• journal_title：Mineralogical Magazine
• Contributor：F. Cámara ; F. Nestola ; L. Bindi ; A. Guastoni ; F. Zorzi ; L. Peruzzo ; D. Pedron
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2012.076.4.01
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：76
• issue：4
• firstpage：827
• section：Articles

Tazzoliite, ideally Ba₂CaSr_0.5Na_0.5Ti₂Nb₃SiO₁₇[PO₂(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 μm 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_0.25formula" id="inline-formula-1">f" /> )Σ₄(Nb_2.88Ti_2.05Ta_0.07Zr_0.01formula" id="inline-formula-2">f" /> )Σ_5.02SiO₁₇[(P_0.13Si_0.12S_0.07)Σ_0.32O_0.66(OH)_0.66][F_0.09(OH)_0.23]Σ_0.32.

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) ų and Z = 8. The crystal structure, obtained from single-crystal X-ray diffraction data, was refined to R₁(F²) = 0.063. It consists of a framework of Nb(Ti) octahedra and BaO₇ polyhedra sharing apexes or edges, and Si tetrahedra sharing apexes with Nb(Ti) octahedra and BaO₇ 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 A2O₈ 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 A1O₈(OH) polyhedra (A1 being occupied by Ba), alternate along [100]. The channels are partially occupied by [PO₂(OH)₂] in two possible mutually exclusive positions, alternating with fully occupied A3O₇ polyhedral pairs [A3 being occupied by Ca(Na)]. The seven strongest X-ray powder diffraction lines [d in Å (I/I₀) (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.