ABSTRACT:  Modstones an.d daystones of the trona-bearing volcano - sedimentary sequence of Neogene- age in. Beypazarı (Ankara) district were examined by x-ray diffraction technique. In addition to smectite-and illite, modstones and claystones consist, mainly of dolomite, calcite, trona, clmoptlolite» matrolite, wairakite» hexahydrite, mageesite» feldspar, rarely cMorite and opal-CX. All are audügenic in origin. Studies have .shown `that `Été fractions finer than 2 fi of trona `bearingPower horizon) mudstones .and `daystones are composed of 67% well. crystallized smectite and 33% Illite. day fractions in non trona bearing mudstones and claystones (upper horizon) are, constituted of 30% poorly crystallized smectite and 70% illite. Smectites and illite s are of trioctaedral and dioctaedral types, respectively. The exchangable interlay er cation of the smectites is Ca++. Depending; upon the geochemical conditions of the environment, clay minerals display a vertical graduational ssoning as; illite (dominant) + moderately crystallized smectite-well crystallized smectite (dominant) + illite-illite (dominant) + •poorly crystallized smectite-illite. Smectite, as well as other authigenic minerals» have been formed by weathering of volcanic glassy material in neutral to alkaline conditions in playa-lake environment