Genesis of Palygorskite in the Eocene Deposit, Central Tunisia

Palygorskite mineralizations are abundant in the Eocene marine Cherahil formation in Khanguet Rheouis (North–South axis, Central Tunisia). Palygorskite is a magnesium clay mineral commonly associated with Eocene dolomites. The origin of these mineralizations of palygorskite related to dolomite in Tunisia is little reviewed in published papers. In this study, we present detailed mineralogical and geochemical investigations exploring the genesis of palygorskite found in Eocene sediments. Current investigators suggest a variety of processes for mineral genesis, such as direct precipitation, detrital transport, and transformation of smectite clay. The mineralogical and morphological characteristics of host rock samples were investigated using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and energy dispersive X-ray microanalysis. The results indicated that dolomite predominates in the samples and is associated with abundant palygorskite and accessory gypsum and quartz. These mineralogical constituents are confirmed through geochemical analyses, whereby, the presence of CaO (11–27%) associated with MgO (7–12%) reflects the presence of dolomite. The presence of SiO2 (9–29% of the total), Al2O3 (1–5%), Fe2O3 (0.55–3%), and MgO appears to confirm palygorskite's dominance. Also, the data obtained by SEM observations indicate that samples are made up of palygorskite fibers, which cover dolomite crystals, bridge them, and fill the pore spaces between the dolomite rhombohedral crystals. EDX analyses revealed that this fibrous clay mainly consists of Si, Mg, Al, and Ca, as confirmed by XRF data. We therefore propose that the genesis of the mineral clay may have directly precipitated from a solution with elevated Mg, Si, and Al and a basic pH in a warm climate. This solution could be related to the weathering of the surrounding formations of the Rheouis diapir structure, which could provide enough dissolved Si and Al, mixed with Mg that are released to the peripheral basin by the dissolution of dolomite. This implies that the palygorskite must have formed after the dolomitization of the Eocene carbonates as a result of direct precipitation from solution. The paleogeography and the tectonic events (reactivation of NE-SW faults and halokinesis) that prevailed during the Eocene probably provided ideal conditions for the palygorskite neoformation. So, these observations and the absence of other clay minerals suggested an authigenic rather than a detrital origin for the palygorskite.