The mineralogical and geochemical characteristics of Eocene continental sediments in south central Tunisia (Chebket Bouloufa and Jebel Hamri) and in north central Tunisia (Jebel Lessouda and Jebel Rhéouis), which
contain considerable amounts of palygorskite, were studied. The clay fraction of the sediments also comprises illite, kaolinite, Mg smectite and Al smectite, together with carbonates (calcite and/or dolomite),
quartz, gypsum and feldspars, all of which are present in extremely variable proportions. The textural characteristics of the samples containing most palygorskite, as well as the chemical composition
of the fibres and the contents of certain trace and rare earth elements suggest that the genesis of this fibrous clay is intimately linked to the diagenetic transformation of illite, mixed-layered minerals
and/or Al smectite, as has also been observed in contemporaneous deposits in Morocco.