Crystal chemistry of diagenetic zeolites in volcanoclastic deposits of Italy

Zeolites from the most important volcanoclastic deposits of Italy include: (1) phillipsite and heulandite from the cinerite of the central northern Apennines; (2) chabazite and phillipsite from phonolitic tephritic ignimbrites contg. black pumices; (3) phillipsite from the tufo lionato of Vulcano Laziale; (4) chabazite and phillipsite from the Campanian ignimbrite; (5) phillipsite from the Neapolitan yellow tuff; and (6) chabazite and phillipsite from the pyroclastics of Monte Vulture. Compared with sedimentary phillipsites and chabazites described in the literature, the chabazites and phillipsites studied have lower Si/Al ratios and higher K contents. These chem. peculiarities are correlated with both the K-rich vesuvianitic-leucititic, latitic-phonolitic, and potassic alkali-trachytic chem. of the ash from which they were derived and, very likely, with the character of the hydrol. open system environment in which they formed. The zeolites of the heulandite-clinoptilolite group from the cinerites of the central northern Apennines is classified as a true heulandite on the basis of its chem. compn. and thermal behavior.