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BY-NC-ND 4.0 license Open Access Published by De Gruyter Open Access December 2, 2017

Geochemistry of Selected Kaolins from Cameroon and Nigeria

  • Nenita N. Bukalo EMAIL logo , Georges-Ivo E. Ekosse , John O. Odiyo and Jason S. Ogola
From the journal Open Geosciences

Abstract

The geochemical characteristics of selected kaolins from Cameroon and Nigeria are presented, with an attempt to elucidate on their possible industrial applications by comparing them to world-known kaolin deposits. Major oxides concentrations were subjected to factor analyses in interpreting their relationships. Geochemical indices, including chemical index of alteration (CIA), chemical index of weathering (CIW) and the index of compositional variability (ICV) were computed and plotted on binary and ternary diagrams to determine the intensity of weathering of the kaolins and discriminate their different source rock types. Kaolinite was the major phase, followed by quartz, illite and goethite as minor phases. Minerals in trace phases included smectite, anatase, muscovite, gibbsite, microcline, palygorskite and calcite. Mean abundances of major oxides in wt% were: SiO2 (56.96)>Al2O3 (24.09)>Fe2O3 (3.78)>TiO2 (1.53)> K2O (1.26)> MgO (0.27)>CaO (0.20)>Na2O (0.17)>P2O5 (0.05)>MnO (0.04). The CIW versus CIA and ICV versus CIA plots showed that most of the kaolins clearly depicted extreme silicate weathering. The current applications of kaolins from Cameroon and Nigeria include ceramics and manufacturing of bricks and tiles. Low MgO, CaO, Na2O, K2O and TiO2 further position the kaolins for pharmaceutics, cosmetics, rubber and plastic applications. Thus, the studied kaolins have the potential to contribute to improved economic development of these countries.

1 Introduction

Twenty-five kaolin occurrences in Cameroon and 40 in Nigeria have been reported [1]. Though these kaolin occurrences could be economically viable, most of them have not been studied. Cameroon is a lower middle income country with a population of 23.3 million people, whereas Nigeria’s population is about 178.5 million people [2]. Cameroon and Nigeria rank 158 and 170 on 189, respectively, on the Doing Business 2015 Report [3]. Chronic poverty stands at about 26% in Cameroon and 30.4% in Nigeria [4]. Boosting Cameroon’s and Nigeria’s economies will require new investments, and kaolin exploration and exploitation could be one of these.

Kaolin deposits could be primary (hydrothermal, residual or mixed hydrothermal and residual deposits) or secondary (formed by erosion and transportation of clay particles and their deposition in lacustrine, paludal, deltaic and lagoonal environments) [5] depending on their genesis [6]. Hydrothermal kaolins are formed from the hydrothermal alteration of alumina-silicate rocks [5]. These types of kaolins are closely related to the tectonic framework of a deposit and the alteration of parent rocks [7]. Residual kaolins usually form in high rainfall, subtropical or tropical climate; because elevated temperatures and high rainfall increase the rate of breakdown of the primary minerals to clay minerals [8].

Some of the world-known and most utilised kaolins are the Capim River kaolin of Brazil [9]; Patagonia kaolins in Argentina [10]; the Georgia kaolins in USA [11]; the Cornwall kaolin, occurring in England [12]; and the Pugu kaolin in Tanzania [13], globally recognised for providing the standard of kaolinite (29-1488) in X-ray diffraction [14]. Other kaolin deposits in Africa include the Makoro and Kgwakgwe primary kaolins in Botswana [6] and the Abu Darag kaolin in Egypt [15].

The wide range of applications of kaolins include construction, agricultural, textile, paper, pharmaceutical, ceramic, electrical, paint, nuclear energy, polymer and petroleum industries [1, 16, 17, 18, 19]. Mineralogical and geochemical characteristics of kaolins are important properties for their quality and potential industrial applications, as physical properties are closely related to the mineralogy and geochemistry [20]. Hence, the objective of this paper was to compare the geochemical characteristics of Cameroon and Nigeria kaolins to known and utilised kaolins from Capim River (Brazil), Pugu (Tanzania), and Kgwakgwe and Makoro (Botswana), and infer on their industrial applications.

2 Geology of studied kaolin sites

2.1 Geology of kaolin sites from Cameroon

The studied kaolins are found in Cameroon and Nigeria (Figure 1). The geology of Cameroon is predominantly made up of Precambrian basement rocks, Cretaceous-Tertiary sedimentary basins, the Tertiary Cameroon Volcanic Line and Quaternary alluviums. The Precambrian mainly comprises the Archean Ntem Complex, the Paleoproterozoic Nyong Series, the Yaounde Group and the Pan-African granitoids [21, 22]. The main sedimentary basins in Cameroon are the Mamfe, the Rio del Rey and the Douala/Kribi-Campo Basins. The description of the geology of these basins are found in [23, 24, 25]. The Cameroon Volcanic Line (CVL) is the main Tertiary feature in Cameroon, made up of volcanic massifs (Pagalu, Bioko, Sao Tome and Principe, Mounts Cameroon, Rumpi, Manenguba, Bamboutos, Mbam, and Oku) [26, 27, 28].

Figure 1 Location map of Cameroon and Nigeria showing sites of studied kaolins
Figure 1

Location map of Cameroon and Nigeria showing sites of studied kaolins

The studied kaolins in both countries were hosted either crystalline rocks (migmatites, gneisses, quartzites and granites) or sedimentary rocks (ironstones, sandstones, shales and limestones). The geology of the localities of the studied kaolins is summarised in Table 1.

Table 1

Summary of the geology of the localities of the kaolin sites

Kaolin sitesLocationHost rocksAgeGenesisSources
C1EdikiSandstones, shales and limestonesCretaceousPrimary[29]
C2MissoleII Marl, clay with lenses of sand, fine to coarse crumbly sandstoneTertiarySecondary[30]
C3BomkoulDeltaic facies interstratified with volcano-clastics layersTertiarySecondary[31]
C4MayouomGranite-gneiss mylonitePrimary[19]
C5Ntamuka[32]
C6BatiePanafrican granitoidsProterozoicPrimary[33]
C7BangouaUndifferentiated gneissTertiaryPrimary[33]
N1OrinMigmatite-gneiss, quartzite, quartz-schist, granite with aplitic intrusion in some places and charnockitesCretaceous-RecentPrimary[34]
N2AbeokutaBasal conglomerate to sand to clay-shale faciesCretaceousPrimary/Secondary[35]
N3IlorinBiotite hornblende gneiss with intercalated amphibolites, porphyritic granites and granite gneissPrecambrianPrimary[36]
N4IyukuSandstones and granitic rocks[37]
N5IbadanMigmatites, gneiss, quartzites, Panafrican granitoidsProterozoic[38]
N6BeninUnconsolidated white to yellowish sandsTertiary[38]
N7AgbajaOolitic ironstonesCretaceousPrimary[39]
  1. C = Samples from Cameroon; N = samples from Nigeria

Sites C1, C2 and C3 are from three different Formations of the Douala Sub-Basin, namely the Mundeck Formation, the Nkappa Formation and the Matanda Formation, respectively. Early Cretaceous Mundeck Formation is made up of sandstones; the Paleocene to Middle Eocene Nkappa Formation is composed of marls, shales and calcareous sandstones; and the Miocene Matanda formation comprises of clays and sands [40]. Site C4 is found at Foumban shear zone, and the kaolin is found on a granite-gneiss mylonite [41]. Sites C6 and C7 are located in western Cameroon. The area is part of the Central Cameroon Shear Zone associated with abundant Panafrican granitoids (C6) and undifferentiated granite (C7), as well as veins of pegmatite, quartzite and aplites with occasionally monzonite, granodiorite and monzodiorite [33].

2.2 Geology of kaolin sites from Nigeria

The geology of Nigeria is made up of about 50% of crystalline rocks and about 50% of sedimentary rocks. The crystalline rocks are mainly made up of migmatite-gneiss complexes; the older metasediments; the younger metasediments; the older granites; the younger granite alkaline ring complexes, while volcanic rocks represent a Cenozoic anorogenic phase [38]. Seven sedimentary basins are found in Nigeria: the Anambra Basin, Benue Trough, Benin Basin, Bida Basin, Bornu Basin, Niger Delta and Sokoto Basin.

Site N1 is located at Orin, whose geology comprises of migmatite-gneiss, quartzite, quartz-schist, granite with aplitic intrusion in some places and charnockites [34] (Table 1). Site N2 is located in Abeokuta, in the Abeokuta Formation, which is made up of basal conglomerate to sand to clay-shale facies [35]. The Ilorin kaolin (N3) was collected from Ilorin [36]. The geology of Ilorin is made up of Eburnean migmatite complex, Pan African older granite series, and the area is covered by dark sandy or clayey loamy top soils [36]. N4 was collected form Iyuku, which is made up of Cretaceous sandstones of the Lokoja Formation and granitic rocks of the older granite series [37] The Ibadan kaolin (N5) occurs in the Migmatite-Gneiss-Quartzite Complex, and the Pan African Granitic Series [38] whereas the Benin kaolin (N6) occurs in the Benin Formation of the Niger Delta, which comprises continental or fluviatile sands, gravels and backswamp deposits [42]. The Bida kaolins (N7) are located in the Agbaja Formation of the Bida Basin, which is made up of oolitic ironstones [39].

3 Methods

Data for this study was collected from 12 articles [29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39]. It is reported in these articles that mineralogical analyses were carried out using the X-Ray Diffraction (XRD) method, whereas major oxides were determined using one of the following: X-Ray Fluorescence (XRF) spectrometry, Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) or Atomic Absorption Spectroscopy (AAS). The methods of analyses of the studied kaolins are described in the articles.

Univariate statistics were applied on the data to determine the means of the major oxides in selected kaolin samples from Cameroon and Nigeria. The concentrations of the major oxides of these kaolins were compared to those of well-known deposit sites worldwide, which included the Capim River, Pugu, Kgwakgwe and Makoro kaolins. Factor analysis was carried out on the data set to interpret the structure within the variance–covariance matrix of a multivariate data collection [43]. The general equation for factor analysis is given in Equation 1 [44]:

zij=af1f1i+af2f2i+af3f3i++afmfmi+efi(1)

Where, z is the measured variable, a is the factor loading, f is the factor score, e is the residual term accounting for errors, i is the sample number, m is the total number of factors. Factors with a total variance less than 4% were not considered, in line with Davis [45]. The Statistical Package for Social Sciences (SPSS) software version 17 was used to carry out correlation and factor analysis.

The chemical index of alteration (CIA), the chemical index of weathering (CIW) and the index of compositional variability (ICV) for each sample were calculated and compared to the CIA and CIW of well-known kaolins so as to identify any variations between chemical indices of the known kaolins to those of Cameroon and Nigeria. The CIA is interpreted as a measure of the extent of the conversion of feldspar to clays and it is a good measure of the degree of weathering [46] (Equation 2); whereas, CIW is a measure of the extent of conversion of feldspar to clays, but does not take into account the K2O, as expressed in Equation 3 [47]. The CIA versus CIW was plotted to determine the intensity of silicate weathering.

CIA=[Al2O3Al2O3+CaO+Na2O+K2O]×100(2)

Where CaO* is the amount of CaO incorporated in the silicate fraction of the rock [48].

CIW=[Al2O3Al2O3+CaO+Na2O]×100(3)

The ICV is used to measure the abundance of Al2O3 relative to the other major oxides in the kaolins (Equation 4). In Equation 4, SiO2 is excluded to eliminate problems of quartz dilution [49]. CIA versus ICV was plotted to discriminate source rock types based on major-element geochemistry of mudstones [50].

ICV=CaO+K2O+Na2O+Fe2O3(t)+MgO+MnO+Ti2OAl2O3(4)

4 Results

4.1 Mineralogy

Kaolinite occurs as the major mineral phase in 12 of the 14 kaolins, and as minor mineral phase in two kaolins. Quartz occurs as the major mineral in two of the kaolins, as a minor mineral in 10 of the kaolins, and as trace mineral in two kaolins. Other minor mineral phases are illite, goethite, microcline and muscovite, which occur in at least one of the kaolins. Illite, anatase, ilmenite, calcite, gibbsite, muscovite, smectite, microcline and palygorskite occur as trace mineral phases in at least one of the kaolins (Table 2).

Table 2

Semi-quantitative mineralogy of studied kaolins

KQIlSmAnMuMiGoCaGiIlPa
C1+++++++++++
C2++++++++
C3+++++++++++++
C4++++++
C5+++++
C6+++++++
C7+++++++
N1+++++
N2+++++
N3++++++++
N4++++++
N5++++++
N6+++++++
N7+++++
  1. (+++): major, (++): minor, (+): trace, K: kaolinite, Q: quartz, Il: illite, Sm: smectite, An: anatase, Mu: muscovite, Mi: microcline, Go: goethite, Ca: calcite, Gi: gibbsite, Ilm: ilmenite, Pa: palygorskite.

Summation of semi-quantitative results was used to determine frequencies of occurrence of minerals in the studied kaolins; such that (+++) = 3, (++) = 2 and (+) = 0. The frequency of occurrence of these minerals in the kaolins are as follows: kaolinite (95%), quartz (69.05%), illite (14.29%), goethite (11.90%), smectite, anatase and microcline (7.14%), palygorskite and gibbsite (4.76%), and calcite and ilmenite (2.38%) (Figure 2).

Figure 2 Frequency of occurrence of minerals in the studied kaolins
Figure 2

Frequency of occurrence of minerals in the studied kaolins

4.2 Geochemistry

Results of the geochemical analyses are shown in Table 3. Concentrations of major oxides (wt.%) in the studied kaolins ranged as follows: SiO2 (47.38-68.39) > Al2O3(17.74-33.03) > Fe2O3 (0.69-8) > K2O (0.06-5.66) > TiO2 (0.39-3.28) > MgO (0.08-0.85) > Na2O (0-0.64) > P2O5(0-0.21) > MnO (0-0.19) > CaO (0-0.14) and SiO2 (40.43-64.45) > Al2O3 (20.28-29.09) > Fe2O3 (0.63-15.76) > TiO2 (0.84-2.94) > CaO (0-1.51) > MgO (0.07-0.94) > K2O (0.03-0.91) > Na2O (0.03-0.66) > MnO (0-0.13) > P2O5 (0-0.12), in kaolins from Cameroon and Nigeria, respectively.

Table 3

Major oxides geochemistry of studied kaolins

KaolinsSiO2Al2O3TiO2Fe2O3MnOMgOCaONa2OK2OP2O5LOITotal
C158.8921.851.092.360.020.850.040.645.660.009.00100.40
C258.0420.881.115.960.010.110.020.060.490.0413.2199.93
C351.7522.191.778.000.030.480.100.080.780.0914.4399.70
C447.3833.033.280.990.010.130.000.000.660.2114.43100.12
C561.8424.880.580.690.000.080.000.040.060.0011.8099.97
C668.3917.740.392.680.190.200.020.134.730.065.3099.83
C758.3321.641.916.270.050.220.140.052.280.108.1499.13
N140.4329.092.0115.760.080.940.000.140.910.1210.4499.92
N260.5525.732.940.640.010.090.610.030.030.039.54100.20
N358.0921.860.922.740.130.221.510.660.790.0013.08100.00
N464.4520.280.840.630.010.120.280.180.420.0012.0299.23
N553.2025.441.842.090.000.130.020.280.400.0015.2898.68
N652.8028.011.852.530.000.150.040.030.340.029.7695.53
N763.3024.600.881.520.010.070.040.030.150.029.76100.38
Mean C57.8023.171.453.850.040.300.050.142.090.0710.9099.87
Mean N56.1225.001.613.700.030.250.360.190.430.0311.4199.13
Capim42.3439.460.360.650.000.080.040.070.060.0713.9299.84
River[a]
Pugu[b]46.3736.931.070.000.000.090.150.080.110.0015.22100.00
Makoro[c]51.0632.031.431.800.030.150.080.140.070.0811.9599.00
Kgwakgwe[d]45.3428.570.682.655.021.060.080.233.870.0712.28100.00
Theoretical kaolinite[e]46.5439.50--------13.96100.00

Figures 3 and 4 show the plots of major oxides concentrations of kaolins from Cameroon and Nigeria, respectively, compared to those from Capim River, Pugu, Kgwakgwe and Makoro. The Cameroon kaolins have SiO2 concentrations higher than the Capim River kaolins (43.34 wt.%), and Al2O3 concentrations lower than those of the Capim River. C6 has the highest SiO2 (68.39 wt.%) and the lowest Al2O3 (17.74 wt.%) contents. Site C4 had the highest TiO2 concentration (3.28 wt.%). Kaolins from Cameroon and those from Capim River, Pugu, Kgwakgwe and Makoro have approximately the same concentrations of CaO, MgO, Na2O and P2O5. Pugu and site N5 have close loss on ignition (LOI) values, 15.22 wt.% and 15.28 wt.%, respectively.

Figure 3 Means of major oxides in kaolins of Cameroon plotted with the major oxides of kaolins from Capim River, Pugu, Makoro, Kgwakgwe and theoretical kaolin
Figure 3

Means of major oxides in kaolins of Cameroon plotted with the major oxides of kaolins from Capim River, Pugu, Makoro, Kgwakgwe and theoretical kaolin

Figure 4 Means of major oxides in kaolins of Nigeria plotted with the major oxides of kaolins from Capim River, Pugu, Makoro, Kgwakgwe and theoretical kaolin
Figure 4

Means of major oxides in kaolins of Nigeria plotted with the major oxides of kaolins from Capim River, Pugu, Makoro, Kgwakgwe and theoretical kaolin

Kaolin from site N1 is the only one having a lower silica content (40.43 wt.%) than Capim River kaolin (42.34 wt.%), though it also has a lower alumina content (29.09 wt.%) than Capim River kaolin (39.46 wt.%). Site N1 kaolin also has the highest Fe2O3 content (15.76 wt.%). Sites N5 and N6 have no amount of MnO, as Capim River and Pugu kaolins.

The relationship between Al2O3 and other oxides is shown in Figure 5. The oxides that mainly varied with an increase in Al2O3 were SiO2, TiO2 and Na2O. An inverse relationship exist between Al2O3 and SiO2, and Al2O3 and Na2O, both having r2 = 0.54; whereas, a positive relationship exists between Al2O3 and SiO2 (r2 = 0.55).

Figure 5 Hacker diagrams showing the correlation between some major elements and Al2O3
Figure 5

Hacker diagrams showing the correlation between some major elements and Al2O3

Factor analysis applied on the data set produced three factors (Table 4). Factor 1 represents 59.64% of the total variance and is dominantly made up of MnO and K2O; factor 2 represents 27.87% of the total variance and is dominantly made up of CaO and Na2O; and factor 3 is made up of Al2O3, TiO2 and P2O5, and represents 7.89% of the total variance. The SiO2 has insignificant loadings in factors 1 and 2, but significant negative loadings (−0.765) in factor 3.

Table 4

Factor analysis results

ElementsFactors
123
SiO20.178–0.008–0.765
Al2O3–0.320–0.1450.917
TiO2–0.188–0.0580.745
Fe2O30.000–0.1200.077
MnO0.5400.269–0.306
MgO0.4790.0470.234
CaO–0.0950.927–0.141
Na2O0.4830.763–0.036
K2O0.979–0.064–0.159
P2O50.090–0.3100.546
CIA–0.939–0.2140.268
CIW–0.145–0.9700.145
% of variance59.64227.8717.893

The plot of the K2O/Al2O3 ratio of kaolins from Cameroon and Nigeria, and Capim River, Pugu, Makoro, and Kgwakgwe shows that site C6 has the highest K2O/Al2O3 ratio (0.27), followed by site C1 (0.26), Kgwakgwe kaolin (0.14) and site C7 (0.11). All the other kaolins have the K2O/Al2O3 ratio less than 0.05. Sites C5 and N2 have a K2O/Al2O3 ratio of 0 as the theoretical kaolin, Capim River, Pugu and Makoro, whose kaolins have a wide range of applications. Sites C1, C6, C7 and Kgwakgwe have the K2O/Al2O3 ratio greater than 0.1, this implies incomplete kaolinisation of K-feldspars and micas.

The plot of CIW versus CIA of the studied kaolin samples (Figure 6) shows that most studied kaolins clearly depict extreme silicate weathering, with those from Nigeria generally being more weathered than the ones from Cameroon. However, sites C1 and C6 are the only two kaolin sites which reflect intermediate silicate weathering. Site N3 has the lowest CIW (90.97), whereas site C4 has the highest CIW (100.00). Site C1 has the lowest CIA (77.51), whereas site C5 has the highest CIA (99.60). Site N7 has the closest CIA and CIW to the Pugu and Makoro kaolins, whereas C5 has the closest CIA and CIW to the Capim River kaolins (Brazil). Site C7 has the closest CIA and CIW to those of Kgwakgwe kaolins.

Figure 6 Chemical index of weathering versus chemical index of alteration of studied kaolin sites
Figure 6

Chemical index of weathering versus chemical index of alteration of studied kaolin sites

Sites C1 and C6 fall in the granite trend field of the CIA versus ICV plot (Figure 7). Sites N3 and N4 are situated between the granite trend and the basalt trend; whereas, sites C2, C3, C4, C5, C7, N1, N2, N5, N6 and N7 are located close to the basalt trend. The CIA versus ICV plot is in agreement with the TiO2-Al2O3 binary plot of studied kaolin sites (Figure 8), which shows that the studied kaolins derived from granitic (or rhyolitic) and basaltic rocks.

Figure 7 Chemical index of alteration versus index of compositional variability. Adapted from [50]
Figure 7

Chemical index of alteration versus index of compositional variability. Adapted from [50]

Figure 8 TiO2-Al2O3 binary plot of studied kaolin sites. Adapted from [55]
Figure 8

TiO2-Al2O3 binary plot of studied kaolin sites. Adapted from [55]

5 Discussion

Kaolins, are secondary minerals, which are products of the alteration of primary minerals from host rocks (igneous or metamorphic rocks) [57]. Most of the host rocks of the studied kaolins in Cameroon are mainly sedimentary and igneous rocks; whereas in Nigeria, they are mainly meta-morphic and igneous rocks (Table 1). The relatively low CaO, Na2O and K2O contents, and their negative correlation coefficients with CIA (−0.15, −0.63 and −0.95, respectively) could be indicative of the loss of Ca2+, Na+ and K+ during high degree of chemical weathering of pyroxenes and K-feldspars [49, 58]. The high values of CIW and CIA suggest that weathering occurred probably under tropical conditions.

The geochemistry of kaolins is affected by the chemistry of their source rocks and minerals. For example, high K2O, and consequently high K2O/Al2O3 ratio in C1, C6 and C7 is due to the presence of muscovite and microcline in C1, and the presence of illite in C6 and C7. From factor analysis, factor 1 is made up of MnO and K2O, which could be attributed to the weathering of K-feldspars. Factor 2 is made up of CaO and Na2O. This factor is attributed to the weathering of plagioclase feldspars. Moreover, the negative correlations between Al2O3 and CaO, and Al2O3 and Na2O (Figure 5) shows that the concentrations of CaO and Na2O in the kaolins is not as a result of an increase in other Na- and Ca-bearing clay minerals such as smectite. Factor 3 is made up of Al2O3, TiO2 and P2O5. This is also sustained by the positive relationship between Al2O3 and TiO2. Therefore, factor 3 could be as a result of weathering of alumina-silicate minerals.

The assumed immobility of Al and higher mobility of Na, K, and Ca, makes CIW increase with the increase of weathering degree [34]. However, as weathering progresses, ICV decreases due to conversion of feldspars to Al-bearing clays, such as kaolins. The ICV and the K2O/Al2O3 ratio may be used to measure the compositional maturity of kaolins [49]. The ICV of studied kaolins ranges between 0.06 (C5) to 0.68 (N1); whereas K2O/Al2O3 ratios ranged between 0-0.27. Sites C5, N2, Capim River, Pugu and Makoro have ICV and K2O/Al2O3 ratio of 0. Such low ICV values and K2O/Al2O3 ratios represent compositionally mature source rocks, rich in kaolinite [50, 59]. This is also supported by the absence of feldspars in the mineralogy of studied kaolins and in the CIA versus ICV plot, which shows that none of the kaolins is from a fresh source rock.

The CIA of the different kaolin samples plotted beside the A-CN-K diagram (Figure 9) shows that the CIA of C1 and C6 corresponds to the presence of Illite in the samples. Though illite was reported in C6 [33], it was not reported in C1 [29]. However, C1 is made up of muscovite (KAl2(AlSi3)O10(OH)2). Site N3 kaolin plots close to smectite, but smectite has not been reported in this kaolin. Site N3 samples have kaolinite as major mineral, quartz and muscovite as minor minerals and anatase and rutile as trace minerals [36]. All the other samples plot in the kaolinite field, with high CIA values.

Figure 9 A-CN-K ternary diagram and chemical index of alteration of studied kaolins. Adapted from [57]
Figure 9

A-CN-K ternary diagram and chemical index of alteration of studied kaolins. Adapted from [57]

The relatively high Fe2O3 averages in Cameroon and Nigeria samples (3.85 and 3.70 wt.%, respectively) are as a result of intense oxidation of iron-bearing silicate minerals, such as biotite present in igneous and meta-morphic rocks, resulting in the formation of more stable iron oxides [60]. This explains the occurrence of goethite (αFeO.OH) in some of the kaolin samples.

The presence of titanium in kaolins reduces the quality of the kaolins. In the present study, titanium is found in ilmenite and anastase. Ilmenite, a titaniferous iron mineral (FeTiO3), usually occurs as an accessory mineral in some rocks [61]. Anatase (TiO2) also is an essential constituent in sedimentary kaolin deposits [62]. For better quality of kaolins, titanium needs to be removed from kaolins.

The chemical composition of the studied kaolins was compared to the standards set for paper coating, paper filler, ceramics, pharmaceutics and cosmetics [63]. The low TiO2 contents (0-1.5 wt.%) of C1, C2, C5, N3, N4, and N7 make them suitable for paper coating and filler, pharmaceutics and cosmetics applications (Table 5). Similarly, the low MgO, CaO, Na2O and K2O contents make C1, C2, C3, C4, C5, C7, N2, N6 and N7 suitable for the pharmaceutics and cosmetics industries; whereas their high SiO2 and Fe2O3 contents make them suitable for paper coating and filling.

Table 5

Chemical composition ranges of kaolins from Cameroon and Nigeria compared with standard chemical composition ranges for different applications

Chemical compositionPaper coating[*]Paper filler[*]Ceramics[*]Pharmaceutics and cosmetics[*]CameroonNigeria
SiO245–4946–4848–5044.6–46.447.38–68.3940.43–63.45
Al2O336–3837–3836–3838.1–39.517.74–33.0320.28–29.09
TiO20.5–1.30.5–1.50.02–0.10–1.40.39–3.280.84–2.94
Fe2O30.5–1.00.5–1.00.6–10.1–.020.69–8.00.63–15.75
MnO----0–0.190–0.13
MgO---0.1–0.20.08–0.850.07–0.94
CaO---0.1–0.20–0.140–1.51
Na2O---0–0.10–0.640.03–0.66
K2O0.5–1.50.5–1.51.2–2.70–0.20.06–5.660.03–0.91
P2O5----0–0.210–0.12
LOI--11.2–12.513.8–13.95.3–14.439.54–15.28

6 Conclusion

This study showed similarities and differences between kaolin sites from Cameroon and Nigeria with world known deposits in Pugu (Tanzania), Capim River (Brazil), Makoro and Kgwakgwe (Botswana). Mineralogically, the major mineral phase is kaolinite and the minor mineral phases are quartz, illite and goethite. Smectite, anatase, muscovite, gibbsite, microcline, palygorskite and calcite are the trace minerals. The geochemistry showed that the concentrations of major oxides in the studied kaolins ranged as follows: SiO2 > Al2O3 > Fe2O3 > K2O > TiO2 > MgO > Na2O > P2O5 > MnO > CaO and SiO2 > Al2O3 > Fe2O3 > TiO2 > CaO > MgO > K2O > Na2O > MnO > P2O5, in kaolins from Cameroon and Nigeria, respectively. The plot of CIW versus CIA shows that except for two of the kaolins from Cameroon which reflect intermediate silicate weathering, the other studied kaolins clearly depict extreme silicate weathering, with those from Nigeria being more weathered than the ones from Cameroon. Sites C5, N2 and N7 have kaolin compositions close to kaolins from Makoro, Pugu and Capim River. The geochemical characteristics of the studied kaolins make them suitable for ceramics, cosmetics, pharmaceutics, rubber and plastic applications; though the kaolins could be beneficiated to improve their quality.

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Received: 2016-10-18
Accepted: 2017-9-12
Published Online: 2017-12-2

© 2017 N. N. Bukalo et al.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

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