As-bearing new mineral species from Valletta mine, Maira Valley, Piedmont, Italy : III. Canosioite, Ba2Fe3+(AsO4)2(OH), description and crystal structure

The new mineral species canosioite, ideally Ba2Fe3+(AsO4)2(OH), has been discovered in the dump of Valletta mine, Maira Valley, Cuneo Province, Piedmont, Italy. Its origin is probably related to the reaction between ore minerals and hydrothermal fluids. It occurs in reddish-brown granules, subhedral millimetre-size crystals, with a pale yellow streak and vitreous lustre. Canosioite is associated with aegirine, baryte, calcite, hematite, bronze Mn-bearing muscovite, unidentified Mn oxides and unidentified arsenates. Canosioite is biaxial (+) with a 2Vmeas = 84(2)°. It is weakly pleochroic with X = brownish yellow, Y = brown, Z = reddish brown, Z > Y > X. Canosioite is monoclinic, P21/m, with a = 7.8642(4), b = 6.1083(3), c = 9.1670(5) Å, β = 112.874(6)°, V = 405.73(4) Å3 and Z = 2. Calculated density is 4.943 g cm-3. The seven strongest diffraction lines of the observed powder X-ray diffraction pattern are [d in Å, (I) (hkl)]: 3.713 (18)(111), 3.304 (100)(211), 3.058 (31)(020), 3.047 (59)(103), 2.801 (73)(112), 2.337 (24)(220), 2.158 (24)(123). Electron microprobe analyses gave (wt.%): Na2O 0.06, MgO 0.43, CaO 0.02, NiO 0.02, CuO 0.03, SrO 0.42, BaO 49.36, PbO 1.69, Al2O3 1.25, Mn2O3 3.89, Fe2O3 6.95, Sb2O3 0.01, SiO2 0.03, P2O5 0.02, V2O5 10.88, As2O5 24.64, SO3 0.01, F 0.02, H2O 1.61 was calculated on the basis of 1 (OH,F,H2O) group per formula unit. Infrared spectroscopy confirmed the presence of OH. The empirical formula calculated on the basis of 9 O apfu, is (Ba1.92Pb0.05Sr0.02Na0.01)∑2.00(Fe3+ 0.52Mn3+ 0.29Al0.15Mg0.06)∑1.02[(As0.64V0.36)∑1.00O4]2[(OH0.92F0.01)(H2O)0.07] and the ideal formula is Ba2Fe3+(AsO4)2(OH). The crystal structure was solved by direct methods and found to be isostructural to that of arsenbrackebuschite. The structure model was refined (R1 = 2.6%) on the basis of 1245 observed reflections. Canosioite is named after the small municipality of Canosio, where the type locality, the Valletta mine, is situated. The new mineral and name were approved by the International Mineralogical Association Commission on New Minerals and Mineral Names (IMA2015-030).