Pedogenesis and clay mineralogical investigation of soils formed on gypsiferous and calcareous materials, on a transect, southwestern Iran

This study examined pedogenesis, as well as the clay mineralogy of soil and rock samples obtained from a transect of gypsiferous and calcareous materials. The main objectives of this research were to study the relationship between clay minerals and physiographic units as well as the relative importance of key pedogenic processes in controlling clay mineralogy. Palygorskite, chlorite, illite, smectite, quartz and interstratified minerals were observed in both soil and rock samples, using XRD, TEM and EDX analyses. Illite, chlorite, and interstratified minerals are inherited largely from parent rocks. Most of the smectite is inherited from highly smectite marl formations, although some may be the product of palygorskite weathering and transformation of illite particularly at the surface horizons. Neoformation of palygorskite, as a result of calcite and gypsum precipitation, seems to be a major pathway for the occurrence of this mineral, especially on plateau physiographic units, although inheritance has less importance. Gypsiferous soils showed more pedogenic palygorskite as compared to calcareous soils. Three morphological forms of palygorskite, related to degree of weathering, were identified in the studied pedons. New great groups and subgroups of Ustepts (Aridic Gypsiustepts and Aridic–calcic Gypsiustepts) are suggested to be included in Soil Taxonomy based on the properties of some of the studied pedons. Dissolution and transport of anhydrite from outcrops, is considered the main source of pedogenic gypsum in these soils. Micromorphological studies, including thin section observations and SEM analyses indicated variable habits of gypsum crystals suggested a dynamic soil environment.