Neustädtelite and cobaltneustädtelite, the Fe3+ - and Co2+ -analogues of medenbachite
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Werner Krause
Abstract
Neustädtelite and cobaltneustädtelite, two new minerals related to medenbachite, were found on samples from the dumps of the Güldener Falk mine near Schneeberg-Neustädtel, Saxony, Germany. The general appearance of the two new minerals is very similar: small tabular crystals up to 0.2 mm in diameter, transparent to translucent, with a brown color and a light brown streak; the lustre is adamantine. Both minerals are optically biaxial negative, 2V = 65(5)°, nx = 2.02(2), ny = 2.09 (calc.), nz = 2.12(2); pleochroism is strong with X = brown to opaque, Y = yellow, Z = pale yellow. Mohs’ hardness is 4.5. The cleavage parallel to {001} is good. The chemical compositions were derived by means of electron-microprobe analyses. Average contents for neustädtelite/cobaltneustädtelite are (in wt%): Bi2O3 52.58/51.54, PbO 0.08/0.08, CaO 0.15/0.32, Fe2O3 13.92/10.90, Al2O3 0.29/0.07, CoO 3.35/5.47, NiO 0.34/1.61, ZnO 0.09/0.39, CuO 0.07/0.00, As2O5 26.82/25.91, P2O5 0.23/0.43, H2O (calc.) 2.56/3.01, total 100.48/99.73. Mössbauer spectra of cobaltneustädtelite and medenbachite confirmed that all of the iron is trivalent. Based on 12 O atoms, the empirical formulae for the neustädtelite and cobaltneustädtelite type materials are (Bi1.94Ca0.02)∑1.96Fe1.00(Fe0.50Co0.38Ni0.04Al0.05Zn0.01 Cu0.01)∑0.99[(OH)2.44O1.40]∑3.84[(AsO4)2.01(PO4)0.03]∑2.04 and (Bi1.91Ca0.05)∑1.96Fe1.02(Co0.63Fe0.16Ni0.19 Zn0.04Al0.01)∑1.03[(OH)2.88O1.12]∑4.00[(AsO4)1.95(PO4)0.05]∑2.00, respectively. As derived from chemical analyses and crystal-structure investigations the ideal end-member compositions are Bi2Fe3+Fe3+O2(OH)2(AsO4)2 (neustädtelite) and Bi2Fe3+Co2+O(OH)3(AsO4)2 (cobaltneustädtelite). Extensive solid solution is observed between these two minerals. Neustädtelite and cobaltneustädtelite crystallize in space group P1̅; the cell parameters refined from powder data are a = 4.556(1)/9.156(1), b = 6.153(2)/6.148(1), c = 8.984(2)/9.338(1) Å, α = 95.43(2)/83.24(1), β = 99.22(2)/70.56(1), γ = 92.95(3)/86.91(1)°, V = 246.9/492.2 Å3, Z = 1/2, density (calc.) 5.81/5.81 g/cm3. Structure investigations were performed using single-crystal X-ray data. In both minerals edge-sharing alternating Fe3+Ø6 and (Fe3+,Co2+)Ø6/(Co2+,Fe3+)Ø6 octahedra (Ø = O,OH) form chains parallel to [010] that are corner-linked by arsenate tetrahedra to layers parallel to (001). The Bi atoms are linked by O atoms to form columns parallel to [100]; these are sandwiched between layers of composition [6]M2(OH)2(AsO4)2 (M = Fe3+,Co2+). In neustädtelite the Bi atoms are site disordered; in cobaltneustädtelite half of the Bi atoms are ordered and half are on a split position. The partial ordering is induced by the presence of three OH groups, as compared to two in neustädtelite. A structural reinvestigation of medenbachite, Bi2Fe3+ (Cu2+,Fe3+)(O,OH)2(OH)2(AsO4)2, proved isotypy with cobaltneustädtelite; the new cell parameters for medenbachite (refined from X-ray powder data) are: a = 9.162(2), b = 6.178(1), c = 9.341(2) Å, α = 83.50(2), β = 71.04(2), γ = 85.15(2)∞, V = 496 Å3, Z = 2.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
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- Hydroxyl in MgSiO3 akimotoite: A polarized and high-pressure IR study
- High-pressure IR-spectra and the thermodynamic properties of chloritoid
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