M. De'Gennaro et al., Genesis of zeolites in the Neapolitan Yellow Tuff: geological, volcanological and mineralogical evidence, CONTR MIN P, 139(1), 2000, pp. 17-35
The study proposes a model by which a thick succession of volcanic tuffs ca
n be zeolitized by alteration of pyroclastic material in the presence of su
fficient eruptive water and at temperatures close to water vapour condensat
ion. In the case of phreatomagmatic products, the model simplifies interpre
tation of problematic deposits that exhibit pronounced vertical and lateral
variation in lithification grade. A major feature of the model is that thi
ck zeolitized tuffs can be formed during emplacement of pyroclastic product
s, in marked contrast to later alteration in an open hydrologic system. Geo
logical, volcanological and mineralogical data for the Neapolitan Yellow Tu
ff, a widespread trachytic pyroclastic deposit outcropping around Campi Fle
grei (Southern Italy), have been used to infer the physicochemical conditio
ns that determined mineral genesis. This tuff shows a reduction in lithific
ation grade towards the base, top and with distance from the vent and very
variable zeolitization within the lithified portion. We suggest that during
initial emplacement the erupted products chilled against the ground, inhib
iting zeolite crystallization. During rapid deposition of the thick, wet su
ccession thermal insulation allowed the persistence of elevated temperature
s for a time sufficient for enhancement of hydration-dissolution processes
in the volcanic glass. The highly reactive alkali-trachytic glass quickly b
uffered the acid pH of the system, favouring phillipsite crystallization fo
llowed by chabazite nucleation. The variable zeolite content reflects fluct
uating emplacement conditions (e.g. changes in water content and temperatur
e). Cooling of the upper and relatively thin distal deposits inhibited the
zeolitization process, thereby preserving the primary unlithified deposit.