Diagenetic chabazites and phillipsites in Italy: Crystal chemistry and genesis

Diagenetic phillipsites and chabazites are the main constituents of many Italian zeolitized volcaniclastic deposits, and represent great economic value. The samples show chem. peculiarities which are closely correlated with the chem. of their parent glasses and diagenetic environments of crystn. The obsd. chem. trends reflect the following zeolitization processes: (1) samples crystallize in "hydrol. open systems" or "geoautoclaves" from alkali-potassic glasses of intermediate acidity (phonolitic, trachytic compns.) or low acidity (tephritic compns.) are peculiarly high in K and display low Si/Al ratios; (2) the low Si/Al ratios and high Na contents of samples from the Vivara hyaloclastites agree with the trachybasaltic compn. of the parent glass and a diagenetic marine environment; and (3) the low Na and high Ca contents of both chabazite and phillipsite from Iblean hyaloclastites are explained by cation exchanges following their crystn. in a marine environment.