The structural order of Cretaceous-Tertiary kaolins of the Douala Sub-Basin in Cameroon was determined in this study. This was achieved using Fourier Transform Infrared spectroscopy (FTIR) with attenuated total reflectance (ATR) on the Bomkoul, Dibamba, Ediki, Logbaba, Missole and Yatchika kaolins. Transmittance spectra of 20 samples were recorded in the mid-infrared regions (MIR). Results show that most of the kaolins had the four distinguishable bands in the hydroxyl (OH) stretching region, though the second transmittance band (3670 cm−1) had low intensities. The hydroxyl stretching of water bands (3457 and 3442 cm−1 for KGa-1b and KGa-2, respectively) were mainly observed in all Logbaba samples at 3443 cm−1 and 3445 cm−1. The two bands reflecting OH deformation of inner-surface hydroxyl and inner hydroxyl groups (937 and 915 cm−1) were quite visible in the Bomkoul, Dibamba, Logbaba and Missole II samples; slightly visible in all Yatchika and two of the Ediki; and not visible in Missole I and one of the Ediki samples. Therefore, based on the empirical classification of the degree of order of kaolinite, 10 of the studied samples had ordered structures. Three displayed partially ordered structures; four had poorly ordered structures, and three exhibited disordered structures. This study revealed that kaolins from Dibamba, Logbaba and Missole II in the Douala Sub-Basin had the best crystallinity, which is one of the important characteristics needed for industrial applications.
Cretaceous-Tertiary Formations are known to host the most exploited kaolins in the world ; namely, the Cornwall (England), Capim River (Brazil), Georgia (USA) and Cape York Peninsula (Australia). Kaolin is a group of clays consisting of four minerals, kaolinite [Al2Si2O5(OH)4], halloysite [Al4Si4O10(OH)8⋅8H2O], nacrite [Al2Si2O5(OH)4] and dickite [Al2Si2O3(OH)4] [2, 3]. It has a wide range of applications, which include construction, agricultural, textile, paper, pharmaceutics, ceramic, electrical, paint, nuclear energy, polymer and petroleum industries [4–7]. These applications are strongly linked to their mineralogical and chemical characteristics, which are important for good quality control of kaolins, as physical properties are closely related to the mineralogy and chemistry .
Minerals of the kaolin group have slight differences in their chemistry. Kaolinite, its main mineral, is a dioctahedral 1:1 phyllosilicate formed by superposition of tetrahedral sheets (mainly containing Si(IV)) and octahedral sheets, occupied mostly by Al(III), Fe(III) or Mg(II) [9, 10]. Distinction among the four kaolin minerals could also be made based on their structural differences. One of which is the orientation and position of the inner and external hydroxyl groups, which can only be detected using infrared spectroscopy .
One of the infrared spectroscopy tools usually used in characterising structural disorder in minerals is Fourier Transform Infrared (FTIR). It is used to characterise functional groups in clay minerals and to fingerprint specific minerals [12, 13]. Unlike X-ray powder diffraction, it has proven to be a rapid and cost-effective technique in determining the degree of structural disorder of clay minerals . FTIR could be done using a transmission or a reflectance technique. Transmission techniques involve the use of KBr-pellet or mull techniques, whereas, reflectance techniques include diffuse reflectance infrared technique (DRIFT), attenuated total reflectance (ATR) and specular reflectance.
The structural order of kaolins can be used to infer their potential industrial applications. Ordered kaolins, which usually are very fine grained, are used in paper, paint, pharmaceutics industries. Poorly ordered kaolins could be used for the production of bricks and in pottery. In an attempt to promote Cameroon’s economy, kaolins of the Douala Sub-Basin in Cameroon have been studied recently. Most studied kaolins in this Sub-Basin are reported to have kaolinite, with illite and/or smectite, quartz, feldspar, muscovite, goethite, hematite and anatase or ilmenite [14–20]. These studies mainly focused on identification and quantification of different mineral phases, chemical composition and genesis of prime kaolins in Cameroon. However, appropriate attention has not been given to the understanding of structural order/disorder of kaolins. This paper addresses the gap using an ATR accessory of FTIR to characterise the structural order of Cretaceous-Tertiary kaolins of the Douala Sub-Basin in Cameroon, based on their transmittance spectra in the mid-infrared regions (400-4000 cm−1).
2 Material and Methods
2.1 Study area and its stratigraphy
Twenty Cretaceous-Tertiary kaolin samples were collected from outcrops in Bomkoul (BKL), Dibamba (DBB), Ediki (EDK), Logbaba (LBB), Missole (MSL) and Yatchika (YTK), all found in the Douala Sub-Basin (Figure 1). Stratigraphically, the Douala Sub-Basin is subdivided into seven Formations, ranging from Cretaceous to Recent.
These Formations, as described by  and , constitute of: the Barremian-Albian Mundeck Formation comprising sandstones; the Cenomanian-Campanian Logbadjeck Formation consisting of sandstones, limestones and microconglomerates; the Maastrichtian Logbaba Formation, characterized by sandstones and clays with intercalations of sands and rare occurrence of limestones; the Paleocene to Middle Eocene Nkapa Formation composed of marls, shales and calcareous sandstones; the Upper Eocene to Oligocene Souellaba Formation, containing sandstones, marls, shales, clayey sands, sands and gravels; the Miocene Matanda Formation, made up of deltaic facies inter layered with volcanic deposits; and the Pliocene-Pleistocene Wouri Formation composed of gravels and clayey sands. Ediki, Logbaba and Yatchika outcrops are found in the Cretaceous Logbadjeck and Logbaba Formations. Whereas, Bomkoul, Dibamba and Missole outcrops are found in Tertiary Nkappa and Matanda Formations.
Three or four representative kaolin samples were collected at each outcrop at different intervals. The method of sampling was judgemental, based on the presence of kaolin deposits in known Cretaceous-Tertiary Formations of the Douala Sub-Basin. The number of samples and sampling distance were a function of the availability of the outcrop, and the size and differences in lithological characteristics of the kaolin occurrences.
Prior to analyses, the collected samples were airdried. Each sample was treated with hydrogen peroxide (H2O2) for organic matter removal; then dispersed with calgon (sodium hexametaphosphate (NaPO3)6 and soda (Na2CO3)) . Clay size fractions (< 2 μm) were obtained by sedimentation and using a centrifuge, following the method described by  and . Sedimentation was based on the principle of Stoke’s Law of sedimentation of spherical particles falling freely at a steady velocity under the influence of gravity, and with the fluid’s viscosity being the only resistance  as in Equation 1:
Where, V = the settling velocity in cm/sec,
D = the particle diameter in cm,
n = the viscosity in poises
g = the gravitational constant (980 cm/sec2)
dp = the density of the liquid in g⋅cm−3
dw = the density of water in g⋅cm−3
The mineralogical composition of the samples was carried out on clay fractions using a Rigaku Ultima IV diffractometer, working with 40 kV and 30 mA, and using a cobalt source (λ = 1.789 Å) and an alpha filter (CoKα). Samples were scanned from 3° 2θ to 90° 2θ at a rate of 2° per minute. The mineral phases present in the samples were identified using X’Pert Highscore Plus Software.
In ATR spectroscopy, a crystal with a high refractive index (ATR crystal) is used to cause an internal reflection . When a sample with lower refractive index (kaolin sample, in this study) is placed in contact with the ATR crystal, the total internal reflection of the beam of light creates an evanescent wave which penetrates into the sample. This evanescent wave is attenuated in the regions of infrared (IR) spectrum, where the sample transmits (absorbs) energy, and an ATR spectrum is generated .
In this study, the method used for ATR spectroscopy is that described by : Each < 2 μm sample was placed in contact with an ATR crystal (ZnSe). The ZnSe crystal is preferably used because it produces the widest transmission range for complete spectra. The IR spectra were recorded using a Brucker Alpha Platinum ATR FTIR spectrometer (made in USA), in the MIR regions (4000-400 cm−1) and with a resolution of 4 cm−1. The peaks obtained were compared to those of the Georgia kaolins (KGa-1b and KGa-2), known for their high grade (high purity and crystallinity), which were also analysed using ATR . The samples were described as ordered, partially ordered and poorly ordered, based on the empirical classification of : (i) Ordered (OH stretching and bending bands are clearly resolved; ii) Partially ordered (individual OH bands at 3670 cm−1,3650 cm−1 and 938 cm−1 have low intensities but can be identified); iii) Poorly ordered (only one band near 3660 cm−1 or inflexions near 3670 cm−1, 3650 cm−1 and 938 cm−1 are observed in the spectra).
3 Results and Discussion
The different mineral phases identified in the clay size fraction of Cretaceous-Tertiary kaolins in the Douala Sub-Basin shown in Table 1. Kaolinite was the most dominant clay mineral in 90% of the samples, with abundances ranging from 8.00 wt% (EDK 03) to 92.17 wt% (DBB CN 01). Except in Ediki samples (EDK 01, EDK 02 and EDK 03), MSL I 01, MSL I 02 and YTK 02A, kaolinite was the most dominant mineral phase (>80 wt% in all samples). Smectite was the most dominant clay mineral in EDK 03 (41.6 wt%) and YTK 02A (20.48 wt%). Illite was the least abundant clay mineral (mean of 4.69 wt%); however, it is the most abundant clay mineral in EDK 03 (21.9 wt%), after smectite (41.6 wt%).
|DBB CN 01||92.17||1.97||2.29||nd||nd||2.80||nd||nd||0.77||100|
|DBB CN 02||88.94||2.49||4.84||nd||nd||2.80||nd||nd||0.93||100|
|MSL I 01||58.50||2.60||16.84||20.16||nd||1.10||nd||nd||0.75||100|
|MSL I 02||31.30||0.90||9.98||57.00||nd||0.10||0.23||nd||0.50||100|
|MSL II 01||87.33||4.94||2.71||0.05||nd||4.28||nd||nd||0.69||100|
|MSL II 02||85.87||6.18||3.05||0.34||nd||3.81||nd||nd||0.74||100|
nd = not detected
The OH stretching of hydroxyl groups is found between 3500 and 4000 cm−1. This band region is important because it provides information on the environment of formation of different minerals, such that mineral groups portray almost similar transmittance/absorption in this region . It also determines the order/disorder in the kaolin mineral’s structure. Kaolin minerals usually have four distinguishable bands in the hydroxyl stretching region (3695, 3670, 3650 and 3620 cm−1), with bands around 3695 and 3620 cm−1 being specific to kaolin minerals .
Figures 2-4 show studied Cretaceous-Tertiary kaolin samples in the Douala Sub-Basin. Tables 2-3 summarise the assignment of the different bands. Most of the studied kaolins had the four distinguishable bands, though the second transmittance band (3670 cm−1) had low intensities. This band usually appears in well crystallised kaolinite . This transmittance band was not present in BKL 03, EDK 03, MSL I 01, MSL I 02 and YTK 02A (Figure 2), which therefore are not well crystallised.
|KGa-1b||KGa-2||BKL 01||BKL 02||BKL 03||DBB||DBB CN 01||DBB CN 02||EDK 01||EDK 02||EDK 03||Assignment|
|3689||3691||3690||3690||3690||3688||3690||3688||3586||3684||3688||OH stretching of inner-surface hydroxyl groups|
|3669||-||3665||3667||-||3667||3667||3667||3665||3667||-||OH stretching of inner-surface hydroxyl groups|
|3651||3650||3649||3649||3647||3651||3649||3651||3647||3647||3645||OH stretching of inner-surface hydroxyl groups|
|3619||3619||3621||3619||3618||3619||3621||3619||3619||3619||3619||OH stretching of inner hydroxyl groups|
|3457||3442||-||-||3439||-||3435||3431||-||-||-||OH stretching of water|
|1635||1630||1636||1634||1634||1640||1640||1634||1636||1636||1636||OH deformation of water|
|1115||1114||1114||1114||1114||1114||1114||1114||1114||1114||1111||Si–O stretching (longitudinal mode)|
|1102||1105||-||-||-||-||-||-||-||-||-||perpendicular Si–O stretching|
|1027||1028||1024||1026||1026||1024||1026||1026||1024||1022||-||in-plane Si–O stretching|
|1005||1004||997||999||999||997||997||997||991||993||983||in-plane Si–O stretching|
|937||935||934||934||936||934||934||934||934||932||-||OH deformation of inner-surface hydroxyl group|
|912||912||910||910||910||910||908||910||908||908||908||OH deformation of inner hydroxyl groups|
(–) not detected
|KGa-1b||KGa-2||LBB 01||LBB 02||LBB 03||MSL I 01||MSL I 02||Assignment|
|3689||3691||3690||3690||3690||3692||3694||OH stretching of inner-surface hydroxyl groups|
|3669||-||3667||3665||3665||-||-||OH stretching of inner-surface hydroxyl groups|
|3651||3650||3649||3647||3649||3649||3651||OH stretching of inner-surface hydroxyl groups|
|3619||3619||3621||3621||3621||3621||3619||OH stretching of inner hydroxyl groups|
|3457||3442||3443||3445||3443||-||-||OH stretching of water|
|1635||1630||1636||1640||1634||1624||1624||OH deformation of water|
|1115||1114||1114||1114||1114||1114||1114||Si–O stretching (longitudinal mode)|
|1102||1105||-||-||-||-||1097||perpendicular Si–O stretching|
|1027||1028||1024||1024||1026||1026||1028||in-plane Si–O stretching|
|1005||1004||997||997||997||1002||1002||in-plane Si–O stretching|
|937||935||934||930||934||934||932||OH deformation of inner-surface hydroxyl group|
|912||912||910||910||910||910||912||OH deformation of inner hydroxyl groups|
(–) not detected
The 3695 cm−1 band had a low intensity in all Ediki samples (EDK). EDK 03 specifically showed broadening of the four hydroxyl stretching bands. The general trend of Ediki samples (EDK) confirm the results obtained by , and it indicates high degree of structural disorder in kaolinite . Transmittance bands 3670 and 3650 cm−1 were totally absent in YTK 02B, this also is indicative of the disorder in the kaolinite’s structure .
OH stretching of water band (3457 and 3442 cm−1 for KGa-1b and KGa-2, respectively) was not present in all the samples. It was mainly observed in all Logbaba samples at 3443 cm−1 for LBB 01 and LBB 03, and 3445 cm−1 for LBB 02 (Figure 3). Besides the band corresponding to the OH stretching of water, all three Logbaba samples had an additional band around 3530 cm−1 (3526, 3530 and 3527 cm−1 for LBB 01, LBB 02 and LBB 03, respectively). This band shows the presence of OH-stretching at the vicinity of 3500 cm−1, similar to the Cretaceous-Tertiary kaolins from Sidi El Bader in Tunisia .
The 3439 cm−1 band in BKL 03 could either be attributed to the OH stretching of water in this sample or to the presence of smectite [13, 30]. Bomkoul kaolins are reported to also be smectitic [15, 19]. This explains the presence of a band around 3426 cm−1. EDK 01 (3425 cm−1), EDK 03 (3421 cm−1), YTK 01 (3433 cm−1), YTK 02A (3400 cm−1) and YTK 03 (3421 cm−1) also had a band around 3426 cm−1.  reported similar results for the Ediki kaolins.
KGa-1b and KG1-2 have OH bending (deformation) of water at bands 1635 and 1630 cm−1, respectively . All the samples had a peak around 1630 cm−1 (Figure 3 and Table 3). The bands of studied samples corresponding to the OH deformation of water varied between 1424 cm−1 (MSL I 01 and MSL I 02) to 1640 cm−1 (DBB, DBB CN 01 and LBB 02). DBB CN 01 showed an additional doublet at 1412 and 1467 cm−1. This doublet can be attributed to the interference of smectite .
Longitudinal mode Si-O stretching bands were observed in all samples at 1114 cm−1, except in EDK 03 (1111 cm−1) and YTK 02B (1118 cm−1); whereas the perpendicular mode Si-O stretching bands was only exhibited in MSL102. The first in-plane Si-O stretching was observed between 983-1002 cm−1 in all samples (Figure 4), and the second in-plane Si-O stretching bands were observed in all the samples between 1022-1028 cm−1. OH deformation of inner-surface hydroxyl and inner hydroxyl groups were observed around 937 and 935 cm−1, respectively for KGa kaolinites . In the studied kaolins, OH deformation of inner-surface hydroxyl groups were observed between 926 cm−1 (MSL II 02) and 936 cm−1 (BKL 03), with most samples having this OH deformation at 934 cm−1. OH deformation of inner hydroxyl groups occurred between 908 and 912 cm−1. These two bands reflecting OH deformation of inner-surface hydroxyl and inner hydroxyl groups were very visible in the BKL, DBB, LBB and MSL II samples; slightly visible in all the YTK samples, EDK 01 and EDK 02; and not visible in MSL I 01, MSL I 02 and EDK 03. Two weak bands of about equal intensities around 795 and 758 cm−1 were observed in BKL, DBB, LBB, EDK 02, MSL II 01, MSL II 02, and YTK 03 samples. These bands are not visible in the remaining samples. These bands are assigned to Si-O.
 used an empirical method to classify the degree of structural order of kaolinite. Four attributes (A-D) were taken into consideration: A) Distinguishable four bands 3695, 3670, 3650 and 3620, B) General broadening of all bands was not observed, C) OH deformation bands around 938 and 916 were visible, D) two weak bands of about equal intensities were found around 795 and 758. Table 5 shows a summary of the classification of studied Cretaceous-Tertiary kaolins of the Douala Sub-Basin. BKL 01, BKL 02, DBB, DBB CN 01, DBB CN 02, LBB 01, LBB 02, LBB 03, MSL II 01 and MSL II 02 had ordered kaolinite structures. The kaolinite content of these ordered kaolins are all above 85.50%. BKL 03, YTK 01 and YTK 03 had partially ordered kaolinite structures, with a kaolinite content generally being < 84%. Samples EDK 01, EDK 02, YTK 02A and YTK 02B exhibited poorly ordered structures. These three samples had a concentration of kaolinite < 70%; though YTK 02B contained 84.07% of kaolinite but a higher concentration of hematite, compared to all other samples. Samples EDK 03, MSL I 01 and MSL I 02 had none of the attributes present. It could therefore be inferred that they have disordered structures. They also have the lowest kaolinite content (< 60%). The poorly and partially ordered kaolins might contain some amounts of smectite and/or illite, causing some variance in their IR spectra .
|KGa-1b||KGa-2||MSL II 01||MSL II 02||YTK 01||YTK 02A||YTK 02B||YTK 03||Assignment|
|3689||3691||3690||3690||3692||3694||3694||3692||OH stretching of inner-surface hydroxyl groups|
|3669||-||3666||3667||3665||-||-||3665||OH stretching of inner-surface hydroxyl groups|
|3651||3650||3649||3649||3651||3647||3649||OH stretching of inner-surface hydroxyl groups|
|3619||3619||3619||3619||3621||3621||3620||3621||OH stretching of inner hydroxyl groups|
|3457||3442||-||-||-||-||-||-||OH stretching of water|
|1635||1630||1636||1630||1636||1630||1634||1636||OH deformation of water|
|1115||1114||1114||1114||1114||1114||1118||1114||Si–O stretching (longitudinal mode)|
|1102||1105||-||-||-||-||-||-||perpendicular Si–O stretching|
|1027||1028||1024||1024||1026||1024||1026||1024||in-plane Si–O stretching|
|1005||1004||997||997||1002||995||999||997||in-plane Si–O stretching|
|937||935||928||926||932||934||-||932||OH deformation of inner-surface hydroxyl group|
|912||912||910||910||910||908||908||910||OH deformation of inner hydroxyl groups|
(–) not detected
|DBB CN 01||Y||Y||Y||Y|
|DBB CN 02||Y||Y||Y||Y|
|MSL II 01||Y||Y||Y||Y|
|MSL II 02||Y||Y||Y||Y|
|BKL 03||N||Y||Y||Y||Partially ordered|
|EDK 01||Y||N||Y||N||Poorly ordered|
Attributes used by , as described in-text. Y = Yes, N = No
Infrared spectra of selected Cretaceous-Tertiary kaolins of the Douala Sub-Basin were studied and compared to those of kaolinites from Georgia (USA), known for their high grade quality and industrial application. The OH stretching bands were best observed in the DBB, DBB CN 01, DBB CN 02, LBB 01, LBB 02, LBB 03, MSL II 01 and MSL II 02 samples. These samples also exhibited best OH deformation and Si-O stretching and bending regions in their spectra. Consequently, according to the empirical classification of the degree of order of kaolinite, these samples were ordered, as Georgia kaolinites. Three of the studied samples (BKL 03, YTK 01 and YTK 03) had partially ordered kaolinite structures; four of the samples (EDK 01, EDK 02, YTK 02A and YTK 02B) had poorly ordered structures; whereas MSL I 01 and MSL I 02 had disordered structures. This study revealed that kaolins from Dibamba (DBB), Logbaba (LBB) and Missole II (MSL II) in the Douala Sub-Basin had the best crystallinity, therefore they can be used for paper, paint, pharmaceutics industries; whereas the other kaolins, which are partially and poorly ordered, could be used in the production of bricks and in pottery.
The financial assistance of the National Research Foundation (NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the authors and not necessarily to be attributed to the NRF.
 Harvey C.C., Murray H.H., Industrial clays in the 21st century: A perspective of exploration, technology and utilisation. Applied Clay Science, 1997, 11, 285-31010.1016/S0169-1317(96)00028-2Search in Google Scholar
 Kearey P., Dictionary of Geology. Penguin Books, 2nd Edition, 2001Search in Google Scholar
 Obaje S.O., Omada J.I., Dambatta U.A., Clays and their industrial applications: Synoptic Review. International Journal of Science and Technology, 2013, 3 (5), 264-270Search in Google Scholar
 Heckroodt R.O., Clay and clay minerals in South Africa. Journal of South African Institute of Mining and Metallurgy, 1991, 91 (10), 343-363Search in Google Scholar
 Ekosse G., The Makoro kaolin deposit, southeastern Botswana: its genesis and possible industrial applications. Applied Clay Science, 2000, 16, 301-32010.1016/S0169-1317(99)00059-9Search in Google Scholar
 Matike D.M.E., Ekosse G.I.E., Ngole V.M., Physico-chemical properties of clayey soils used traditionally for cosmetics in Eastern Cape, South Africa. International Journal of the Physical Sciences, 2011, 6 (33), 7557-7566. 10.5897/IJPS11.1249Search in Google Scholar
 Tassongwa B., Nkoumbou C., Njoya D., Njoya A., Tchop J.L., Yvon J., Njopwouo D., Geochemical and mineralogical characteristics of the Mayouom kaolin deposit, West Cameroon, Earth Science Research, 2014, 3 (1), 94-10710.5539/esr.v3n1p94Search in Google Scholar
 Cravero F., Gonzalez I., Galan E., Dominguez E., Geology, mineralogy, origin and possible applications of some Argentina kaolins in the Neuquen basin. Applied Clay Science, 1997, 12, 27-4210.1016/S0169-1317(96)00035-XSearch in Google Scholar
 Frost R.L., Yang J., Cheng H., Liu Q., Du X., Influencing factors on kaolinite-potassium acetate intercalation complexes. Applied Clay Science, 2010, 50 (4), 476-48010.1016/j.clay.2010.09.011Search in Google Scholar
 Ruiz Cruz M.D., Genesis and evolution of the kaolin-group minerals during the diagenesis and the beginning of metamorphism. In: Fernando Nieto and Juan Jiménez-Millán, eds. “Diagenesis and Low-Temperature Metamorphism. Theory, Methods and Regional Aspects” Seminarios SEM, 2007, 3, 41-52Search in Google Scholar
 Madejová J., Komadel P., Baseline studies of the clay minerals society source clays: Infrared methods. Clays and Clay Minerals, 2001, 49 (5), 410-43210.1346/CCMN.2001.0490508Search in Google Scholar
 Ekosse G.-I.E., Fourier transform infrared spectrophotometry and X-ray powder diffractometry as complementary techniques in characterising clay size fraction of kaolin. Journal of Applied Science and Environmental Management, 2005, 9 (2), 43-4810.4314/jasem.v9i2.17289Search in Google Scholar
 Ngon Ngon G.F., Bayiga E., Ntamack-Nida M.J., Etame J., Noa Tang S., Yongue-Fouateu R., Trace elements geochemistry of clay deposits of Missole II from the Douala sub-basin in Cameroon (Central Africa): A provenance study. Sciences, Technologie & Développement, 2012, 13 (1), 20-35Search in Google Scholar
 Ngon Ngon G.F., Etame J., Ntamack-Nida M.J., Mbog M.B., Mpondo A.M.M., Gerard M., Yongue-Fouateu, R., Bilong P., Geological study of sedimentary clayey materials of the Bomkoul area in the Douala region (Douala sub-basin, Cameroon) for the ceramic industry. Comptes Rendus Geoscience, 2012, 344, 366-37610.1016/j.crte.2012.05.004Search in Google Scholar
 Diko M.L., Ekosse G.E., Physicochemical and mineralogical considerations of Ediki sandstone-hosted kaolin occurrence, South West Cameroon. International Journal of the Physical Sciences, 2012, 7 (3): 501-507. 10.5897/IJPS11.1506Search in Google Scholar
 Diko M.L., Ekosse G.E., Characterisation of two kaolin facies from Ediki, Southwest Cameroon, Scientific Research and Essays, 2013, 8 (18), 698-704Search in Google Scholar
 Logmo E.O., Ngon Ngon G.F., Samba W., Mbog M.B., Etame J., Geotechnical, mineralogical and chemical characterisation of the Missole II clayey materials of Douala Sub-Basin (Cameroon) for construction materials, Open Journal of Civil Engineering, 2013, 3, 46-5310.4236/ojce.2013.32A006Search in Google Scholar
 Ngon Ngon G.F., Mbog M.B., Etame J., Ntamack-Nida M.J., Logmo E.O., Gerard M., Yongue-Fouateu R., Bilong P., Geochemistry of the Paleocene-Eocene and Miocene-Pliocene clayey materials of the eastern part of the Wouri River (Douala sub-basin, Cameroon): Influence of parent rocks. Journal of African Earth Sciences, 2014, 91, 110-12410.1016/j.jafrearsci.2013.12.005Search in Google Scholar
 Diko M., Ekosse G., Ogola J., Fourier transform infrared spectroscopy and thermal analyses of kaolinitic clays from South Africa and Cameroon. Acta Geodyn. Geomater., 2016, 13 (2), xx1–x1010.13168/AGG.2015.0052Search in Google Scholar
 Chavom B.M., Njike Ngaha P.R., Bitom D.L. Sedimentary facies and depositional environments of Cenozoic sedimentary Formations cropping out at the central part of the Douala Basin. American Journal of Geosciences, 2014, 4 (1): 8-2310.3844/ajgsp.2014.8.23Search in Google Scholar
 Effoudou-Priso E.N., Onana V.L., Boubakar L., Beyala V.K.K., Ekodeck G.E., Relationships between major and trace elements during weathering processes in a sedimentary context: Implications for the nature of source rocks in Douala, Littoral Cameroon, Chemie der Erde, 2014, http://dx.doi.org/10.1016/j.chemer.2014.05.00310.1016/j.chemer.2014.05.003Search in Google Scholar
 Van Reeuwijk L.P., Procedures for soil analysis. International Soil Reference and Information Centre, Wageningen, The Netherlands, Tech. Paper 9, 2002Search in Google Scholar
 Jackson M.L., Soil Chemical Analysis – Advanced Course 2nd ed., published by the author, University of Wisconsin, Madison, WI, 1979, 497 pagesSearch in Google Scholar
 Gaspe A., Messer P., Young P., Selection and preparation of clay bodies for stove manufacture. A manual clay/non clay ratio measurement technique. Clay Testing, 1994, 10 pages10.3362/9781780443980Search in Google Scholar
 Fringeli U.P., ATR and reflectance IR spectroscopy, applications. In: Lindon, J.C., Tranter, G.E., Holmes J.L. (Eds), Encyclopedia of spectroscopy and spectrometry. Academic Press, London, 2000, 58-7510.1006/rwsp.2000.0013Search in Google Scholar
 Khoshhesab Z.M., Reflectance IR Spectroscopy. In: Theophile T. (Ed.), Infrared Spectroscopy-Materials Science, Engineering and Technology. InTech, Rijeka, 2012, 233-24410.5772/37180Search in Google Scholar
 Vaculíková L., Plevová E., Vallová S., Koutník I., Characterisation and differentiation of kaolinites from selected Czech deposits using infrared spectroscopy and differential thermal analysis. Acta Geodyn. Geomater., 2011, 8 (161), 59-67Search in Google Scholar
 Fehli M., Tlili A., Gaied M.E., Montacer M., Mineralogical study of kaolinitic clays from Sidi El Bader in the far north of Tunisia, Applied Clay Science, 2008, 39, 208-21710.1016/j.clay.2007.06.004Search in Google Scholar
 Ojima J., Determining of crystalline silica in respirable dust samples by infrared spectrophotometry in the presence of interferences. Journal of Occupational Health, 2003, 45, 94-10310.1539/joh.45.94Search in Google Scholar PubMed
© 2017 N. N. Bukalo et al.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.