## 4 Results and Discussion  
### Mineralogy  
The results obtained from the analysis of the X-ray diffraction of the bulk clay materials showed the same mineralogical phases with varying concentrations (Figure 4). The mineralogical composition of the studied samples included quartz (SiO\({}_{2}\)), kaolinite (Al\({}_{2}\)(OH)\({}_{4}\)Si\({}_{2}\)O\({}_{5}\)), illite ((K,H\({}_{3}\)O(Al,Mg,Fe)\({}_{2}\)(Si,Al)\({}_{4}\)O\({}_{10}\)[(OH)\({}_{2}\),(H\({}_{2}\)O)]), hematite (Fe\({}_{2}\)O\({}_{3}\)), muscovite (KAl\({}_{2}\)(AlSi\({}_{3}\)O\({}_{10}\))(F,OH)\({}_{2}\)), feldspar (KAlSi\({}_{3}\)O\({}_{8}\)-NaAlSi\({}_{3}\)O\({}_{8}\)-CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\)), geothite (FeOOH) and gibbsite (Al(OH)\({}_{3}\)). Figure 5 presents the XRD patterns of the <2 \(\upmu\)m fraction of the analyzed samples and Table 1 estimates the semi-quantitative mineralogical composition of the mineral phases of the clay materials based on the peak intensities of the XRD patterns. This revealed an important proportion of kaolinite (48.3-69.2 wt.%), quartz (20.5-41.2 wt.%) and appreciable amounts of illite (2.4-10.2 wt.%), which are in relation. In Ndogpassi, kaolinite was the most abundant mineral followed by quartz. In Yansoki and Japoma, kaolinite was also the main mineral, but in lower proportions here than in Ndogpassi. Illite remained the same in Ndogpassi. This clay mineral was shown to be present by the less intense peaks at 10.3 A and 3.34 A, which indicate low crystallinity. Illite is known for its cohesive, swelling and plastic properties as ceramic products [43; 44]. The Y1C2 and N1C4 samples stood out from other samples due to the presence of hematite (3.5 and 3.8%, respectively), which at these proportions gave a yellow color to these samples [19; 20]. The amounts of the other mineral phases were: hematite (0-3.8 wt.%), goethite (0-0.8 wt.%), feldspar (0-3.4 wt.%), muscovite (0-2.7 wt.%) and gibbsite (0-1.8 wt.%), respectively. These identified minerals are characteristic of marine deposits in Cameroon's coastal basins [28; 45]. In Figure 5, kaolinite is characterized by diffraction peaks between 7.11 and 7.18 A in the normal state (N) and after treatment with ethylene glycol (EG), which then disappears after heating to 500 \({}^{\circ}\)C (CH). This clay mineral had a sharp diffraction peak, indicating that it is crystalline. However, illite was indicated by low peaks between 9.56 and 10.3 A at normal (N) and did not change after being treated with ethylene glycol (EG) and heated to 500 \({}^{\circ}\)C (CH). These minerals are mainly accompanied by quartz. According to [PERSON] [46], kaolinite and quartz peaks on the diffractograms are well expressed, indicating that the minerals are well crystallized. However, other similar peaks were identified in the raw clay materials with broad peaks or were weakly expressed, which is characteristic of minerals with a lower degree of crystallinity. The mineralogical composition of the samples in this study is similar to that of the Douala clay materials [25; 26]. However, the results obtained were compared with other clay materials that were formed under the same conditions, notably the raw clay materials of Ketou Benin [47], Lokoundje Cameroon [48] and Tamazert, Hadj Ali and Chekfa from Algeria [49]. The same mineralogical characteristics of the mineral assemblage were observed, except the rutile concentration was found in Lokoundje Kribi Cameroon [48]. The results from the XRD analysis can be confirmed by FTIR spectra (Figure 6), indicating the presence of kaolinite, quartz, illite and oxyhydroxic minerals. The absorption peak found between 3700 cm\({}^{-1}\) and 3600 cm\({}^{-1}\) (3692.60 cm\({}^{-1}\), 3615.23 cm\({}^{-1}\)) in all samples attests the presence of kaolinite [28; 48]. The lack of a well-defined peak at 3664 cm\({}^{-1}\) may be due to the existence of disordered kaolinite [50].