Finding of a Cs-rich pharmacosiderite-like mineral: preliminary data Petrini Elisa 1, Bellatreccia Fabio 1 and Cavallo Andrea 2 1University of ROMA3, Dept. Earth Sciences, L.go S.Leonardo Murialdo 1, Roma, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, Italy In north western Argentina (Jujuy Province), in some volcanic areas, soils and waters are affected by high natural background levels of arsenic (As). Elevated As content can create serious health hazard to the populations settled in the surrounding areas. In this area As is mainly present in the pyroclastic rocks of the volcano Tuzgle (5486 m) whose activity is mainly characterized by emission of andesitic and rhyodacitic products. High concentration levels of As associated with solphur containing Fe and Pb that characterized many deposits along the Andine Cordillera belt TUZGLE volcano During the study of rhyolitic rock, collected near hydrothermal spring, it was observed a Cs-rich arsenate phase. The mineral occurs as a thin crusts into the rock cavities formed by very small tetrahedral shape crystals (> 2  m) covering the walls of cavities rock Preliminary SEM/EDS and EMPA analysis on thin section, suggested the mineral to be very close to pharmacosiderite KFe 4 (AsO 4 ) 3 (OH) 4 · 6-7(H 2 O), but with anomalously high Cs contents The structure of pharmacosiderite [1] consists of FeO 6 octahedra and AsO 4 tetrahedra connected to each other to form a three-dimensional network of channels. The pore has 8-membered ring openings, with alternating As tetrahedra and Fe octahedra. Each channel, approximately 3 Å in diameter, is occupied by charge neutralizing K + extra-framework cations and H 2 O molecules [2]. Buerger et al. [1] investigated a synthetic Cs-pharmacosiderite obtained treating natural pharmacosiderite with an aqueous solution of CsOH at room temperature. So, it was observed the K  Cs cationic exchange because of their ionic radius are very similar and the large canals can host the Cs cation The images obtained by means of FEG-FIB Dual Beam (LIME) shows the morphology of Cs-mineral that is clearly constituited by tetrahedra. (photo: D. De Felicis) References: [1] Buerger M.J., Dollase W.A., Garaycochea-Wittke I. Z. Kristallogr. 125, 92 (1967) [2] Yakubovich O.V., Massa W., Dimitrova O.V. Crystallography Reports 53, 409 (2008 [3] Behrens E.A., Sylvester P., Clearfield A. Environ. Sci. Technol. 32, 101 (1998) Acknowledgements: A. Mottana and M.F. Brigatti Where is the Cesium? However, until now the occurrence of a natural Cs-pharmacosiderite was not yet observed. If confirmed, K  Cs (> 50%) cationic exchange, this will imply the definition of a new mineral belonging to the pharmacosiderite group. Moreover, the occurrence of a new Cs-bearing natural phase Could be relevant to development for new 137Cs isotope selective absorbers from solutions of radioactive wastes and storage units for radioactive metals disposal [3]. CONCLUSIONS The formula unit, calculated on the basis of 3 As+P apfu, is (Cs 0.47 K 0.42 ) ∑=0.89 Fe (As 0.96 P 0.04 O 4 ) 3 (OH) 4 · 6.43H 2 O Rapresentative EMPA analysis expressed in wt% (sum 100%) As 2 O P2O5P2O F2O3F2O K2OK2O2.17 Cs 2 O7.18 H 2 O [calc] 16.57