Chemical Composition and Genesis of Analcime from Tuffs of Karaj Formation (Eocene), Central Alborz, Iran

Analcime and other associated minerals occur in tuffs of Karaj and Kond Formations of Eocene age in Cheheltan Mountain in Central Alborz, Iran. Details of field occunence, petrography and geochemical studies of the analcimes are presented in this paper. The unit cell formula of analcime of the region is Na^Al^Si^C^14 0 H₂0. Analcime was found with quartz in the lower layers, with clinoptilolite and halite in middle layers and with caicite and dolomite in celestine bearing samples of low-K tuffs. The suitable conditions were available to form analcime without any zeohtic precursors in the samples of high K/Na ratio. The absence of high silicon zeolites, as precursors, in the samples of high K/Na ratio was attributed to the very high pH conditions. It is likely that the zeohtic precursors were necessary for formation of anaicime in alkaline conditions of not very high pHs. The absence of pumpellyite in association with analcime, the relatively low thickness of the Karaj Formation in the study region and other analytical evidences are in favor of diagenetic character of the analcime in the east central Alborz.