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1 Henriette-Lott-Weg 8, D-50354 Hürth, Germany
2 Institut für Mineralogie, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany
3 Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
4 Institut für Mineralogie und Kristallographie, Universität Wien, Althanstrasse 14, A-1090 Vienna, Austria
Correspondence: * E-mail: herta.silvia.effenberger@univie.ac.at
| The first 20% of the full text of this article appears below. |
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 P
; 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, 
= 95.43(2)/83.24(1), ß = 99.22(2)/70.56(1), 
= 92.95(3)/86.91(1)°,
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