Geological setting, geochemical, textural, and genesis of palygorskite in Eocene carbonate deposits from Central Tunisia

Abstract

The Eocene deposits belonging to the Khanguet Rheouis area, North–South Axis, Central Tunisia, consist of three lithological units deposited in a shallow water environment that is congruent to the Eocene regression. The middle unit represents a massive white dolomitic clay with gypsum intercalations that increase towards the top, containing significant amounts of palygorskite. SEM–EDX, XRD, and XRF analytical techniques were deployed to better understand the mineralogical and geochemical properties of the collected samples. Microtextural characteristics were analyzed using SEM and polarizing microscopy. Mineral spectral characteristics were determined by the Fourier Transform Infrared (FTIR) spectroscopy. The XRD analysis results show the predominance of dolomite with variable amounts of palygorskite and low quartz contents. Samples contained low concentrations of Nb, Th, Zr, Y, Rb, Zn, Cr, and V, which showed a positive correlation with the major oxides SiO₂, Al₂O₃, Fe₂O₃, K₂O, and TiO₂, and a negative correlation with MgO and CaO, indicating that trace elements were exclusively retained in palygorskite rather than dolomite. SEM observations of palygorskite showed dense mats of short, interwoven fibers that bridged and filled the pore spaces between the dissolved dolomite. These textural characteristics suggest that the formation of palygorskite is post-dates dolomitization. Petrographic observations illustrate the existence of micro-tectonic structures such as stylolites and microcracks, playing an important role in the distribution of palygorskite. The obtained results are consistent with the hypothesis of a neoformed origin of palygorskite by direct chemical precipitation from solution, after partial dissolution of the dolomite.