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Favreauite, a new selenite mineral from the El Dragόn mine, Bolivia

Mills, Stuart J. and Kampf, Anthony R. and Christy, Andrew G. and Housley, Robert M. and Thorne, Brent and Chen, Yu-Sheng and Steele, Ian M. (2014) Favreauite, a new selenite mineral from the El Dragόn mine, Bolivia. European Journal of Mineralogy, 26 (6). pp. 771-781. ISSN 0935-1221. doi:10.1127/ejm/2014/0026-2405.

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Favreauite, ideally PbBiCu_6O_4(SeO_3)_4 • H_2O, is a new secondary selenite mineral from the El Dragón mine, Antonio Quijarro Province, Potosí Department, Bolivia. The mineral occurs in vughs in a matrix of (Co, Cu)-rich penroseite, dolomite and goethite. Associated minerals are: ahlfeldite, allophane, calcite, chalcomenite, malachite, molybdomenite and an unnamed Al selenite. Favreauite forms tiny green square tabular crystals, flattened on {001}, up to 0.1 mm on edge and 0.01 mm thick, occurring in subparallel and divergent groups. The Mohs hardness of favreauite is estimated as ≈3; it has perfect cleavage on {001}, an irregular fracture and a vitreous lustre. The calculated density based on the empirical formula is 4.851 g cm^(−3). Favreauite is uniaxial (−), with mean refractive index estimated as 1.854 from the Gladstone–Dale relationship. It is pleochroic in shades of green, O < E. Electron microprobe analyses gave the empirical formula Pb_(0.95)Ca_(0.17)Bi_(0.90)Cu_(5.81)Se_(4.10)O_(16)(OH) • 1H_2O, based on 18 O pfu. The Raman spectrum shows strong SeO_3 bands at 847 cm^(−1) (ν_1), 764 and 795 cm^(−1) (ν_3), 493 and 542 cm^(−1) (ν_2), and 320 and 392 cm^(−1) (ν_4). Favreauite is tetragonal, space group P4/n, with the unit-cell parameters: ɑ = 9.860(4) Å, c = 9.700(5) Å, V = 943.0(9) Å^3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d_(obs)/Å (I) (hkl)]: 5.67(100)(111), 3.470(76)(220,202), 3.190(35)(003), 2.961(40)(311,113), 2.709(33)(302,203), 2.632(34)(231,312), 2.247(36)(331,133), and 1.6652(33)(305,513,531). The crystal structure was refined to R_1 = 0.0329 for 1354 observed reflections [F_o > 4σF_o] and 0.0356 for all 1432 unique reflections. Favreauite is a close structural relative of nabokoite, KCu_7Te^(4+)O_4(SO_4)_5Cl, and atlasovite, KCu_6Fe^(3+)BiO_4(SO_4)_5Cl. In all cases, oxygen-centred tetrahedra share edges to form corrugated [Cu_6MO_4] layers (M = Bi or Te) which can be derived from the framework structure of murdochite, Pb^(4+)Cu^(2+)_6O_(8-x)(Cl,Br)_(2x) by selective deletion of atoms. In favreauite, additional OH and H_2O between the layers are weakly bound to Cu, giving it Jahn-Teller distorted 4 + 2 coordination. The Cu–Bi–O layer is braced by SeO_3 pyramids. The Bi^(3+) and interlayer Pb^(2+) form an approximately face-centred cubic array analogous to the Pb^(4+) sites in murdochite. Unlike Bi^(3+), Pb^(2+) is in a site with nonpolar Formula point symmetry, which suppresses the stereoactivity of its lone pair.

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Kampf, Anthony R.0000-0001-8084-2563
Additional Information:© 2014 E. Schweizerbart’sche Verlagsbuchhandlung Science Publishers. Received 4 May 2014. Modified version received 4 July 2014. Accepted 4 July 2014. First published online September 2, 2014. Editor-in-chief Sergey Krivovichev and two anonymous referees are thanked for their helpful comments on the manuscript, which improved it greatly. Peter Williams and Peter Leverett are thanked for their help with the crystal structure. The structure data collection was carried out at ChemMatCARS Sector 15, Advanced Photon Source at Argonne National Laboratory. ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1346572. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The Caltech EMP analyses were supported by a grant from the Northern California Mineralogical Association. Part of the remainder of this study was funded by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County.
Funding AgencyGrant Number
Department of Energy (DOE)DE-AC02-06CH11357
John Jago Trelawney EndowmentUNSPECIFIED
Subject Keywords:favreauite; nabokoite; murdochite; selenite; crystal structure; anion-centred tetrahedra; synchrotron; El Dragόn mine; Bolivia.
Issue or Number:6
Record Number:CaltechAUTHORS:20150121-084017265
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Official Citation:Stuart J. Mills, Anthony R. Kampf, Andrew G. Christy, Robert M. Housley, Brent Thorne, Yu-Sheng Chen, and Ian M. Steele Favreauite, a new selenite mineral from the El Dragón mine, Bolivia Eur J Mineral, December 2014, v. 26, p. 771-781, First published on September 2, 2014, doi:10.1127/ejm/2014/0026-2405
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:53921
Deposited By: Ruth Sustaita
Deposited On:21 Jan 2015 22:43
Last Modified:10 Nov 2021 20:24

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