As-bearing new mineral species from Valletta mine, Maira Valley, Piedmont, Italy: II. Braccoite, NaMn2+5[Si5AsO17(OH)](OH), description and crystal structure

The new mineral species braccoite, ideally NaMn2+5[Si5AsO17(OH)](OH), has been discovered in the Valletta mine dumps, in Maira Valley, Cuneo province, Piedmont, Italy. Its origin is probably related to the reaction between ore minerals and hydrothermal fluids. It occurs as subhedral crystals that occurs in brown-red coloured thin masses, with pale yellow streak and vitreous to resinous luster. Braccoite is associated with tiragalloite, of which new data is provided, as well as gamagarite, hematite, manganberzeliite, palenzonaite, quartz, saneroite, tokyoite, unidentified Mn oxides, organic compounds, and Mn arsenates and silicates under study. Braccoite is biaxial positive with refractive indices α 1.749(1), β 1.750(1), γ 1.760(1). It is triclinic, space group P1, with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) Å, α = 92.691(2)°, β = 117.057(4)°, γ = 105.323(3)°, V = 740.37(4) Å3 and Z 2. Its calculated density is 3.56 g/cm3. The ten strongest diffraction lines of the observed X-ray powder diffraction pattern are [d in Å, (I), (hkl)]: 3.055 (69)(221), 3.042 (43)(102), 3.012 (65)(321), 2.985 (55)(231), 2.825 (100)(213), 2.708 (92)(220), 2.627 (43)(232), 2.381 (58)(411), 2.226 (25)(214), and 1.680 (433)(36). Chemical analyses by WDS electron microprobe gave (wt%): Na2O 4.06, CaO 0.05, MnO 41.76, MgO 0.96, Al2O3 0.04, CuO 0.02, SiO2 39.73, As2O5 6.87, V2O5 1.43, SO3 0.01, and F 0.04. H2O 2.20 was calculated on the basis of 2OH groups p.f.u. Raman spectroscopy confirmed the presence of (SiO4)4-, (AsO4)3- and OH groups. The empirical formula calculated on the basis of Σ cations-(Na,K) = 11 p.f.u., in agreement to the results of crystal structure, is Na1.06(Mn2+4.46Mn3+0.32Mg0.19V3+0.01Al0.01Ca0.01)[Si5(As0.48Si0.37V5+0.15)O17 (OH)](OH0.98F0.02), the simplified formula is Na(Mn,Mg,Al,Ca)5[Si5(As, V, Si)O17(OH)](OH,F). Single crystal X-ray diffraction allowed us to solve the structure by direct methods and revealed that braccoite is the As-dominant analogue of saneroite. The structure model was refined on the basis of 4389 observed reflections to R1 3.47 %. Braccoite is named in honor of Dr. Roberto Bracco (b. 1959), a systematic collector with a special interest in manganese minerals. The new mineral was approved by IMA 2013-093.