DIAGENETIC MORDENITE FROM PONZA, ITALY

Citation
E. Passaglia et al., DIAGENETIC MORDENITE FROM PONZA, ITALY, European journal of mineralogy, 7(2), 1995, pp. 429-438
Citations number
40
Categorie Soggetti
Mineralogy
ISSN journal
09351221
Volume
7
Issue
2
Year of publication
1995
Pages
429 - 438
Database
ISI
SICI code
0935-1221(1995)7:2<429:DMFPI>2.0.ZU;2-U
Abstract
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.