Bianchiniite, Ba-2(Ti4+V3+)(As2O5)(2)OF, a new diarsenite mineral from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy

The new mineral bianchiniite, Ba-2(Ti4+V3+)(As2O5)(2)OF, has been discovered in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as brown {001} tabular crystals, up to 1 mm across, with a vitreous lustre. It is brittle, with a perfect {001} cleavage. Streak is brownish. In reflected light, bianchiniite is grey, with orange-yellow internal reflections. It is weakly bireflectant, with a very weak anisotropy in shades of grey. Minimum and maximum reflectance data for COM wavelengths [R-min/R-max (%), (lambda, nm)] are: 5.0/5.8 (470), 5.7/6.5 (546), 5.7/7.0 (589) and 5.2/6.3 (650). Electron microprobe analyses gave (wt.% - average of 10 spot analyses): TiO2 10.34, V2O3 3.77, Fe2O3 3.76, As2O3 44.36, Sb2O3 0.22, SrO 0.45, BaO 34.79, PbO 0.28, F 1.77, sum 99.74, -O = F -0.75, total 98.99. On the basis of 12 anions per formula unit, the empirical formula of bianchiniite is (Ba2.00Sr0.04Pb0.02)(Sigma 2.06)(Ti1.144+V0.443+Fe0.423+)(Sigma 2.00)[(As3.96Sb0.02)(Sigma 3.98)O-10](O1.18F0.82)(Sigma 2.00). Bianchiniite is tetragonal, space group I4/mcm, with unit-cell parameters a = 8.7266(4), c = 15.6777(7) angstrom, V = 1193.91(12) angstrom (3) and Z = 8. Its crystal structure was refined from single-crystal X-ray diffraction data to R-1 = 0.0134 on the basis of 555 unique reflections with F-o > 4 sigma (F-o) and 34 refined parameters. The crystal structure shows columns of corner-sharing [Ti/(V,Fe)]-centred octahedra running along c, connected along a and b through (As2O5) dimers. A {001} layer of Ba-centred [10+2]-coordinated polyhedra is intercalated between (As2O5) dimers. Bianchiniite has structural relations with fresnoite- and melilite-group minerals. The name honours the two mineral collectors Andrea Bianchini (b. 1959) and Mario Bianchini (b. 1962) for their contribution to the knowledge of the mineralogy of pyrite +/- baryte +/- iron-oxide ore deposits from the Apuan Alps.