Knowledge of soil types and their properties is highly important in determining constraints and potential as well as better utilization of soils. Clay minerals are important fractions of the soil solid phase and many soil physical and chemical characteristics are controlled by clay minerals. Better understanding of characteristics of clay minerals and their relationship to soil physico-chemical properties help researchers to better predict behavior of soils in the fields of agriculture and environment. Various studies conducted on the clay minerals in the arid and semiarid environment, showed the existence of, chlorite, illite, smectite, vermiculite, and palygorskite minerals. The purpose of this research was to study mineralogical composition of soils developed under dry climatic condition in Karaj and Eshtehard regions located in central part of Iran. Eight sites were selected from dominant soil series developed under arid and semiarid condition. The soils were classified under Aridisols and Entisols orders. The composite soil samples were taken from 0-25 cm. Some soil properties such as particle size distribution, pH, EC, SAR, OM, CEC, CCE, water soluble Na, K, Ca and Mg were measured by conventional methods. The clay mineral was studied using an X-ray diffractometer. Transmission Electron and Scanning Electron Microscopy techniques was utilized to identify palygorskite and sepiolite clay minerals in eight dominant soil series. X-ray diffraction analysis indicated that illite and chlorite clay minerals were the dominant clay minerals in the soils studied. The origin of illite and chlorite was mainly from the parent materials. The amount of vermiculite and smectite were higher in the soil 4 developed under irrigated orchards. This could be the result of the removal of interlayered K and the leaching of K ions from the illitic clay minerals due to intensive irrigation and cultivation practices. Illite and chlorite clay minerals were also higher in the soils formed in the higher elevation on the calcareous parent material. Palygorskite and smectite were found in the higher amounts in the lower areas under the saline and sodic condition. This study indicated that the clay minerals including illites and chlorites were the major clay minerals in most of the soil studied, but palygorskite was present in higher amounts in the highly saline -sodic soils developed on the marl formation during the Miocene period.
