Nickel- and Fe 3+ -rich oxy-dravite from the Artana Mn prospect, Apuan Alps (Tuscany, Italy)

[摘要] Nickel- and Fe3+-rich oxy-dravite was identified on a specimen collected in the Artana Mn prospect, Carrara, Apuan Alps,Tuscany, Italy. Oxy-dravite occurs as brownish-orange prismatic crystals, up to 0.3 mm in length, associated with quartz,carbonates, and hematite. Electron microprobe analysis gave (in wt. % – average of 7 spot analyses): SiO2 35.81, TiO2 0.41,B2O3(calc) 10.38, Al2O3 29.36, V2O3 0.78, Cr2O3 0.09, Fe2O3 3.32, FeO 0.33, MgO 8.04, CaO 0.39, MnO 0.34, NiO 3.46, ZnO0.40, Na2O 2.84, F 0.29, H2O(calc) 3.00, O = F –0.12, total 99.12. The Fe3+/Fetot ratio was calculated based on optical absorption spectroscopy. The empirical ordered formula of the studied sample is (with rounding errors) X(Na0.92Ca0.07□0.01)Σ1.00 Y(Mg2.01Ni2+0.47Fe3+0.33Ti0.05Mn2+0.05Fe2+0.05Zn0.05)Σ3.00Z(Al5.80V0.10Cr0.01Fe3+0.09)Σ6.00Si6O18(BO3)3V(OH)3W[O0.50(OH)0.35F0.15]Σ1.00.This is an intermediate member of the dravite–oxy-dravite series. In naming it, the prefix oxy- was preferred since WO isvery close to being larger than 0.5 atoms per formula unit. Infrared spectroscopy revealed the occurrence of significantamounts of W(OH), and allowed to propose a specific short-range arrangements around the O(1) and O(3) sites. Unitcell parameters are a = 15.9349(11), c = 7.2038(5) Å, V = 1584.1(2) Å3, space group R3m. The crystal structure wasrefined by single-crystal X-ray diffraction data to R1 = 0.0146 on the basis of 1138 unique reflections with Fo > 4σ(Fo)and 94 refined parameters. The optimized crystal-chemical formula is X(Na0.92Ca0.07□0.01)Σ1.00Y(Mg1.21Al0.80Ni2+0.47Fe3+0.26Ti0.05Mn2+0.05Zn0.05V0.10Cr0.01)Σ3.00Z(Al5.00Mg0.80Fe3+0.16Fe2+0.05)Σ6.00Si6O18(BO3)3O(3)(OH)3O(1)[O0.50(OH)0.35F0.15]Σ1.00. Nickel isordered at the Y site, in agreement with results obtained on synthetic tourmalines. Oxy-dravite is likely the result of themetamorphic recrystallization of Mn-rich layers at the top of the Liassic carbonates belonging to the Marble Formationof the Apuan Alps Metamorphic Complex.

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[关键词] tourmaline supergroup;oxy-dravite;nickel;crystal structure;Apuan Alps;Italy