Diagenetically altered rhyolitic rocks consisting of major mordenite a
nd minor cristobalite and smectite occur beneath a thick (about 90 m)
smectite-rich layer in an abandoned quarry at Cala Fontana, in the nor
thwestern part of the Ponza Island (Thyrrenian Sea, Gulf of Gaeta). Th
e structure of the mordenite was refined by full-profile Rietveld anal
ysis on X-ray powder diffraction data. Quantitative mineralogical anal
ysis of the specimen having the highest zeolite content from the Rietv
eld-refined phase-scale factors yield: mordenite 72.8%, smectite 2.0%,
cristobalite 5.9%, illite 3.0%, sanidine 13.9%, augite 2.4%. The chem
ical composition of the mordenite from electron microprobe analysis re
sulted in a content of tetrahedral atoms [Si/(Si + Al) = 0.83] in agre
ement with the range (0.80-0.85) known for this zeolite. Both the chem
ical analysis and the structure refinement showed a high K content amo
ng the extraframework cations. The assemblage of the authigenic minera
ls is interpreted as the result of diagenetic alteration of the rhyoli
tic glass by meteoric or ground water in an hydrologically open system
, and the crystallization of mordenite instead of the most common smec
tite is interpreted as due to locally high pH conditions.