During a survey of the As environmental distribution in NW Argentina, we observed a Cs-rich arseniate phase. The mineral occurs in a rhyolitic rock, collected near one hydrothermal spring on the slope of Tuzgle volcano (Puna region, Jujuy Province), as a thin crusts consisting of very small tetrahedral crystals (average < 2 m), covering the walls of the rock cavities. Preliminary SEM/EDS and microdiffraction X-ray analysis in thin section, suggested the mineral to be very close to pharmacosiderite KFe4(AsO4)3(OH)4•6-7H2O, but with anomalously high Cs contents. Complete WDS-EMP analyses gave a composition compatible with the formula (Cs0.47K0.42)0.89Fe4.25(As0.96P0.04O4)3(OH)4•6.43H2O, thus confirming the similarity of the Cs-bearing phase with pharmacosiderite. The structure of pharmacosiderite  consists of FeO6 octahedra and AsO4 tetrahedra connected to each other to form a three-dimensional network of channels. The pore has 8-membered ring openings, with alternating arsenic tetrahedra and iron octahedra. Each channel, approximately 3.5 Å in diameter, is occupied by charge neutralizing K+ extra-framework cations and H2O molecules . All the structural sites can be occupied by many other elements: Al, Fe, Ge, Mo, Ti at the octahedral sites, and As, Ge, P, Si at the tetrahedral sites . The extra-framework cations can be Ag, Ba, Cs, H, K, Li, Na, NH4, Pb, Rb and Tl. This makes possible the existence of many microporous inorganic compounds, isostructural with pharmacosiderite and characterized by remarkable ion-exchange properties. Buerger et al.  investigated a synthetic Cs-pharmacosiderite obtained treating natural pharmacosiderite with an aqueous solution of CsOH at room temperature. However, until now the occurrence of a natural Cs-pharmacosiderite was not yet observed. If confirmed, the observed Cs K cationic exchange will imply the definition of a new mineral belonging to the pharmacosiderite group. Moreover, the occurrence of a new Cs-bearing natural phase would be relevant to the search for new materials that can be used as selective absorbers for the 137Cs isotope from solutions of radioactive wastes and as containers for disposal of radioactive metals .  Buerger M.J., Dollase W.A., Garaycochea-Wittke I. Z. Kristallogr. 125, 92 (1967)  Yakubovich O.V., Massa W., Dimitrova O.V. Crystallography Reports 53, 409 (2008)  Baur W.H. Mitt. Österr. Miner.Ges. 148 (2003)  Behrens E.A., Sylvester P., Clearfield A. Environ. Sci. Technol. 32, 101 (1998)
Petrini, E., Bellatreccia, F., Cavallo, A. (2011). Finding of a Cs-rich pharmacosiderite-like mineral: preliminary data, 4, 92.