Crystal-chemistry and synchrotron Rietveld refinement of two different clinoptilolites from volcanoclastites of North-Western Sardinia

Two clinoptilolites from genetically different volcanoclastic formations ha ve been studied. Both deposits are located in the Logudoro region (North-We stern Sardinia, Italy) within epiclastic and pyroclastic formations related to calcalkaline volcanic activity. Chemical characterization performed by electron microprobe analysis on both samples showed a quite similar composi tion ([Si/(Si+Al)] = 0.81 divided by 0.82), with the exception of some extr a-framework cations. In particular the epiclastic clinoptilolites turned ou t to be much higher in Mg content (0.72 vs. 0.26 a.f.u.) and considerably l ower in K (1.10 vs. 2.37 a.f.u.) than the pyroclastic clinoptilolites. Previous investigations evidenced a different behaviour of the two clinopti lolite samples towards cation exchange processes and a substantial differen ce between the theoretical and experimental CEC. In this frame, Rietveld re finements based on synchrotron data allowed to define a different cationic population within the four exchangeable cation sites.