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Fianelite occurs in iron and manganese ores at Fianel, a small mine in Val Ferrera, Graubunden, Switzerland. It is associated with quartz, aegirine, and limonite in thin open fractures crosscutting quartz + rhodonite + kutnohorite + palenzonaite ± aegirine ± parsettensite ± saneroite ± pyrobelonite veinlets. Only a small amount of very minute, tabular, (010)-flattened fianelite crystals have been collected. Fianelite, with ideal formula Mn 2 V(V,As)O 7 ·2H 2 O, crystallizes in the P2 1 /n space group with a = 7.809(2), b = 14.554(4), c = 6.705(4) Å and β = 93.27(3)°. The strongest lines in the X-ray powder pattern are d 022,230  = 3.039, d 120 = 5.32, d 231 = 2.721, and d 163,362 = 1.593 Å. The orange-red mineral is transparent; its optical sign is unknown, and 2V is small. Fianelite shows a strong pleochroism from yellow to red. Microprobe analyses revealed a partial substitution of As for V, leading to the empirical formula Mn 1.98 (V 1.57 As 0.44 Si 0.01 )O 7 ·2H 2 O. The crystal structure of fianelite, refined to R = 0.058, consists of Mn in octahedral chains bound together by pairs of V in tetrahedra. Among the two V positions, only one hosts As. H is present in H 2 O groups belonging to the octahedra and occupying channels extending through the structure along [001]. No mineral having a chemical formula with the same atomic proportions as fianelite shows similar structural features.

Description and crystal structure of fianelite, Mn 2 V(V,As)O 7 ·2H 2 O, a new mineral from Fianel, Val Ferrera, Graubünden, Switzerland