Diagenetically altered rhyolitic rocks consisting of major mordenite and minor cristobalite and smectite occur beneath a thick (about 90 m) smectite-rich layer in an abandoned quarry at Cala Fontana, in the northwestern part of the Ponza Island (Thyrrenian Sea, Gulf of Gaeta). The structure of the mordenite was refined by full-profile Rietveld analysis on X-ray powder diffraction data. Quantitative mineralogical analysis of the specimen having the highest zeolite content from the Rietveld-refined phase-scale factors yield: mordenite 72.8%, smectite 2.0%, cristobalite 5.9%, illite 3.0%, sanidine 13.9%, augite 2.4%. The chemical composition of the mordenite from electron microprobe analysis resulted in a content of tetrahedral atoms [Si/(Si + Al) = 0.83] in agreement with the range (0.80-0.85) known for this zeolite. Both the chemical analysis and the structure refinement showed a high K content among the extraframework cations. The assemblage of the authigenic minerals is interpreted as the result of diagenetic alteration of the rhyolitic glass by meteoric or ground water in an hydrologically open system, and the crystallization of mordenite instead of the most common smectite is interpreted as due to locally high pH conditions.