Michèle Vanessa Bomolomo^{1, 2,}, Ivan Emmanuel Fotsing Mouafo¹, Christian Bouba Mana¹, Boris Gouott Bekonga Secke^{1, 2}, Prisca-Gaëlle Bien à Nwos¹, François Ndong Bidzang¹

The samples used in this study were collected in the Mayo Kebbi region (western Chad). From a geological point of view, the Mayo Kebbi is a region with great potential. A lot of work has been done in this part of Chad. The aim of this study is to determine the proportions and types of minerals present in the <2µm fraction of the study area, their physico-chemio-mineralogical properties, their origin and the paleoenvironmental conditions that prevailed during their formation. Mineralogical analyses carried out on bulk rock and the positive hydrochloric acid test revealed that the fresh rocks of the two profiles studied are limestones. X-ray diffractometric analysis combined with ethylene glycol treatments and heating performed on the <2µm fraction of the collected materials, reveal the presence of smectitic type clay. The results obtained from these analyses reveal that smectite is the major mineral in these materials, with contents varying between 63 and 98%. It is associated with very low proportions of kaolinite (1 to 30%), illite (1 to 2%), and chlorite (1 to 5%). Quartz is the only non-clay accessory mineral identified. Geochemistry of these samples revealed that they consist primarily of SiO₂ (1.3-78.67 wt. %), CaO (50.81-0.3 wt. %), Al₂O₃ (18.25-0.37 wt. %), and Fe₂O₃ (0.09-6.52 wt. %). The pH is slightly alkaline for these materials. The CEC (Cation Exchange Capacity) and exchangeable bases oscillate between 9.23-13.95 and 6.4-10.53 meq/100 respectively. The saturation indice is between 6 and 24 and the residual moisture values between 1.94 and 10.99%. These mineralogical and physico-chemical properties of these clay-rich Mayo Kebbi’s materials indicate that they can be used in many conventional clay applications. Furthermore, the findings of the geochemistry, the mineralogy of the bulk and <2µm fractions, added to the consideration of some geological parameters suggest that the smectites of the Mayo Kebbi's area, in Western Chad, have a dual origin. One part of these smectites is formed during partial hydrolysis and erosion of the formations located on the slopes near the mayo. The second part is formed in situ. Moreover, the results also suggested that the smectites in the study area were formed in an area with low topography and in which the dominant alteration is bisiallitization.

smectite, mineralogy, physicochemical properties, sudano-sahelian area, Mayo Kebbi west, Chad