This work focuses on the interactions between fungus-growing termites (Isoptera, Macrotermitinae), clay particles and soil organic matter (SOM). As major bioturbators in tropical ecosystems, termites create biogenic structures (galleries, sheetings, nests, mounds, fungus-comb chambers) that strongly influence the physical and chemical properties of soils. Different kinds of substrates were given to Pseudacanthotermes spiniger, one of the main famous Macrotermitinae termite species in West Africa: (i) clay (mainly illite), (ii) a mix of sand and clay, or (iii) sand. When we proposed both clay and sand, we observed a high selection of clay by termites. Observation of the properties of biogenic structures by Xray analysis showed that illite contained in the sheetings was strongly modified when compared to the bulk soil. We argue that the termite's saliva and/or the action of stimulated associate microorganisms have extracted un-exchangeable potassium, then leading to the creation of smectite layers. Thus, the termites P. spiniger, thanks to their building activity, can be seen as weathering agents of clay minerals. SOM content of the biogenic structures was found to be highly variable depending on the type of soil used (clay vs. sand) with enriched or impoverished C and N contents. Finally, we discuss how the auto-ecological requirements of termites (the balance between the cost of organic matter incorporation and the stability of the rehandled substrates) can affect soil and ecosystem functioning.