Zeolitization processes at the pyroclastic deposit “Tufo rosso a scorie nere” (Auctorum), Vico Volcano, Italy

This paper focuses on the authigenic mineralizations acting in the “Tufo Rosso a Scorie Nere” (Auctorum) (hereafter TRS), i.e. one of the main pyroclastic units of the Vico Volcanic Apparatus. Vico is a stratovolcano located in northern Latium (Italy) beonging to the potassium-rich Roman Volcanic magmatic province. Vicanian volcanic products show a ultrapotassic character and extremely variable chemical rock composition, from leucite-bearing tefrites, trachybasalts, tephri-phonolites, to trachites. The “Tufo Rosso a Scorie Nere” belongs to the Ignimbrite “C” formation, i.e. the largest pyroclastic ow of the vicanian plateau and one of the most widespread pyroclastic body of Lazio region, extending on a surface of about 1300 Km2, also known as “Sutri Formation” of the “Lago di Vico Synthem” (Perini & Conticelli, 2002). TRS is used with pro t in animal feeding (de Gennaro & Langella, 1996) and for the synthesis of zeolites for the chemical industry (Novembre et al., 2004). The cation exchange capacity of this material, due to both chabazite and phillipsite presence, ranges around 2 meq/g (Giampaolo et al., 2008). The pyroclastic deposits appear in general massive and made of “black vitreous vesciculated juvenile scoriae”, immersed inside a lithi ed ashy matrix after zeolitization processes. The main minerals are chabazite and phillipsite and the zeolitic content is locally variable, reaching peaks of 68 wt %. Zeolites grow replacing the amorphous fraction and occur both inside matrix and scoriae. Concerning scoriae, zeolitization moves from the rim to the core of the scoriaceous fragment as a function of the temperature of the uids and permeability of water inside the structure. Composition of parental fresh glass and those of zeolitized rocks is referred to trachyte, lightly undersaturated in SiO2, and the alteration processes modi ed the parental rock chemical character. The zeolitic genesis is ascribed to a “geoautoclave system”, zeolites displaying a Si/Al ratio similar to that of the parental glasses.