Distribution and genesis of palygorskite in selected Tertiary deposits of central Iran

Palygorskite is a widely distributed clay mineral that is commonly found in soils and sediments of arid regions, particularly in the Middle East. This study aimed to determine the distribution of palygorskite in major parent materials of soils developed on central Iranian Tertiary sediments. Based on field observations and geologic maps, 12 geological formations with Tertiary age were sampled and analyzed for mineralogical composition using x-ray diffraction (XRD) and transmission and scanning electron microscopy (TEM and SEM). Results showed that sediments of Miocene and Pliocene ages had the highest amount of palygorskite in the study area. In contrast, no trace of palygorskite was found in samples of Paleocene, Eocene and Oligocene ages. Smectites were highly associated with palygorskite. Stability diagrams for smectite-palygorskite system in Tertiary sediments of diff erent ages showed that geochemical conditions were not conductive for pakygorskite formation during the Late Oligocene. Occurrence of palygorskite in the Tertiary sediments of Central Iran suggests that the most possible source of this mineral in soils of the study area, and probably those of other areas with similar conditions, is the inheritance from parent materials. It seems that the geochemical conditions of the Tethys Sea became gradually suitable for palygorskite formation in the Tertiary sediments of central Iran. Today's arid environment prevailing in central, eastern and southern Iran has led to the stability of palygorskite in soils developed on Tertiary sediments.