As-bearing new mineral species from Valletta mine, Maira Valley, Piedmont, Italy: IV. Lombardoite, Ba2Mn3+(AsO4)2(OH) and aldomarinoite, Sr2Mn3+(AsO4)2(OH), description and crystal structure

Lombardoite, ideally Ba2Mn3+(AsO4)2(OH), and aldomarinoite, ideally Sr2Mn3+(AsO4)2(OH), are two new minerals of the arsenbrackebuschite group in the brackebuschite supergroup, discovered in Fe-Mn ore in metaquartzites of the abandoned mine of Valletta, Canosio, Val Maira, Cuneo Province, Piedmont, Italy. They occur as red-brown and orange brown, respectively, as subhedral crystals (< 0.5 mm) in thin masses, associated to quartz, aegirine, baryte, calcite, hematite, muscovite and Mn minerals such as cryptomelane, braunite and manganberzeliite. Both minerals are translucent, have yellow-orange streak and vitreous luster. Both are brittle. Estimated Mohs hardness is 6-6½ for lombardoite (by analogy to canosioite), and 4½-5 for aldomarinoite (by analogy to tokyoite). Calculated densities are 5.124 g/cm3 for lombardoite and 4.679 g/cm3 for aldomarinoite. Both minerals are biaxial (+). Lombardoite shows 2Vz(meas.) = 78(4)° and is pleochroic with X = yellowish brown, Y = brown, Z = reddish brown (Z > Y > X). Aldomarinoite has 2Vz(meas.) = 67.1(1)°, and is pleochroic with X = brown, Y = brownish orange, Z = yellowish brown (Z > Y > X). Chemical point analyses by WDS-EPMA resulted in the empirical formula (based on 9 O anions): (Ba1.96Sr0.17Pb0.04Na0.02Ca0.02)2.20 (Mn3+0.62Fe3+0.13Al0.06Mg0.11)0.92 [(As0.87V0.12P0.01)1.00O4]2(OH) for lombardoite, and (Sr1.93Ca0.21Ba0.04Pb0.01)2.19(Mn3+0.48Al0.35Fe3+0.21Mg0.01)1.05 [(As0.92V0.03)0.95O4]2(OH) for aldomarinoite. The absence of H2O was confirmed by Raman spectroscopy and infrared spectroscopy. Both minerals are monoclinic, P21/m, with unit-cell parameters a = 7.8636(1) Å, b = 6.13418(1) Å, c = 9.1197(1) Å, β = 112.660(2)°, V = 405.94(1) Å3, for lombardoite and a = 7.5577(4) Å, b = 5.9978(3) Å, c = 8.7387(4) Å, β = 111.938(6)°, V = 367.43(3) Å3, for aldomarinoite. The eight strongest X-ray powder diffraction lines are [d Å (Irel; hkl)]: 6.985 (39; 10–1), 3.727 (33; 111), 3.314 (100; 21–1), 3.073 (24; 020), 3.036 (33; 21–2, 10–3), 2.810 (87; 12–1, 112), 2.125 (20; 301,11–4), 1.748 (24; 321) for lombardoite and 3.191 (89; 21–1), 2.997 (45; 020), 2.914 (47; 21–2, 10–3), 2.715 (100; 112), 2.087 (39; 12–3), 1.833 (32; 31–4), 1.689 (36; 321), 1.664 (21; 132) for aldomarinoite. The minerals are isostructural with brackebuschite: infinite chains of edge sharing octahedra running parallel to the b-axis and decorated with AsO4 groups are connected along the a and c axes through Ba and Sr atoms in lombardoite and aldomarinoite, respectively. The minerals are named after Bruno Lombardo (1944-2014), geologist and petrologist at C.N.R. (National Research Council of Italy), and the mineral collector and founding member of the AMI - Italian Micromineralogical Association, Aldo Marino (b. 1942).