Schneebergite and nickelschneebergite are new members of the tsumcorite group; they represent the bismuth analogues of cobaltlotharmeyerite and nickellotharmeyerite. The two minerals were discovered on samples from the former mining area “Am Roten Berg”, Schneeberg, Saxony, Germany; associated minerals are scorodite, barium-pharmacosiderite, ferrilotharmeyerite (cobaltoan, bismuthian), preisingerite, and waylandite. Schneebergite and nickelschneebergite are very similar in appearance; they form crystal aggregates up to 1 mm and small tabular crystals (≤ 0.5 mm), elongated parallel to [010]. The colour is brown to beige (depending on the iron content), the streak is pale brown; Vickers hardness is VHN15 = 250 kg/mm2 corresponding to a Mohs' hardness of 4 to 4 1/2. Crystallographic forms are {2̄01} (dominant), {001}, {101}, {1̄01}, {1̄02}, {1̄11}. The optical orientation is Y parallel to [010], X ≈ [001], pleochroism is weak to distinct (depending on the iron content) with X = Z = pale yellow and Y = brown (for both minerals). Schneebergite is biaxial positive, 2V = 85(5)°, nx = 1.93(calc.), ny = 1.95(1), and nz = 1.98(2); nickelschneebergite is biaxial negative, 2V = 77(5)°, nx = 1.92(calc.), ny = 1.95(1), and nz = 1.97(2). Empirical formulas calculated from electron microprobe analyses and based on 10 oxygen atoms are (Bi0.74Ca0.29)Σ1.03(Co1.06Ni0.75Fe0.22)Σ2.03 [(AsO4)1.99(SO4)0.01]Σ2.00 [(OH)1.09(H2O)0.91]Σ2.00 for schneebergite, and (Bi0.73Ca0.28Pb0.01)Σ1.02 (Ni1.18Co0.64Fe0.16)Σ1.98 (AsO4)1.99 [(H2O)1.10(OH)0.93]Σ2.03 for nickelschneebergite; the ideal formulas are BiCo2(AsO4)2[(H2O)(OH)] and BiNi2(AsO4)2 [(H2O)(OH)], respectively. There is a probably complete solid solution involving schneebergite, nickelschneebergite, cobaltlotharmeyerite, and nickellotharmeyerite. Schneebergite/nickelschneebergite are monoclinic, space group C2/m, with a = 9.005(1)/8.995(1), b = 6.211(1)/6.207(1), c = 7.440(1)/7.462(1) Å, β = 115.19(1)/115.00(1)°, V = 376.5(1)/377.6(1) Å3, Z = 2, Dcalc = 5.28/5.23 g/cm3. Structure investigations for schneebergite (R1 = 0.038) and nickelschneebergite (R1 = 0.044) based on single-crystal X-ray data confirmed isotypy with the tsumcorite-type structure.

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