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 smectitepalygorskite 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 Mg 2+ 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.

Rock alteration in alkaline cement waters over 15 years and its relevance to the geological disposal of nuclear waste

“…For example, saponite was recognised as an alteration product of bentonite by Cuevas (2004), secondary Mg-enriched mineral phases have been observed at the Tournemire analogue site (Tinseau et al, 2006;Techer et al, 2012) and palygorskite was identified as an Mg-rich alteration product of bentonite by high pH groundwaters in the Troodos natural analogue site (Alexander et al, 2012). The formation of these phases at these high pH conditions is not unexpected as they are commonly associated with alkaline, saline lake environments (Yeniyol, 2007(Yeniyol, , 2012Hojati et al, 2010;Akbulut and Kadir, 2003) where they form from post-sedimentary alteration of soils and rocks in high pH fluids in the presence of dolomitic material (Derkowski et al, 2013;Xie et al, 2013;Schwarzenbach et al, 2013;Birsoy, 2002). Their formation has also been identified in cement-aggregate systems where dedolomitisation in the presence of Ca(OH) 2 and amorphous silica resulted in the formation of a mixed calciummagnesium silica gel (Gali et al, 2001).…”