Bonacinaite, Sc(AsO₄)  ⋅  2H₂O, the first scandium arsenate

Abstract. The new mineral bonacinaite (IMA2018-056), Sc(AsO4) ⚫ 2H2O, was found on the dumps of the Varenche Mine (Saint-Barthélemy, Nus, Aosta Valley, Italy), an old manganese mine, where it occurs as a low-temperature hydrothermal mineral associated mainly with quartz, granular braunite, undefined manganese oxides, arseniopleite, manganberzeliite and thortveitite. Bonacinaite forms colourless (with faint to distinct violet tints), pseudohexagonal, thick tabular crystals, up to 0.25 mm in size, sometimes with annular internal zones showing violet tinges, or as small, faintly violet lath-shaped crystals. The crystals are transparent and brittle, with vitreous lustre. The calculated density of an almost pure bonacinaite crystal is 2.82 g cm−3. Optically, bonacinaite is biaxial negative, α=1.598(4), β=1.618(3), and γ=1.638(3) (measured with a Na light source, 589 nm); 2V (measured) is large, and 2V (calculated) = −88.9°. The empirical formula, based on six O atoms per formula unit is (Sc0.90Mn3+0.08Fe3+0.01Pb0.01)Σ1.00[(As0.95P0.06)Σ1.01O4] ⚫ 2H2O. Bonacinaite has monoclinic symmetry, with space group P21/n and unit-cell parameters (single-crystal data / powder diffraction data) a=5.533(1)/5.521(1), b=10.409(2)/10.336(3), c=9.036(2)/9.059(4) Å, β=91.94(3)/91.97(4), V=520.1(2)/516.7(3) Å3 and Z=4. The crystal structure was refined from single-crystal intensity data obtained from a distinctly Al- and P-bearing crystal to R1(F) = 3.7 % for 1178 reflections. Bonacinaite is isotypic with the other members of the metavariscite group: kolbeckite, metavariscite and phosphosiderite.