Diagenetic chabazites and phillipsites in Italy; crystal chemistry and genesis

Diagenetic phillipsites and chabazites are the main constituents of many Italian zeolitized volcanoclastic deposits. Due to their high zeolite contents and broad extent, many of them are of great economic potential. An overview of the chemistry of these phillipsites and chabazites is presented here. The samples show chemical peculiarities which are closely correlated with the chemistry of their parent glasses and diagenetic environments of crystallization. The observed chemical trends reflect the following zeolitization processes: a) samples crystallized in "hydrologically open systems" or "geoautoclaves" from alkali-potassic glasses of intermediate acidity (phonolitic, trachytic compositions) or low acidity (tephritic compositions) are peculiarly high in K and display, respectively, low or very low Si/Al ratios; b) the low Si/Al ratios and high Na contents of samples from the Vivara hyaloclastites agree with the trachybasaltic composition of the parent glass and a diagenetic marine environment; c) the low Na and high Ca contents of both chabazite and phillipsite from Iblean hyaloclastites are explained by cation exchanges successive to their crystallization in a marine environment.