Sedimentary zeolites clinoptilolite, mordenite and erionite occur as diagenetic products of ash falls of rhyolitic composition from the Oligocene Chichindaro Formation at the limits of the Mexican Volcanic Belt with the Mexican Highland. These zeolitic tuffs were formed from open hydrologic environments where rhyolitic glass altered to clinoptilolite + opal-C and to clinoptilolite + mordenite + erionite + opal-C. The process of diagenesis was hydrolysis associated with the removal of SiO2, K2O, and Na2O from the rhyolitic precursor and enrichment of Al2O3, MgO, and CaO. The crystallization of clinoptilolite occurs at ratios of SiO2/Al2O3 between 4.91 and 7.14 and (K2O + Na2O)/(MgO + CaO) from 2.19 to 0.79. Formation of clinoptilolite appears to be directly from glass and through a vitreous proto-zeolite intermediate. The zeolitic tuff is in contact with a vitric tuff where ash falls were devitrified to K-feldspar and diagenetically altered to smectite + opal-C.
