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Clinoptilolite is a natural micro-porous zeolite composed of silica and alumina tetrahedra that commonly forms as a devitrification product of glass in volcanic tuffs. Besides applications in the agriculture, building and manufacturing industries, clinoptilolite has been widely studied in veterinary and human medicine because of its positive effects on health. Based on field observations and microstructural investigations, we show, for the first time, strain localization features along faults in natural zeolite tuffs in a clinoptilolite-tuff open-pit mine at Nižný Hrabovec (Slovak Republic). The faults are localized along pre-existing joints with plumose structures that acted as a pathway for the infiltration of mainly Mn- and K-rich fluids. Displacement along the faults created structures that are indicative of both velocity hardening (accommodated by dissolution precipitation creep on SC/SCC′ foliation) and velocity weakening (accommodated by multi-generations of ultracataclasites along principal slip surfaces) behaviors. Rock-fluid interaction was characterized by a high-mobility of K-rich fluids, with K-feldspar decorating the SC/SCC′ foliation, infiltrating fractures in the damage zone of the faults and precipitating as idiomorphic crystals in open cavities and along the fault surface. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast-cortex grains in cataclasites and truncated grains along principal slip surfaces probably suggest that at least some of the faults formed during seismic slip. • We present for the first-time fault localization in clinoptilolite (zeolite).. • Field observations show that the faults in these porous rocks are extremely localized.. • Microstructures are indicative of velocity hardening (SC/SCC′ foliation) and velocity weakening (principal slip surfaces in ultra-cataclasites).. • Microstructures along principal slip surfaces might indicate seismic slip. [ABSTRACT FROM AUTHOR]