Abstract Medenbachite occurs in association with mixite, preisingerite, alunite, goethite, reichenbachite, and malachite in a silicified barite vein from Reichenbach near Bensheim, Odenwald, Hesse, Germany. It forms aggregates of small tabular crystals. No cleavage was observed; fracture is conchoidal and Mohs hardness is 4.5. The color varies from yellow to brown-yellow; the luster is vitreous to adamantine. Medenbachite is biaxial negative, 2 V = 43(3)°, dispersion r > v, α = 2.03(2), β = 2.09 (calc.), and γ = 2.10(2)°. Observed crystal forms are {001}, {1̅01}, {101}, {1̅10}, {01̅1}, and {131̅}. Medenbachite is triclinic, space group P1̅, with a = 4.570(1), b = 6.162(1), c = 8.993(1) Å, α = 94.56(1), β = 99.69(1), γ = 94.28(1)°, V = 247.9 Å3, Z = 1, Dcalc = 5.90 g/cm3. The six strongest lines of the powder diffraction pattern are [d(Å),(I/Io),(hkl)]: 8.823(62)(001), 3.749(100)(01̅2,101), 3.596 (77)(1̅11), 3.468(58)(110, 111̅), 2.903(69)(102), and 2.810(51)(021). Electron microprobe analyses gave Bi2O3 53.36, Fe2O3 8.99, FeO 1.00 (calc.), CuO 7.85, As2O5 25.32, H2O (calc.) 3.07%, sum 99.59%. Microchemical tests indicate the presence of both Fe3+ and Fe2+ ions. On the basis of crystal-chemical considerations, the Fe3+-Fe2+ distribution was calculated from the total iron content to achieve equal numbers of Fe3+ and (Cu2+ + Fe2+) ions, for two crystallographically different positions. This yields the empirical formula Bi2.04Fe1.003+Cu0.88Fe0.122+)Σ1.00O1.11(OH)3.03(AsO4)1.96,or ideally Bi2Fe3+(Cu,Fe2+)(O,OH)2(OH)2- (AsO4)2. The structure refinement converged at R = 0.079 for 1124 unique reflections. Edge-sharing octahedra with an ordered occupation of alternating Fe3+ and (Cu2+ ,Fe2+) ions form columns parallel to [010], which are linked parallel to (001) by arsenate tetrahedra. The Bi3+ is probably distributed over two sites, which are 0.463(5) Å apart. Refinement of the site-occupancy factors indicates 50% probability for the location of Bi3+ on each position.
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