Two As-bearing new mineral species from Valletta mine: grandaite and braccoite

The Valletta Mine (Canosio, Maira Valley, Piedmont, Italy) is a small iron deposit with subordinate manganese. A systematic study of the mineral phases in the quartzites with quartz veins revealed that those contain a large variety of mineral phases rich in arsenic, vanadium, barium and strontium. Among those, two mineral species have been found and approve by IMA-CNMMC: braccoite, ideally NaMn2+ 5[Si5AsO17(OH)](OH), and grandaite, ideally Sr2Al(AsO4)2(OH). Braccoite (IMA 2013-093) occurs as subhedral crystals in brown-red colour thin masses, with pale yellow streak and vitreous to resinous luster, and is associated with tiragalloite, gamagarite, hematite, manganberzeliite, palenzonaite, quartz, saneroite, tokyoite, unidentified Mn oxides, organic compounds, and Mn arsenates and silicates under study. Its crystal structure have been solved and refined. Braccoite is triclinic, space group P 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. Chemical analyses by WDS electron microprobe gave the empirical formula calculated on the basis of Σ cations-(Na,K) = 11 p.f.u. Na1.06(Mn2+4.46Mn3+0.32Mg0.19V3+0.01Al0.01Ca0.01)[Si5(As0.48Si0.37V5+0.15)O17(OH)](OH0.98F0.02). Braccoite Is isostructural with sanneroite, but has dominance of As over V. Braccoite is named in honour of Roberto Bracco, a systematic collector with a special interest in manganese minerals. Grandaite (IMA 2013-059) occurs in quartz veins as thin masses of bright orange to salmon to brown colour, or infrequently as fan-like aggregates of small (< 1 mm) crystals, with reddish brown streak and waxy to vitreous lustre. Grandaite is associated with aegirine, baryte, braunite, hematite, tilasite, quartz, unidentified Mn-oxides, and Mnsilicates under study. Its crystal structure has been solved and refined. Grandaite is monoclinic, space group P21/m, with a 7.5764(5), b 5.9507(4), c 8.8050(6) Å, β 112.551(2)°, V 366.62(4) Å3. Chemical analyses by electron microprobe gave the empirical formula calculated on the basis of 9 O a.p.f.u., (Sr1.41Ca0.64Ba0.05Pb0.01)S=2.11(Al0.68Fe3+0.14Mn3+0.12Mg0.13)S=1.07[(As0.96V0.01)S=0.97O4]2 (OH). Grandaite is isostructural with brackebuschite and is named after the informal appellation of the province in which the type locality is located.