Abstract: Beidellite is a dioctahedral smectite mineral, which contains principally (Al) ions in octahedral site. Beidellite mineral was foundin the Ayvacık bentonite deposits, which were formed within Miocene volcanic rocks. In the Ayvacık region; volcanic rocks are composed of mainly highly fractured andesite, trachyandesite and highly porous tuffs. In the study area, hydrothermal solution sascended through the fracture systems and altered volcanic rocks to bentonite. During bentonitization, beidellite mineral was occurred as an alteration product of plagioclase in andesitic rocks and vitric tuffs. X-ray diffraction data indicate an unusually ordered layer stacking arrangement of beidellite. Minor amount ofillite, halloysite, kaolinite and cristobalite were determined during X-ray diffraction analysis of clay size.