The natural hydrous sodium silicates from the northern bank of Lake Chad : occurrence, petrology and genesis

Hydrous sodium silicates sometimes associated with zeolites, form in an alkaline environment, in which there is a high concentration of dissolved silica. Such an environment existed during the Holocene in N'Guigmi interdunal depressions (Lake Chad), which led to the precipitation of various types of hydrous sodium silicates, including magadiite, kenyaite, and zeolites. Scanning electron and optical microscope observations allow several microstructures to be distinguished. These microstructures result from either precipitation sequences or a transformation along a diagenetic gradient. New petrological, microstructural and geochemical data confirm the transformation of magadiite into kenyaite during its diagenetic evolution, of which the final stage is probably Magadi-type chert. The study of various deposits of these minerals (hardened beds, scattered isolated crystals, mineralized plant debris, irregular concretions) have been used for paleo-environmental reconstruction. The decrease in the abundance of magadiite concretions in the sedimentary sequence can probably be explained by the climatic evolution of the region.