MEDENBACHITE, BI2FE(CU,FE)(O,OH)(2)(OH)(2)(ASO4)(2), A NEW MINERAL SPECIES - ITS DESCRIPTION AND CRYSTAL-STRUCTURE

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 agg regates of small tabular crystals. No cleavage was observed; fracture is conchoidal and Mohs hardness is 4.5. The color varies from yellow t o brown-yellow; the luster is vitreous to adamantine. Medenbachite is biaxial negative, 2V= 43(3)degrees, dispersion r > nu, alpha = 2.03(2) , beta = 2.09 (calc.), and gamma = 2.10(2)degrees. Observed crystal fo rms are {001}, {<(1) over bar 01>}, {101}, (<(1)over bar 10>), {0(1) o ver bar1$}, and {13(1) over bar}. Medenbachite is triclinic, space gro up P (l) over bar, with a = 4.570(1), b = 6.162(1), c = 8.993(1)Angstr om, alpha = 94.56(1), beta = 99.69(1), gamma = 94.28(1)degrees, V = 24 7.9 Angstrom(3), Z = 1, D-calc = 5.90 g/cm(3). The six strongest lines of the powder diffraction pattern are [d(Angstrom),(I/I-0),(hkl)]: 8. 823(62)(001), 3.749(100)(<0(1)over bar2>,101), 3.596 (77)(<(1)over bar 11>), 3.468(58)(110,11(1) over bar), 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%. Microc hemical 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 p ositions. This yields the empirical formula Bi2.04Fe1.003+(Cu0.88Fe0.1 22+)(Sigma 1.00)O-1.11(OH)(3.03)(AsO4)(1.96), or ideally Bi2Fe3+(Cu,Fe 2+)(O,OH)(2)(OH)(2)(AsO4)(2). The structure refinement converged at R = 0.079 for 1124 unique reflections. Edge-sharing octahedra with an or dered occupation of alternating Fe3+ and (Cu2+,Fe2+) ions form columns parallel to [010], which are linked parallel to (001) by arsenate tet rahedra. The Bi3+ is probably distributed over two sites, which are 0. 463(5) Angstrom apart. Refinement of the site-occupancy factors indica tes 50% probability for the location of Bi3+ on each position.