The bentonites of Aftar region, with average of 15 meters thickness and about 10 km long, are volcanoclastic sequence of Semnan Formation that formed in a shallow sea water environment. Based on the field, XRD data, microscopic and scanning electron microscopic (SEM) studies, as well as chemistry, the bentonite beds contain zeolites (clinoptilolite and mordenite), gypsum, calcite, celestine, opal, quartz, crystobalite and aragonite associated with clay minerals. XRD data of air-dried, glycolated and heated of clay fractions show the majority of the clay minerals are of swelling and of dioctahedral, smectite types. FTIR spectra, in agreement with chemical analyses data, reveal that the smectites are of Wyoming type (SWy-2) montmorillonite. The structural formula unit of representative clay of Aftar region, based on 11 oxygens, is:

(Ca_0.057Na_0.270K_0.030)(Al_1.515Mg_0.313Fe_0.109Ti_0.010)(Si_4.015O₁₀)(OH)₂.

Based on the composition of tuff and bentonites, the bentonites appear to be derived from alteration of acidic (dacitic to rhyolitic) tuffs of Eocene age in a relatively basic environment. Relatively stable smectites and zeolites are formed by dissolutim of less stable glass of vitric ash that was present in tuff during variation in basic environmental conditions.