Selected samples of large cavity filling and vein-type fibrous zeolites from Eocene volcanic rocks in the Kahrizak region, northern Iran, have been studied for their mineralogical and chemical characteristics. X-ray powder diffraction and electron microprobe analyses confirmed the presence of natrolite, mesolite and scolecite with compositions of [Na14.922Ca0.202K0.015Ba0.002] [Al15.697Si24.267O80nH2O, [Ca15.714Na14.224][Al46.431Si73.398O240nH2O and [Ca7.804Na0.142K0.024Ba0.012Mg0.006][Al15.320Si24.437O80nH2O, respectively. In addition, examination of textural relationships in thin sections and back-scattered electron images reveals a paragenetic order in which the Ca-rich zeolites crystallized first. It is most probable that the fibrous zeolites of Kahrizak were formed during two pulses of hydrothermal activity in the area. Scolecite and mesolite were precipitated from Ca-rich solution, whereas the second stage Na-rich, low-temperature fluid crystallized natrolite and reacted with Ca-species.

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