Mineral Constituents and Kaolinite Crystallinity of the <2 μm Fraction of Cretaceous-Paleogene/Neogene Kaolins from Eastern Dahomey and Niger Delta Basins, Nigeria

2.1 Study Areas

Four Cretaceous-Paleogene/Neogene kaolin deposits in Nigeria were selected for this study, namely,

1. The Cretaceous Lakiri and Eruku kaolins occur within Ise Formation of the Abeokuta Group in the Eastern Dahomey Basin (Fig. 1). The Abeokuta Group has three formations, namely, Ise Formation (oldest), Afowo Formation, and Araromi Formation (youngest). The Ise Formation is a sequence of continental sands, grits and siltstones with a basal conglomerate overlying the Basement Complex. Interbedded kaolinitic clays occur up to metres in some places. Sporomorphs recovered from the shell-BP paleontologists including Cicatricosisporites sp. cf. C. mohrioides, Pilosisporites trichopapillosus, Klukisporites pseudoreticulatus, Aequitriradites aff. verucosus and Stapilinisporites caminus indicate an Early Cretaceous (probably Valanginian – Barremian) age for the formation [12].

   

   Figure 1

   Geologic Map of Eastern Dahomey Basin showing the study areas [14].

2. The Paleogene/Neogene Ubulu-Uku and Awo-Omama kaolins occur within the Ogwashi-Asaba Formation of Niger Delta Basin (Fig. 2). The Ogwashi-Asaba Formation consists of white, blue, and pink clays, cross-bedded sands, carbonaceous mudstones, shales and seams of lignite. The type locality is at Eke Mgbalimgba in Ogwashi-Asaba, Delta State but extends from Okitipupa Ridge through Onitsha, Ozubulu, Nnewi, Ikot Ekpene, Uyo, and Calabar where it is over-lapped by the Benin Formation. Ogwashi-Asaba Formation contains some plant remains, which indicate an Oligocene – Miocene age and a continental environment of deposition for it [13].

Figure 2

Geologic Map of Niger Delta Basin showing the study areas [13].

2.2 Sampling

To avoid contaminations from recent weathering products or external leached materials, the outcrop faces were dug back to at least 60 cm to gain access to fresh surfaces before sampling. Twenty-eight (28) bulk representative kaolin samples composed of nine (9) from Eruku (EP), andsix (6) from Lakiri (LP) deposits at 2 m interval, respectively; six (6) from Awo-Omama (AL), seven (7) from Ubulu Uku (UL) deposits at 1 m interval, respectively (Fig. 3) were collected, stored in polythene bags carefully labeled, and transported to the laboratory for XRD and FTIR analyses.

Figure 3

Vertical profiles showing sampling depths and lithologic units of the studied kaolin deposit.

2.3 Sample Preparation and Clay Fraction Analyses

The <2 μm fraction was obtained based on the principle of sedimentation according to Stoke’s law which allow individual spherical particles to fall freely at a steady velocity under the influence of gravity, resisted only by the viscous drag of the medium [15]. The mineralogy of the <2 μm fraction of the kaolins was determined by using the XRD method after the removal of organic matter [16]. In order not to alter the kaolinite crystallinity, the dried samples were gently crushed in an agate mortar to a fine texture [17]. The powder samples were scanned in the Bruker AXS D8 Advance PSD system which was operated at 40 kV and 40 mA, using a Cu-Kα radiation, graphite monochromator by taking recordings at step size of 0.02° 2θ scanned from 2 to 32° 2θ with a counting time of 2 s/step.

The mineral phases were identified using Bruker EVA software and compared with data and patterns available in the Mineral Powder Diffraction File [18] for confirmation. The relative phase amounts (weight %) were estimated using the Rietveld method. The infrared spectra (IR) were obtained using a Bruker Alpha Platinum-ATR spectrometer. To achieve high quality spectra, good contact between the sample and ATR crystal was ensured. The Bruker’s spectroscopy software OPUS allowed real time monitoring of the spectral quality after applying pressure on the sample. The IR peaks were reported based on % transmittance to given wavelengths. The crystallinity of <2 μm fraction of the studied kaolins was evaluated using the Hinckley Index (HI) [5, 19] from X-ray diffractogram, IR - empirical (IRE) and IR - numerical (IR-N) approaches [4] from IR spectra. The HI computation is illustrated in Fig. 4. Normal HI values ranges from <0.5 (disordered) to 1.5 (ordered) [5]. The IR-N approach is based on crystallinity indices CI₁ and CI₂ defined as CI₁ = I(v₁)/I(v₃) and CI₂ = I(v₄)/I(v₁) where I(v₁) and I(v₄) are intensities of OH stretching bands at 3691/3689 cm^–1 and 3619 cm^–1, respectively, and I(v₃) is the intensity of OH bending band at 912 cm^–1. The kaolinite structures are classified as poorly ordered, if CI₁ <0.7 and CI₂>1.2; partially ordered; if 0.7<CI₁ <0.8 and 0.9<CI₂ <1.2; and ordered, if CI₁ <0.8 and CI₂<0.9 [20, 21].

Figure 4

Calculation of the HI from the ratio of the height above background of the 110 (A) and 111 (B) peaks above the band of overlapping peaks occurring between 20-23 ~ compared to the total height of the 110 above background (At). The abscissa is in terms of 2 theta and the ordinate is on a relative scale [7].

3.1 X-ray Diffraction

The X-ray diffractograms for the <2 μm fraction were not significantly different, hence only one per deposit is presented for Awo-Omama, Ubulu-Uku, Eruku, and Lakiri deposits, respectively, (Figs 5 and 6). Tables 1 and 2 give the summary of mineral constituents contained in the different <2 μm fraction kaolin samples. Of all the minerals present, kaolinite alone constitutes between 67 – 91 % in the Paleogene/Neogene deposits and 87 – 99 % in the Cretaceous deposits. Quartz and anatase on the other hand, varies from 4 – 28 % and 1 – 6 % in Paleogene/Neogene deposits and 1 – 6 % and 1 – 8 % in Cretaceous deposits, respectively. Hematite and goethite were present in minor quantities. The Cretaceous deposits generally show less quartz contents and higher kaolinite contents compared to the Paleogene/Neogene deposits. The presence of anatase, hematite, and goethite coupled with the absence of pyrite and marcasite indicate that kaolin is associated with oxidizing depositional environment [22, 23]. The average kaolinite percentages obtained for the Paleogene/Neogene (82 %) were comparable to the value reported for Cretaceous Red Sea kaolins in Egypt (84 %) [24] whereas the 96 % average percentage for the Cretaceous deposits is comparable to those reported for the <5 μm fraction of Paleogene/Neogene Maoming kaolins (97%) in China [25], <4 μm fraction of Paleogene/Neogene Georgia kaolins (98%) in the USA and Cretaceous Poveda kaolins in Spain [5]. A plot of the average percentages of minerals present in each of the deposits on the ternary diagram for general mineralogical classification of economic kaolin deposits categorised the deposits into kaolin types (Fig. 7). The Paleogene/Neogene Awo-Omama and Ubulu-Uku deposits plotted in the region of sandy kaolin whereas the Cretaceous Eruku and Lakiri deposits plotted in the region of pure kaolin show a decrease in quartz content with age.

Figure 5

Representative X-ray diffractogram of the <2 μm fraction of the kaolin from Awo-Omama (AL1 2m) and Ubulu-Uku (UL2 0m) deposits showing kaolinite (k), quartz (q), anatase (a), and hematite (h).

Figure 6

Representative X-ray diffractogram of the <2 μm fraction of the kaolin from Eruku (EP2 4m) and Lakiri (LP1 4m) deposits showing kaolinite (k), quartz (q), and hematite (h).

Figure 7

Kaolin type mineralogical classification for the deposits [26].

The calculated Hinckley index (HI) percentages from the XRD pattern of the <2 μm fraction are presented in Tables 1 and 2. The HI values obtained for the kaolinites from Awo-Omama range between 0.27 and 0.41 with an average value of 0.31, which corresponds to low kaolinite structural order whereas the HI values for the kaolinites from Ubulu-Uku, Eruku, and Lakiri ranges from 0.58 and 1.36 with an average value of 0.97; 0.8 and 1.38 with an average value of 0.98; and 0.75 and 1.09 with an average value of 0.99, respectively, which is classified under the medium to high kaolinite structural order. From the average HI values, the kaolinites are partially ordered except for the Awo-Omama kaolinites. The increasing kaolinite structural order is Awo-Omama (with least structural orderliness) < Ubulu-Uku < Eruku < Lakiri (with best structural orderliness). This implies that the Cretaceous kaolinites are more ordered than the Paleogene/Neogene kaolinites. The average HI value of 0.75 obtained for the <2 μm fraction of Cretaceous Red Sea kaolinites in Egypt [24] is lower than the average values obtained for Ubulu-Uku, Eruku, and Lakiri deposits. Kaolins that have undergone more intense oxidative weathering typically contain a higher percentage of well-ordered kaolinite and therefore a higher HI value [23].

3.2 FTIR Spectroscopy

The IR spectra for the <2 μm kaolin fraction from Awo- Omama, Ubulu-Uku, Eruku, and Lakiri deposits are presented in Figs. 8 – 11. The summary of the assignment of the absorption bands in measured IR is presented in Tables 3-6.

Figure 8

The IR spectra of the <2 μm kaolin fraction from Awo- Omama between 3900 to 3550 cm^–1 and 1200 to 500 cm^–1.

Figure 9

The IR spectra of the <2 μm kaolin fraction from Ubulu-Uku between 3900 to 3550 cm^–1 and 1200 to 500

Figure 10

The IR spectra of the <2 μm kaolin fraction from Eruku between 3900 to 3550 cm^–1 and 1200 to 500 cm^–1.

Figure 11

The IR spectra of the <2 μm kaolin fraction from Lakiri between 3900 to 3550 cm^–1 and 1200 to 500 cm^–1.

The identification of absorption bands at about 3691/3689, 3669, 3651/3650, and 3619 by visual estimation (IR-E) can be used to infer kaolinite structural order [4, 8]. According to the IR-E classification, kaolinite structure is considered ordered, if the four OH stretching and bending bands were clearly resolved; partially ordered, if the individual OH bands at 3669, 3651/3650 and 937/935 cm^–1 could be identified but intensities were low; and poorly ordered, if only one band near 3660 cm^–1 or inflexions near 3669 cm^–1, 3651/3650 cm^–1 and 937/935 cm^–1 were observed in the spectra [4]. From the IR spectra obtained for the samples, the four bands in the OH stretching region were observed with the 3669 band showing weak inflexions which is more pronounced particularly for the <2 μm kaolin fraction from Awo-Omama deposit (Fig. 8). This indicates partially ordered structure for the samples. Based on the IR spectrum for the deposits, the order of increasing structural order is Awo-Omama, Ubulu-Uku, Eruku, and Lakiri deposits, respectively. Characteristic bands at 1116-1114 and 692-683 indicative of quartz interference in the samples were identified. Muscovite interference was identified in all the samples at bands between 1028-1023 cm^–1. The 500-600 cm^–1 bands due to the presence of iron oxide and Ti-O bond vibrations [27] were prominent at bands between 532-520 cm^–1 in all the samples. From the foregoing, it therefore means that the IR spectra showed the presence of kaolinite, quartz, muscovite, hematite, goethite, and anatase in the samples, most of which had earlier been identified by XRD in some of the samples. This further demonstrates the complementary role of both FTIR and XRD techniques in characterising kaolin clay fraction, though the latter did not show the presence of muscovite in the samples.

The calculated CI₁ and CI₂ values obtained for the kaolinites correspond to partially ordered structures based on the IR-N classification (Table 7). There is an agreement between the IR-E and IR-N classifications in that the kaolinite structures were classified as partially ordered. This agreement is due to the absence of clay minerals like illite or smectite which could affect the IR pattern in OH stretching and bending region [4]. The IR and HI classifications agree with each other for the Ubulu-Uku, Eruku, and Lakiri deposits except for Awo-Omama deposit. This accordance further confirm that FTIR is a sensitive tool in the estimation of kaolinite crystallinity [28].

3.3 Industrial Applications

The industrial utility of kaolin is determined by its physico-chemical, mineralogical, and geochemical characteristics. The <2 μm fraction is important in accessing the industrial applications of kaolins because most applications require between 40 – 95 % clay fraction [29]. Based on the mineralogical composition (Fig. 12), the pure kaolin type deposits will possibly be useful as raw materials for refractory, fiberglass and ceramic applications whereas the sandy kaolin type deposit will possibly be useful for pigment applications. For the pure kaolin-type, the major colouring minerals are anatase, hematite, and goethite. The presence of Fe and Ti phases could cause whiteness problem in ceramics [30]. This deduction can be substantiated by matching the chemistry of these kaolin samples with various industrial specifications.

Figure 12

Possible industrial applications of the kaolin based on their mineralogical classification [26] described in Fig. 7.