Structural and chemical contrasts between prismatic and fibrous fluoro-edenite from

Fluoro-edenite [NaCa2Mg5(Si7Al)O22F2], a new end-member of the calcic amphibole group, is present with both prismatic and fibrous morphologies in the altered volcanic products of the Monte Calvario locality, Biancavilla, on the flanks of Mount Etna, Sicily, Italy. Whereas the crystal chemistry of the prismatic variety was described recently, the fibrous variety has not been well characterized. We report original crystal-chemical and structural data on this fibrous amphibole. Because of the difficulty in analyzing this micrometric material, specific procedures of sample preparation were developed. SEM–EDS microanalyses and Mössbauer spectroscopy indicate a variable composition of the fibers, ranging mostly from fluoro-edenite to, subordinately, winchite, tremolite, and richterite compositions. Given the impossibility of using a single-crystal X-ray method, the powder-diffraction data were evaluated by the Rietveld method. By combining the microchemical data, Mössbauer Fe3+/Fetot values, and with a refinement of the structure, the following chemical formula was obtained from an averaged composition of all the fibrous amphibole analyzed: (Na0.307K0.157)S0.464 (Ca1.505Na0.495)S2.000 (VIAl0.104Fe3+0.333Fe2+0.162Mg4.255Ti0.062Mn0.063)S4.980 (Si7.520IVAl0.480)S8.000 O22 (F1.970Cl0.020)S1.990. The cell parameters, positional parameters for all the atoms, and site scattering for O3, M1, M2, M3, M4, A, and A(m) were refined. In agreement with the site occupancies of cations based on both XRPD and Mössbauer data, Fe2+ was assigned to the M2 and M3 sites, and all Fe3+ was assigned to the M2 site. In particular, Fe2+ content of each M2 site is far greater (0.125 apfu) than that of M3 (0.037 apfu). The analytical procedures utilized here can be adopte