Tazzoliite : a new mineral with a pyrochlore-related structure from the Euganei Hills, Padova, Italy

Tazzoliite, ideally Ba 2CaSr 0.5Na 0.5Ti 2Nb 3SiO 17[PO 2(OH) 2] 0.5, is a new mineral (IMA 2011-018) from Monte delle Basse, Euganei Hills, Galzignano Terme, Padova, Italy. It occurs as lamellar pale orange crystals, which are typically a few m m thick and up to 0.4 mm long, closely associated with a diopsidic pyroxene and titanite. Tazzoliite is transparent. It has a white streak, a pearly lustre, is not fluorescent and has a hardness of 6 (Mohs' scale). The tenacity is brittle and the crystals have a perfect cleavage along 010. The calculated density is 4.517 g cm -3. Tazzoliite is biaxial (-) with 2V meas of ∼50°, it is not pleochroic and the average refractive index is 2.04. No twinning was observed. Electronmicroprobe analyses gave the following chemical formula: (Ba 1.93Ca 1.20Sr 0.52Na 0.25Fe 2+0.10) Σ4(Nb 2.88Ti 2.05Ta 0.07Zr 0.01V 5+0.01) Σ5.02SiO 17[(P 0.13Si 0.12S 0.07) Σ0.32O 0.66(OH) 0.66] [F 0.09(OH) 0.23] Σ0.32. Tazzoliite is orthorhombic, space group Fmmm, with unit-cell parameters a = 7.4116(3), b = 20.0632(8), c = 21.4402(8) Å, V = 3188.2(2) Å 3 and Z = 8. The crystal structure, obtained from single-crystal X-ray diffraction data, was refined to R 1(F2) = 0.063. It consists of a framework of Nb(Ti) octahedra and BaO 7 polyhedra sharing apexes or edges, and Si tetrahedra sharing apexes with Nb(Ti) octahedra and BaO 7 polyhedra. The structure, which is related to the pyrochlore structure, contains three Nb(Ti) octahedra: two are Nb dominant and one is Ti dominant. Chains of A2O 8 polyhedra [A2 being occupied by Sr(Ca, Fe)] extend along [100] and are surrounded by Nb octahedra. Channels formed by six Nb(Ti) octahedra and two tetrahedra, or four A1O 8(OH) polyhedra (A1 being occupied by Ba), alternate along [100]. The channels are partially occupied by [PO 2(OH) 2] in two possible mutually exclusive positions, alternating with fully occupied A3O 7 polyhedral pairs [A3 being occupied by Ca(Na)]. The seven strongest X-ray powder diffraction lines [d in Å (I/I 0) (hkl)] are: 3.66 (60) (044), 3.16 (30) (153), 3.05 (100) (204), 2.98 (25) (240), 2.84 (50) (064), 1.85 (25) (400) and 1.82 (25) (268). Raman spectra of tazzoliite were collected in the range 150-3700 cm -1 and confirm the presence of OH groups. Tazzoliite is named in honour of Vittorio Tazzoli in recognition of his contributions to the fields of mineralogy and crystallography.