 0.01°/s over a 2θ range varying between 5° and 85°. The acceleration voltage was 40 kV and the current was 45 mA. The diffractograms obtained were analysed using ExCEL software and the Powder Diffraction File (PDF) of Brown and Brindley (1980).

### 3.2.2. Geochemical analysis by X-ray fluorescence (XRF)

X-ray fluorescence spectrometry was carried out in the laboratory of the University of Bejaia in South Africa. XRF is used to determine the chemical composition of the oxides present in a sample.

3.2.2.1. *Sample preparation.* the samples were oven-dried at 105°C for 24 h to remove any residual moisture. They were then finely ground using a ball mill to obtain a homogeneous powder with a particle size of less than 75 micrometres. This powder was used to make press pellets. To prepare the pellets, approximately 5 grams of powder were mixed with a binder, then compressed under a pressure of 10 to 20 t using a hydraulic press to form solid, homogeneous discs. The pellets were then analysed directly by XRF spectrometry.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Geological map of the study area (extract from the geological map of Delor et al., 2021).</figcaption></figure>

3.2.2.2. Principle of the method. X-ray fluorescence spectrometry is based on the excitation of atoms by a beam of X-rays. When these rays strike the sample, the excited atoms re-emit some of this energy in the form of characteristic secondary X-rays called X-ray fluorescence. The intensity and energy of the rays emitted make it possible to identify and quantify the elements present in the sample, particularly in the form of oxides (SiO₂, Al₂O₃, Fe₂O₃, Na₂O, K₂O, CaO and other oxides).

3.2.3. Mineralogical data processing methods

The X-ray diffractograms were analysed both qualitatively and semi-quantitatively. The qualitative method consists of searching for the minerals present in the material analysed on the ba-

sis of the peaks emitted by the latter. The Brown and Brindley (1980) charts were used for this purpose. The quantitative method adopted is the peak area method. This involves indexing all the peaks of the minerals determined qualitatively. The area of each peak is then related to the sum of the areas of the diffracgram peaks (see Eq. 1). The result obtained corresponds to the proportion of the mineral in the sample. If a peak carries more than one mineral, its area is related to the number of minerals it carries. The proportion of mineral i in the material is given by the relationship:

$$P_i = \frac{1}{A} \sum_{k=1}^{n} \frac{A_p}{N_i} \quad (1)$$
<figure><img src="image_7.png" /><figcaption>Fig. 5. Soil sampling map.</figcaption></figure>

With $P_i$: proportion of mineral i; A: total area of the diffractogram; $A_p$: area of peak p; $N_i$: number of minerals on a peak p.

### 3.2.4. Methods for processing geochemical data

3.2.4.1. S/R ratio. the $SiO_2/(Fe_2O_3 + Al_2O_3)$ ratio is the weight ratio between the $SiO_2$ content of the material and the sesquioxide content ($Fe_2O_3 + Al_2O_3$). This ratio is used to define the degree of lateritisation of materials in a soil profile. S/R < 1.33: true laterites; 1.33 < S/R < 2: lateritic rocks; S/R > 2: non-lateritic materials.

3.2.4.2. Chemical weathering index. the chemical weathering index (CIA; Nesbitt and Young, 1982) is the most frequently used to characterise the chemical evolution of soils or sediments (Onana et al.,

2016; Nyassa Ohandja et al., 2020). The molar ratio used to calculate the values of this parameter is:

$$CIA = 100 \times \frac{Al_2O_3}{Al_2O_3 + CaO* + Na_2O + K_2O} \quad (2)$$

where $CaO* = amount of CaO presents only in silicates.

This index is used to assess the mobility of cations ($Ca^{2+}$, $Na^+$ and $K^+$) in relation to $Al^{3+}$. Fresh, unaltered material will have a low CIA (~50%). High CIA values (>80%) indicate intense alteration of the material studied. Its maximum value is 100%.

In general, CIA values for slightly altered igneous and metamorphic rocks are of the order of 55 or less (values of ~50 correspond to potassium feldspars). Heavily altered rocks, which produce residual clay minerals (kaolinite and chlorite), correspond to CIA
values close to 100. Values of 70-75 characterise clays (shales) due to the high proportion of clay minerals (Nesbit and Young, 1982) and CIA ~ 56 are typical of greywackes (McLennan et al., 2003; Garcia et al., 2014). In the case of this study the average CIA value is 96.38% which allows us to deduce that our materials are highly weathered.

### 3.2.4.3. Mafic index of alteration.
The mafic index of alteration (MIA) complements the chemical alteration index because it extends the CIA equation to include the Mg and Fe elements (Babechuck et al., 2014), many mafic minerals (pyroxene, olivine) are sensitive to chemical alteration, resulting in a loss of Mg in alteration profiles. Mg loss can be monitored independently using the Mg Index (MgI; maynard, 1992). Unlike Mg, the fate of Fe during weathering of most mafic minerals depends on redox. In reducing environments, ferrous iron (Fe²⁺) can be mobile and leached with Mg during the weathering of mafic minerals. In oxidising alteration environments, however, Fe is generally retained by the formation of highly insoluble ferric iron oxides or oxyhydroxides (Fe³⁺) (Driese, 2004) and thus enriched with Al. Because of this dichotomous redox behaviour of Fe, an MIA layout is proposed for each of the end-member alteration environments.

When the weathering environment is oxidising and Fe is retained, total Fe is considered as an immobile element with Al (Al₂O₃) and the MIA is calculated as follows:

$$MIA(O) = 100 \times \frac{(Al_2O_3 + Fe_2O_3)}{Al_2O_3 + Fe_2O_3(T) + MgO + CaO* + Na_2O + K_2O} \quad (3)$$

When the alteration environment is reductive and Fe is leached with Mg, total Fe is considered as a mobile element with Mg, Ca, Na and K and the MIA is calculated as follows:

$$MIA(R) = 100 \times \frac{Al_2O_3}{Al_2O_3 + Fe_2O_3(T) + MgO + CaO* + Na_2O + K_2O} \quad (4)$$

### 3.2.4.4. Parker alteration index.
Parker alteration index (WIP) is best applied to an alteration profile developed on a heterogeneous felsic rock. It takes into account base cations, the most mobile elements, which means that there is a significant contrast between the values calculated for the rock and those for the alteration profile. The downside of using only cations is that the WIP can quickly fall to zero or no longer evolve in highly altered profiles such as bauxites or ferrites. The data are in molar percentages. Eq. (5) is used to determine the Parker alteration index (WIP).

$$WIP = 100 \times \frac{2Na_2O}{0.35} + \frac{MgO}{0.90} + \frac{2K_2O}{0.25} + \frac{CaO}{0.70} \quad (5)$$

The WIP index takes into account the individual mobility of cations (Mg²⁺, Ca²⁺, Na⁺ and K⁺) as a function of the strength of their bond with oxygen atoms. It is greater than 100 for fresh material and drops to zero when there are no more cations.

### 3.2.4.5. Lateritisation index.
knowledge of the CIA (Chemical Index of Alteration) and MIA (mafic index of alteration) indices alone is not sufficient to measure the stages of chemical alteration. Another approach is based on quantifying the phenomenon of lateritisation (Babechuck et al., 2014). During this phenomenon, the dissolution of quartz, the formation of kaolinite and the enrichment of iron oxides, are the dominant processes mainly resulting in a relative loss of Al, Si and Fe. Laterisation characterised by the laterisation index (IOL: Index of Laterisation) is defined using the oxides SiO₂, Al₂O₃ and Fe₂O₃ as indicated by the following Eq. (6):

$$IOL = 100 \times \frac{(Al_2O_3 + Fe_2O_3)}{(SiO_2 + Al_2O_3 + Fe_2O_3)} \quad (6)$$

The data for the oxides are in mass percentages. The IOL index completes the SAF ternary diagram (Babechuck et al., 2014).

### 3.2.4.6. Loss on ignition (LOI).
The volatile element content of a sample, known as loss on ignition, is proportional to the quantity of hydrated minerals. During chemical alteration, the primary magmatic minerals, which are largely anhydrous, are replaced by phyllosilicates (smectites, illites and kaolinites). Loss on ignition is a useful parameter for assessing the degree of weathering (Duzgoren-Aydin et al., 2002; Ndzié Mvindi, 2019), although mineralogy ultimately controls the degree of hydration (Duzgoren-Aydin et al., 2002).

## 4. Results

### 4.1. Morpho-structural description of the lateritic soils of Ngaoundel

The borrow material was identified using hand-dug pits. The borrow is covered by topsoil with an average thickness of 0.20 m. A total of 15 shafts were dug and 15 reworked samples were taken. These samples were taken from an average depth of between 0.10 m and 1.50 m. The macroscopic descriptions of the lateritic soil samples taken in Ngaoundel are shown in Table 2.

After visual analysis in the field and particle size analysis in the laboratory, the size of the lateritic gravel sampled at Ngaoundel ranged from 0.08 to 50 mm. In the Ngaoundel locality, the percentage of the fraction of particles smaller than 0.08 mm made up of silts and clays varies between 14.6% and 24.7%, with an average of 19.9%; the proportion of elements with a size ranging from 2 mm to 80 µm (sand) varies between 1.6% and 21.8%, with an average value of 10.06%; Coarse elements are divided into two groups (gravel and pebbles); gravel content varies between 53% and 76.6% with an average value of 64.92%; pebbles content varies between 1.8% and 10.8% with an average value of 5.10% (Fig. 6).

#### 4.1.1. Soil mineral association

Analysis and interpretation of the various diffractograms enabled us to determine the different mineral phases found in each soil sample analysed. In ascending order we have: quartz, goethite, gibbsite, kaolinite, haematite, magnetite, anatase and boehmite. Fig. 7 shows the diffractograms for the seven samples.

The diffractograms obtained show the signals from the mineral compounds detected, corresponding to characteristic peaks for specific 2θ values. These are the peaks used for semi-quantification Results. The x-axis represents the diffraction angle 2θ (two theta), where θ is the angle between the incident X-rays and the crystal planes. The y-axis represents the intensity of the diffraction peaks, which indicates the amount of X-rays diffracted at each angle.

Quartz is the most abundant mineral, followed by goethite. The results of the semi-quantitative analysis presented in Table 3 show that the quartz content varies from 24.02% (NG02) to 73.3% (NG29), giving an average of 38.27%. Goethite content varies between 12.6% (NG19) and 16.77% (NG54), with an average of 13.98%. Gibbsite content varies from 6.04% (NG54) to 13.71% (NG02) with an average value of 10.59%. Kaolinite varies from 3.2% (NG29) to 11.45% (NG39) and haematite from 2.4% (NG29) to 12.19% (NG19). magnetite (2.68% to 9.49%), anatase (2.3% to 11.57%) and boehmite (2.68% to 9.5%) are the least represented minerals in the study area. However, magnetite and boehmite were not identified at site NG29.

The high dominance of quartz in samples NG29 and NG54 reflects weaker alteration in these samples. Conversely, high levels of gibbsite and goethite in NG19 suggest an advanced stage of alteration with enrichment in sesquioxides. This spatial variation reflects the influence of microenvironments on alteration trajectories.
<table><caption>**Table 2**
Detailed description of Ngaoundel lateritic soil samples (Aboubakar et al., 2024).</caption><thead><tr><th>Samples</th><th>Description</th></tr></thead><tbody><tr><td>NG02</td><td>Overall reddish colour (2.5 YR 5/8), clay matrix with dark brown nodules. Dark brown. Grain size varies from millimeter to centimeter with rounded to sub-rounded.</td></tr><tr><td>NG11</td><td>Lumpy structure.</td></tr><tr><td>NG06</td><td>Overall colour dark brown (7.5 YR 5/8), clay matrix consisting of ferruginous nodules with black cortex.</td></tr><tr><td>NG40</td><td>The shape of the nodules is rounded to sub-rounded.<br>Nodules millimetric (60 %) to multi-centimetric (40 %).<br>Lumpy structure.</td></tr><tr><td>NG13</td><td>Overall colour reddish yellow (7.5 YR 6/8), sandy-clay matrix consisting of rounded to sub-rounded nodules.<br>The nodules are millimeter to multi-centimeter in size.<br>Lumpy structure.</td></tr><tr><td>NG08</td><td>Overall colour dark brown (7.5 YR 5/6), sandy-clay matrix.</td></tr><tr><td>NG29</td><td>Two types of nodules can be observed: red ferruginous nodules ranging in size from a millimeter to a centimeter, and small nodules with the presence of a few millimeters of quartz crystals.</td></tr><tr><td>NG54</td><td>Some nodules are crumbly on the hand.<br>Lumpy structure.</td></tr><tr><td>NG18</td><td>Overall color dark brown (7.5 YR 5/8), clay matrix made up of two types of nodules: ferruginous nodules with black cortex with a red colour and a nodule with a dark brown color.</td></tr><tr><td>NG19</td><td>Grain size varies from a millimeter to a centimeter, with rounded to sub-rounded shapes.</td></tr><tr><td>NG50</td><td>Overall color orange-brown, sandy-clay matrix with dark brown nodules of millimeter nodules and millimeter to multi-centimeter orange nodules.</td></tr><tr><td>NG28</td><td>The nodules are rounded to sub-rounded in shape.</td></tr><tr><td>NG42</td><td>The nodules are rounded to sub-rounded in shape.<br>Lumpy structure.</td></tr><tr><td>NG48</td><td>Overall colour yellowish brown (10YR 5/8), sandy clayey matrix.<br>Rounded to sub-rounded nodule.<br>Millimetric to centimetric nodule 60-70.<br>30-40 % multi-centimeter nodule.<br>Lumpy structure, nodules crumble by hand.</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Sieve size curves for various Ngaoundel soil samples.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. Diffractograms of soil samples.</figcaption></figure>

<caption>**Table 3**</caption>

Semi-quantitative estimate of the mineralogical composition of Ngaoundal soil samples in percent (%) (Aboubakar et al., 2024).

<table><caption>**Table 4**</caption><thead><tr><th>Samples</th><th>Quartz</th><th>Goethite</th><th>Gibbsite</th><th>Kaolinite</th><th>Hematite</th><th>Magnetite</th><th>Anatase</th><th>Boehmite</th><th>Total</th></tr></thead><tbody><tr><td>NG02</td><td>24.02</td><td>14.24</td><td>13.71</td><td>11.17</td><td>9.49</td><td>9.49</td><td>10.05</td><td>8.10</td><td>100.27</td></tr><tr><td>NG13</td><td>31.39</td><td>13.30</td><td>13.31</td><td>10.23</td><td>8.19</td><td>8.19</td><td>7.84</td><td>7.50</td><td>99.95</td></tr><tr><td>NG19</td><td>18.18</td><td>12.60</td><td>11.98</td><td>11.36</td><td>12.19</td><td>12.60</td><td>11.57</td><td>9.50</td><td>99.98</td></tr><tr><td>NG28</td><td>35.01</td><td>13.20</td><td>9.33</td><td>8.94</td><td>8.94</td><td>8.17</td><td>9.3</td><td>7.39</td><td>100.28</td></tr><tr><td>NG29</td><td>73.30</td><td>14.40</td><td>7.20</td><td>3.20</td><td>2.40</td><td></td><td>2.3</td><td></td><td>102.80</td></tr><tr><td>NG31</td><td>24.30</td><td>13.40</td><td>12.56</td><td>11.45</td><td>10.61</td><td>9.20</td><td>9.77</td><td>8.35</td><td>99.64</td></tr><tr><td>NG54</td><td>61.74</td><td>16.77</td><td>6.04</td><td>4.02</td><td>3.35</td><td>2.68</td><td>2.68</td><td>2.68</td><td>99.96</td></tr><tr><td>Average</td><td>38.27</td><td>13.98</td><td>10.59</td><td>8.62</td><td>7.88</td><td>8.38</td><td>7.64</td><td>7.25</td><td>100.41</td></tr></tbody></table>

Weight percentage of major element oxides in Ngaoundal soil samples (Aboubakar et al., 2024).

<table><thead><tr><th>Oxides</th><th>NG02</th><th>NG13</th><th>NG19</th><th>NG28</th><th>NG29</th><th>NG31</th><th>NG54</th><th>Max</th><th>Min</th><th>Ave.</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>22.79</td><td>22.63</td><td>16.12</td><td>30.648</td><td>68.09</td><td>25.23</td><td>69.38</td><td>69.38</td><td>16.12</td><td>37.8</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>19.71</td><td>22.93</td><td>18.11</td><td>20.196</td><td>11.14</td><td>20.61</td><td>11.37</td><td>22.93</td><td>11.14</td><td>17.5</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>43.28</td><td>35.82</td><td>50.34</td><td>32.21</td><td>10.67</td><td>37.84</td><td>10.259</td><td>50.34</td><td>10.259</td><td>31.2</td></tr><tr><td>CaO</td><td>0.12</td><td>0.86</td><td>0.25</td><td>0.659</td><td>0.43</td><td>0.107</td><td>0.141</td><td>0.86</td><td>0.107</td><td>0.39</td></tr><tr><td>MgO</td><td>0.047</td><td>0.065</td><td>0.044</td><td>0.097</td><td>0.208</td><td>0.078</td><td>0.137</td><td>0.208</td><td>0.044</td><td>0.103</td></tr><tr><td>SO<sub>3</sub></td><td>0.016</td><td>0.007</td><td>0.012</td><td>0.007</td><td>0.014</td><td>0.009</td><td>0.016</td><td>0.016</td><td>0.007</td><td>0.011</td></tr><tr><td>K<sub>2</sub>O</td><td>0.084</td><td>0.14</td><td>0.053</td><td>0.191</td><td>0.6</td><td>0.26</td><td>0.269</td><td>0.6</td><td>0.053</td><td>0.25</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.011</td><td>0.02</td><td>0.015</td><td>0.013</td><td>0.029</td><td>0.019</td><td>0.016</td><td>0.029</td><td>0.011</td><td>0.018</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.19</td><td>0.14</td><td>0.2</td><td>0.14</td><td>0.057</td><td>0.178</td><td>0.058</td><td>0.2</td><td>0.057</td><td>0.135</td></tr><tr><td>TiO<sub>2</sub></td><td>1.09</td><td>1.06</td><td>1.05</td><td>1.082</td><td>1.04</td><td>1</td><td>0.887</td><td>1.09</td><td>0.887</td><td>1.02</td></tr><tr><td>ZnO</td><td>0.002</td><td>0.002</td><td>0.002</td><td>0.002</td><td>0.002</td><td>0.002</td><td>0</td><td>0.002</td><td>0</td><td>0.001</td></tr><tr><td>Mn<sub>2</sub>O<sub>3</sub></td><td>0.022</td><td>0.029</td><td>0.019</td><td>0.017</td><td>0.073</td><td>0.033</td><td>0.035</td><td>0.073</td><td>0.017</td><td>0.035</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.082</td><td>0.13</td><td>0.155</td><td>0.071</td><td>0.036</td><td>0.088</td><td>0.025</td><td>0.155</td><td>0.025</td><td>0.085</td></tr><tr><td>PAF</td><td>12.51</td><td>16.13</td><td>13.59</td><td>14.65</td><td>7.57</td><td>14.52</td><td>7.38</td><td>16.13</td><td>7.38</td><td>12.2</td></tr><tr><td>Total</td><td>99.954</td><td>99.963</td><td>99.96</td><td>99.983</td><td>99.959</td><td>99.974</td><td>99.973</td><td>99.983</td><td>99.954</td><td>99.96</td></tr><tr><td>S/R</td><td>0.35</td><td>0.36</td><td>0.22</td><td>0.55</td><td>2.8</td><td>0.41</td><td>2.98</td><td>2.98</td><td>0.22</td><td>1.09</td></tr></tbody></table>

### 4.1.2. Concentration of major elements in soils (%)

The silica/sesquioxide weight ratio (S/R) provides an approximation of the degree of lateritization of a lateritic deposit. The higher the ratio, the less lateritized the profile. The chemical composition (major and minor elements) of the soil samples taken, together with the S/R ratio, are given in Table 4. Table 4 shows an average of 36.41 % SiO₂, followed by 31.48 % Fe₂O₃ and 17.72 % Al₂O₃, and a low content of 1.02 % TiO₂ and 0.36 % CaO.

At this level, the high dominance of SiO₂ in samples NG29 and NG54 reflects weaker alteration in these samples. Conversely, high Fe₂O₃ contents in NG19 suggest an advanced stage of alteration

with enrichment in sesquioxides. This spatial variation reflects the influence of microenvironments on alteration trajectories.

### 4.1.3. Weathering indices and parameters

The values of indices and parameters used to characterize weathering are given in Table 5. In the Ngaoundal laterites, the S/R ratio values range from 0.22 to 2.98, with an average of 1.09. This average value of between 1.33 and 2.98 indicates that the soil samples taken are lateritic materials. AIC values varied between 91.31 and 96.38 % in laterites, with an average of 96.38 %. These values, close to 100 %, suggest the disappearance of mobile elements
<table><caption>**Table 5**
Index values and chemical parameters in laterites in the study area.</caption><thead><tr><th>Samples</th><th>MIA (%)</th><th>WIP</th><th>IOL (%)</th><th>CIA (%)</th><th>PF</th><th>S/R</th></tr></thead><tbody><tr><td>NG02</td><td>99.58</td><td>95.85</td><td>73.43</td><td>98.92</td><td>12.51</td><td>0.35</td></tr><tr><td>NG13</td><td>98.18</td><td>253.5</td><td>72.19</td><td>95.74</td><td>16.13</td><td>0.36</td></tr><tr><td>NG19</td><td>99.47</td><td>91.57</td><td>80.93</td><td>98.27</td><td>13.59</td><td>0.22</td></tr><tr><td>NG28</td><td>98.2</td><td>265.14</td><td>63.09</td><td>95.9</td><td>14.65</td><td>0.55</td></tr><tr><td>NG29</td><td>94.5</td><td>581.11</td><td>24.26</td><td>91.31</td><td>7.57</td><td>2.8</td></tr><tr><td>NG31</td><td>99.21</td><td>242.8</td><td>69.84</td><td>98.16</td><td>14.52</td><td>0.41</td></tr><tr><td>NG54</td><td>97.46</td><td>259.7</td><td>23.76</td><td>96.38</td><td>7.38</td><td>2.98</td></tr><tr><td>Moy.</td><td>98.09</td><td>255.67</td><td>58.21</td><td>96.38</td><td>12.33</td><td>1.09</td></tr><tr><td>Min.</td><td>94.5</td><td>91.57</td><td>23.76</td><td>91.31</td><td>16.13</td><td>0.22</td></tr><tr><td>Max.</td><td>99.58</td><td>581.11</td><td>80.93</td><td>98.92</td><td>7.38</td><td>2.98</td></tr></tbody></table>

PF: Fire loss; CIA = 100 × [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)], Nesbitt et Young (1982); MIA = 100 × [(Al₂O₃ + Fe₂O₃(T))/(Al₂O₃ + Fe₂O₃(T) + MgO + CaO* + Na₂O + K₂O)], Babechuk et al. (2014); IOL = 100 × [(Al₂O₃ + Fe₂O₃(T)/(SiO₂ + Al₂O₃ + Fe₂O₃(T))], Babechuk et al. (2014); WIP = 100 × [2Na₂O/0.35 + MgO/0.90 + 2K₂O/0.25 + CaO/0.70], Parker (1970).

in these laterites and indicate the presence of kaolinite and gibbsite (Nyassa Ohandja et al., 2020). MIA values range from 94.50 to 99.58%, with an average of 98.09%. These average values, which are close to 100%, indicate complete removal of labile elements in laterites (Babechuk et al., 2014). PF values range from 7.38 to 16.13%, with an average of 12.33%. IOL values range from 23.76 to 80.93%, averaging 58.21%. Parker's weathering index (WIP) values range from 91.57 to 581.11%, with an average of 255.67%.

## 5. Discussion

The lateritic soils of Ngaoundal are nodular with a silty or clayey matrix. The average thickness of the layer is 1.3 m. The dominant colour is dark brown (7.5YR 5/6). This morphology is commonly observed in the equatorial zone (Ekodeck, 1984; Gidigasu, 1983; Kamtchueng et al., 2015 and Onana et al., 2015).

The laterites from the Ngaoundal locality are silico-ferrugino-aluminous. These observations corroborate those made on laterites derived from basalts in West Cameroon (Hyoumbi et al., 2018) and on those derived from sedimentary rocks in the Douala coastal basin (Ndome et al., 2014). The moderate Al₂O₃ and Fe₂O₃ contents are linked to the mineralogical composition of the parent rock. SiO₂ contents decrease from sound rocks to laterites while, Al₂O₃ and Fe₂O₃ contents increase. This behaviour is different from that observed in laterites on pyroxenites in south-east Cameroon (Ebah Abeng, 2013), Brazil (Colin et al., 1990) and laterites on ultrabasic rocks in New Caledonia (Traoré et al., 2006). The average Fe₂O₃ content in Ngaoundal laterites (31.2%) is due to the high proportions of iron nodules in these materials.

Rossiter (2004) established a classification of soils according to the degree of laterisation by evaluating the silica - sesquioxide (S-S) ratio (SiO₂ / (Fe₂O₃ + Al₂O₃)). Consequently, soils with an S-S ratio > 2 are considered to be non-lateritic. For lateritic soils in the study area, this S-S ratio is between 1.33 and 2, and for true laterites, this ratio is <1.33. The position of the materials on the straight line representing the S/R ratio (Fig. 8) shows that the lateritic gravelly soils in the Ngaoundal region are, on the one hand, 'true laterites' for NG19, NG02, NG13, NG31 and NG28; these remarks corroborate those of Mvindi (2011), Ntoh (2013), Nyemb et al. (2013) and Kamtchueng et al. (2015) on materials developed respectively on Batchenga amphibolites, Mbouda basalts, Bamendjou - Batchum basalts and Mfou micaschists; and secondly, materials NG29 and NG54 are 'non-lateritic materials'. These remarks corroborate those of the materials studied by Milogo et al. (2008).

The binary diagram PF versus CIA enables us to appreciate the mineralogical transformations in the laterites of the Ngaoundal lo-

cality. The laterites studied are characterized mainly by minerals resistant to alteration (Fig. 9).

The predominant pedogenetic process in laterites from the Ngaoundal locality is ferruginization (Fig. 10). These alteration processes are confirmed by the SiO₂-Al₂O₃-Fe₂O₃ (SAF) diagram, which differentiates between kaolinitized, lateritized and bauxitized materials.

Associated with MIA and IOL, the weathering process can also be quantified graphically in the systems A-Fe-Mg-Ca-Na-K and S-A-F with the help of 02 ternary diagrams (Fig. 11 and Fig. 12) according to Nesbitt and Young (1982) and Babechuk et al. (2014). The A-L-F diagram (Fig. 11) shows a very significant decrease in labile elements and an overall moderate to strong lateritization of the lateritic gravels studied.

The representation of the chemical parameters of the Ngaoundal soil samples in the SiO₂-Al₂O₃-Fe₂O₃ ternary system diagram (Schellmann, 1986) indicates that these soils fall into three classes, namely: the "strong lateritization" class for sample NG19, then the "moderate lateritization" class for samples NG31, NG13, NG28 and NG02, and finally the "Kaolinitized" class for samples NG29 and NG54 (Fig. 12). Laterites in the study area are moderately to strongly lateritized, while some laterite samples are kaolinitized (Fig. 12). This evolution of the lateritization process in the laterites studied differs from observations made on laterites derived from basalts in the Southwest Cameroon region (Nyemb, 2021) and, carbonate rocks in China (Xiao et al., 2014). Indeed, laterites on basalts in Southwest Cameroon show the lowest levels of lateritization, which are low to moderate, while those in China show high levels. On the other hand, this evolution is similar to that encountered in migmatites (Nzabakurikiza et al., 2012), charnockites (Onana et al., 2017), chloritoschists (Onana et al., 2016) and granites (Ndzié Mvindi, 2019), in the South Cameroon Plateau zone.

Associated with CIA, the A-CN-K diagram in Fig. 13 shows that the lateritic gravelly soils follow an evolution parallel to A-K. This corresponds to a total evacuation of Ca and Na. This corresponds to a total evacuation of Ca and Na. This evacuation is accentuated in all materials in the study area. K enrichment is low and almost non-existent in the study area. This evolution is similar to those encountered in lateritic gravels developed on gneiss at Obala in Central Cameroon (Ndzie Mvindi, 2019), on basalt of Bidar in India (Babechuk et al., 2014) in West Cameroon (Hyoumbi et al., 2018), on basalt in South West Cameroon (Nyemb, 2021) in Cameroon. However, this evolution is different from that observed in laterites derived from the Chhindwara basalts in India (Babechuk et al., 2014).

The position of the lateritic soils studied in the AF-CNK-M diagram (Fig. 14), shows that the dissolution of feldspars (Ca, Na and K losses) is proportional to that of mafic minerals (Mg loss). This fact is corroborated by the equal values of CIA and MIA.

DRX showed that soils in the study area consist on average of 38.27% quartz, 13.98% goethite, 10.59% gibbsite, 8.62% kaolinite, 7.88% hematite, 8.38% magnetite, anatase and boehmite. These results are close to those obtained by Nzabakurikiza et al. (2016), who obtained 40.77% quartz, 10.9% goethite, 7.14% gibbsite and 9.9% hematite on lateritic gravels of granitic origin in the east; at this level only kaolinite, which has a very low percentage compared with Ngaoundal. These results are also in line with work carried out on lateritic soils in the South Cameroon region (Ekodeck, 1984; Onana, 2010; Lemougna et al., 2011; Tsozué et al., 2012; Kamtchueng et al., 2015). However, in the work of Onana et al. (2015), these results differ from those of Ngaoundal. Tsozué et al. (2012) showed that iron oxides in southern Cameroon soils are always associated with Kaolinite; this observation is also valid for the various Ngaoundal soil samples. Mineralogical investigations also indicated the absence of swelling clays such as mont-
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Representation of the chemical parameters of Ngaoundal soils on the S/R diagram after Rossiter (2004).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Covariation of loss on ignition (LOI) and chemical alteration index (CIA) in laterites from the study area in the diagram by Babechuk et al. (2014).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Comparative covariation loss on ignition (LOI) - lateritization index (LOI) in laterites from the study area in the diagram by Babechuk et al. (2014).</figcaption></figure>

morillonite, whose undesirable swelling characteristics are not expected in these soils. The presence of gibbsite (less than 11%) indicates that these materials evolve in an environment rich in Al₂O₃ and in a region with high rainfall, as shown by Ngaoundal's annual rainfall, which exceeds 1300 mm (PNDP, 2015).

Given that the parent rock at Ngaoundal is a granite that is rich in quartz (SiO₂), feldspar (orthose: KAISi₃O₈; plagioclase: Na or CaAlSi₃O₈) and mica (biotite/muscovite) and that after the alteration processes we obtained kaolinite, gibbsite and goethite, this indicates a leaching that could be total or partial depending on the type of product obtained. In some cases, the base and silica are

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Comparative position of the different degrees of weathering of laterites from the study area and those of other authors in the A-L-F molar diagram of Babechuk et al. (2014).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Position of Ngaoundal lateritic soils in the SiO₂-Al₂O₃-Fe₂O₃ ternary diagram (Schellemann, 1986).</figcaption></figure>

completely removed, while in other cases the base is partially removed and the silica completely removed.

The formation of kaolinite (formula) means that in the original parent rock there has been a partial leaching of the base (KAl, NaAl, CaAl) in the feldspars of the rock and a total leaching of the silica from these feldspars (reference) which is generally characteristic of alteration in tropical regions (reference: some authors who have worked in the same circumstances).
<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Comparative position of the different degrees of weathering of laterites from the study area and those of other authors in the Al₂O₃-CaO⁺ + Na₂O-K₂O (A-CN-K) molar diagram by Nesbitt and Young (1982).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 14.** Comparative illustration of the different degrees of weathering of Ngaoundal lateritic soils in the AF-CNK-M diagram.</figcaption></figure>

In hot, humid climates, the potassic feldspars and plagio-classes present in granitoids alter by hydrolysis into kaolinite (via monosiallitisation), then into gibbsite under prolonged leaching. This process is accentuated by effective drainage, which encourages the leaching of basic cations and the concentration of sesquioxides".

# 6. Conclusions

The study area, which is the locality of Ngaoundal, is located at 137 km south of the town of Ngaoundéré by rail. The prevailing climate is Sudano-Guinean, characterized by two contrasting seasons of unequal duration. The hydrographic network is well organized in dendritic form, with low density. Ferralitic soils are the most widespread. The aim of this study was to assess the weatherability of Ngaoundal soils through mineralogical (XRD) and geochemical (XRF) studies. Fieldwork was used to identify, locate and collect lateritic soil samples in the study area. Fifteen samples of lateritic soils developed from granitic rocks were taken from the nodular

level and subjected to morpho-structural observations. Seven samples were then subjected to geochemical and mineralogical analyses.

The results show that Ngaoundal's lateritic soils are nodular with a silty or clayey matrix. The average thickness of the laterite layer is 1.3 m. The dominant color is dark brown (7.5YR 5/6) with a lumpy structure and a silty or clayey texture. These materials are silico-ferruginous-aluminous, with S/R ratios ranging from 0.22 to 2.98, averaging 1.09. Ngaoundal soils are composed of quartz (38.27%), goethite (13.98%), gibbsite (10.59%), kaolinite (8.62%), hematite (7.88%), magnetite (8.38%), anatase and boehmite. The ternary alteration quantification diagrams were exploited on the assumption of the existence of 07 lateritic gravelly samples in the study area. The laterites studied are characterized mainly by minerals resistant to weathering. The predominant pedogenetic process in laterites from the Ngaoundal locality is ferruginization. These laterites are moderately to strongly lateritized, while some laterite samples are kaolinitized. Alterological diagrams indicate that monosiallitization and alloying are the dominant hydrolytic processes, with alloying predominating. Mineralogical investigation indicates the absence of swelling clays such as smectite and montmorillonite. Chemical analyses show that these soils are "true laterites" with a high concentration of sesquioxides. Quantitative geological parameters indicate that the materials studied are tropical lateritic soils.

# Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

# CRediT authorship contribution statement

**Abdoul Aboubakar:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Ilalie Bertille Kentsa Manefouet:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation. **Loique Rodolph Mbounou Azefack:** Validation, Supervision, Software, Resources, Project administration, Methodology. **Yannick Langollo Tchedele:** Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation. **Dalil Adoulko:** Resources, Project administration, Methodology, Investigation, Conceptualization. **Demonstel Yvan Ntomb:** Writing – original draft, Visualization, Supervision, Software, Resources. **Abakar Mahamat:** Validation, Supervision, Software, Resources, Methodology. **Jacques Wadjou Wassouo:** Software, Project administration, Methodology, Investigation. **Quentin marc Anaba Fotze:** Writing – original draft, Visualization, Supervision, Resources, Project administration, Methodology.

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