Nickel- and Fe3+-rich oxy-dravite from the Artana Mn prospect, Apuan Alps (Tuscany, Italy)

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Nickel-and Fe 3+ -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): SiO 2 35.81,TiO 2 0.41, B 2 O 3(calc) 10.38, Al 2 O 3 29.36,V 2 O 3 0.78, Cr 2 O 3 0.09, Fe 2 O 3 3.32, FeO 0.33, MgO 8.04, CaO 0.39, MnO 0.34, NiO 3.46, ZnO 0.40, Na 2 O 2.84, F 0.29, H 2 O (calc) 3.00, O = F -0.12, total 99.12.The Fe 3+ /Fe tot ratio was calculated based on optical absorption spectroscopy.The empirical ordered formula of the studied sample is (with rounding errors) X (Na 0.92 Ca 0.07 □ 0.01 ) Σ1.00 Y (Mg 2.01 Ni 2+ 0.47 Fe 3+ 0.33 Ti 0.05 Mn 2+ 0.05 Fe 2+ 0.05 Zn 0.05 ) Σ3.00 Z (Al 5.80 V 0.10 Cr 0.01 Fe 3+ 0.09 ) Σ6.00 Si 6 O 18 (BO 3 ) 3 V (OH) 3 W [O 0.50 (OH) 0.35 F 0.15 ] Σ1.00. This is an intermediate member of the dravite-oxy-dravite series.In naming it, the prefix oxy-was preferred since W O is very close to being larger than 0.5 atoms per formula unit.Infrared spectroscopy revealed the occurrence of significant amounts 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)Å 3 , space group R3m.The crystal structure was refined by single-crystal X-ray diffraction data to R 1 = 0.0146 on the basis of 1138 unique reflections with F o > 4σ(F o ) and 94 refined parameters.The optimized crystal-chemical formula is X (Na 0.92 Ca 0.07 □ 0.01 ) Σ1.00 Y (Mg 1.21 Al 0.80 Ni 2+ 0.47 Fe 3+ 0.26 Ti 0.05 Mn 2+ 0.05 Zn 0.05 V 0.10 Cr 0.01 ) Σ3.00 Z (Al 5.00 Mg 0.80 Fe 3+ 0.16 Fe 2+ 0.05 ) Σ6.00 Si 6 O 18 (BO 3 ) 3 O(3) (OH) 3 O(1) [O 0.50 (OH) 0.35 F 0.15 ] Σ1.00 .Nickel is ordered at the Y site, in agreement with results obtained on synthetic tourmalines.Oxy-dravite is likely the result of the metamorphic recrystallization of Mn-rich layers at the top of the Liassic carbonates belonging to the Marble Formation of the Apuan Alps Metamorphic Complex.