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 alk aline environment, in which there is a high concentration of dissolved sili ca. 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. Sca nning electron and optical microscope observations allow several microstruc tures to be distinguished. These microstructures result from either precipi tation sequences or a transformation along a diagenetic gradient. New petro logical, microstructural and geochemical data confirm the transformation of magadiite into kenyaite during its diagenetic evolution, of which the fina l stage is probably Magadi-type chert. The study of various deposits of the se minerals (hardened beds, scattered isolated crystals, mineralized plant debris, irregular concretions) have been used for paleo-environmental recon struction The decrease in the abundance of magadiite concretions in the sed imentary sequence can probably be explained by the climatic evolution of th e region. (C) 2001 Elsevier Science B.V. All rights reserved.