Palygorskite Formation Under the Influence of Saline and Alkaline Groundwater in Central Iranian Soils

Abstract

Few studies have investigated the formation or instability of palygorskite under the influence of groundwater. This study aims to identify the effects of a saline and sodic groundwater on the formation of palygorskite and the associated clay minerals. Four pedons along a calcareous catena with different depths of groundwater were sampled and analyzed using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and inductively coupled plasma mass spectrometry. Chemical compositions of both the groundwater and soil samples were also determined. Results revealed the presence of palygorskite, smectite, illite, chlorite, quartz, and feldspar in the soils studied. The highest amount of palygorskite was found in horizons with permanent groundwater. In surface horizons, palygorskite could not be detected by X-ray diffraction, but its presence, although in trace amounts, was confirmed by transmission electron microscopy. In most of the pedons studied, smectite showed an inverse trend, decreasing with depth, as compared with palygorskite. Stability diagrams for the smectite-palygorskite system showed that geochemical conditions were conducive to the formation of palygorskite from smectite under the influence of Mg-rich groundwater. Growth of palygorskite fibers on halite and calcite crystals, as revealed by scanning electron microscopy observations, clearly indicated in situ formation of palygorskite. The chemical composition of groundwater in the study area showed high concentrations of soluble Mg2+ and silica, both of which are necessary for the formation of palygorskite from solution. In conclusion, palygorskite seems to have been formed in partially or entirely submerged horizons through smectite transformation or in situ formation.