Eur. J. Mineral., 32, 449–455, 2020 https://doi.org/10.5194/ejm-32-449-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Luxembourgite, AgCuPbBi4Se8, a new mineral species from Bivels, Grand Duchy of Luxembourg Simon Philippo1, Frédéric Hatert2, Yannick Bruni2, Pietro Vignola3, and Jiríˇ Sejkora4 1Laboratoire de Minéralogie, Musée National d’Histoire Naturelle, Rue Münster 25, 2160 Luxembourg, Luxembourg 2Laboratoire de Minéralogie, Université de Liège B18, 4000 Liège, Belgium 3CNR-Istituto di Geologia Ambientale e Geoingegneria, via Mario Bianco 9, 20131 Milan, Italy 4Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 9, Prague, Czech Republic Correspondence: Frédéric Hatert ([email protected]) Received: 24 March 2020 – Revised: 30 June 2020 – Accepted: 15 July 2020 – Published: 12 August 2020 Abstract. Luxembourgite, ideally AgCuPbBi4Se8, is a new selenide discovered at Bivels, Grand Duchy of Lux- embourg. The mineral forms tiny fibres reaching 200 µm in length and 5 µm in diameter, which are deposited on dolomite crystals. Luxembourgite is grey, with a metallic lustre and without cleavage planes; its Mohs hard- ness is 3 and its calculated density is 8.00 g cm−3. Electron-microprobe analyses indicate an empirical formula Ag1:00.Cu0:82Ag0:20Fe0:01/61:03Pb1:13Bi4:11.Se7:72S0:01/67:73, calculated on the basis of 15 atoms per formula unit. A single-crystal structure refinement was performed to R1 D 0:0476, in the P 21=m space group, with 3 a D 13:002.1/, b D 4:1543.3/, c D 15:312.2/Å, β D 108:92.1/◦, V D 782:4.2/Å , Z D 2. The crystal struc- ture is similar to that of litochlebite and watkinsonite and can be described as an alternation of two types of anionic layers: a pseudotetragonal layer four atoms thick and a pseudohexagonal layer that is one atom thick. In the pseudotetragonal layers the Bi1, Bi2 ,Bi3, Pb, and Ag1 atoms are localised, while the Cu2 and Bi4 atoms occur between the pseudotetragonal and the pseudohexagonal layers. Bi1, Bi2, and Bi3 atoms occur in weakly distorted octahedral sites, whereas Bi4 occurs in a distorted 7-coordinated site. Ag1 occupies a fairly regular octahedral site, Cu2 a tetrahedral position, and Pb occurs on a very distorted 8-coordinated site. 1 Introduction During winter 2012, excavation works were realised by the “Société électrique de l’Our” at Bivels, in the north of the The term “Luxembourg” designates a natural region com- Grand Duchy, exposing some fragments of reddish schists posed of the Grand Duchy of Luxembourg and by the Lux- containing mineralised veins. The veins were constituted by embourg Province in the South of Belgium. The Ardennes translucent dolomite crystals reaching 2 mm in diameter, as mountains are in the north of this region and are named “Oes- well as by siderite. A careful examination under the binocular ling” in the Grand Duchy. This massif is constituted of Lower microscope also showed needles of a grey metallic mineral, Devonian rocks, mainly schists and quartzites, which were with a habit similar to that of millerite. Preliminary Energy- affected by the Variscan metamorphism. Pressures and tem- Dispersive X-ray Spectrometry (EDS) measurements indi- ◦ peratures were estimated to be 500 C and 3–4 kbar in the cated that these fibres contained Cu, Bi, Ag, Pb, and Se as region of Libramont, but the metamorphic conditions pro- essential constituents, thus corresponding to an intermediate gressively decrease to the east, reaching 400 ◦C and 2 kbar between litochlebite, Ag2PbBi4Se8 (Sejkora et al., 2011) and at Bastogne (Beugnies, 1986; Theye and Fransolet, 1993; watkinsonite, Cu2PbBi4Se8 (Topa et al., 2010). Hatert, 2005). Electron-microprobe analyses indicated a Cu = Ag ra- tio very close to 1.0 in the mineral, in agreement with Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC. 450 S. Philippo et al.: Luxembourgite, AgCuPbBi4Se8 the formula AgCuPbBi4Se8. A single-crystal X-ray diffrac- Table 1. Electron-microprobe analysis of luxembourgite. tion study showed a crystal structure similar to that of watkinsonite but with significant differences compared to Wt. % Range (wt. %) Cation numbers litochlebite. Moreover, the ordering of Cu and Ag at two dif- S 0.01 0.00–0.02 0.005 ferent crystallographic sites indicated that the mineral was Fe 0.02 0.00–0.05 0.006 not only an intermediate member between litochlebite and Pb 11.95 10.50–12.79 1.133 watkinsonite but a separate mineral species. This assumption As 0.00 0.00 0.000 was confirmed by the very limited solid solution between the Ag 6.60 5.68–9.04 1.202 new species and litochlebite. Cu 2.66 2.23–2.81 0.821 The mineral was named luxembourgite for the city of Lux- Bi 43.73 42.66–44.26 4.111 embourg, close to the locality where it was discovered; it Se 31.04 30.37–32.04 7.722 is the first new species found in the Grand Duchy of Lux- Total 96.01 embourg. The new mineral and its name were approved by Average of five-point analyses. Cation numbers were calculated on the the IMA-CNMNC under the number IMA 2018-154. A part basis of 15 atoms per formula unit. of the type sample used for electron-microprobe analyses is stored in the collection of the Natural History Museum of Luxembourg (catalogue number FD040). Another part of 4 Chemical composition the type used for the single-crystal structure determination is stored in the collection of the Laboratory of Mineralogy, Uni- Electron-microprobe analyses of luxembourgite (Table 1) versity of Liège, catalogue number 21302. A detailed miner- were performed with a Jeol JXA-8200 WDS instrument alogical characterisation of this new species is given in the located in Milan, Italy. Acceleration voltage was 15 kV, present paper. with a probe current of 5 nA and a beam diameter of 1 µm. Standards used were galena (Pb, S), nickeline (As), and pure metals (Fe, Ag, Cu, Bi, Se). The empirical for- 2 Occurrence mula, calculated on the basis of 15 atoms per formula unit, is Ag .Cu Ag Fe / Pb Bi .Se S / . Luxembourgite was found on dumps produced by the con- 1:00 0:82 0:20 0:01 61:03 1:13 4:11 7:72 0:01 67:73 The ideal formula is AgCuPbBi Se , which requires struction of a tunnel by the “Société Electrique de l’Our”, 4 8 Ag 5.58, Cu 3.44, Pb 11.22, Bi 45.28, and Se 34.22 for a total at Bivels, north of the Grand Duchy of Luxembourg (co- of 100.00 wt %. ordinates: 49◦570800 N, 6◦1004100 E). Associated minerals are dolomite and siderite. The mineral occurs in red schists con- taining veins of finely crystallised dolomite and siderite. The 5 X-ray diffraction and crystal structure dolomite crystals reach 2 mm in diameter and sometimes show tiny fibres of luxembourgite. The veins were formed by X-ray powder diffraction data of luxembourgite (Table 2), as a hydrothermal process, which is also responsible for other well as X-ray single-crystal data, were collected on a Rigaku famous sulfide mineralisations in the area, such as the cop- Xcalibur four-circle diffractometer using the MoKα radia- per mine of Stolzembourg (Philippo et al., 2007) and the tion (λ D 0:71073 Å) and equipped with an EOS Charge- antimony mine of Goesdorf (Philippo and Hanson, 2007; Coupled Device (CCD) detector. Unit cell parameters were Philippo and Hatert, 2018). refined from the powder diffraction data, using the LCLSQ software (Burnham, 1962): a D 13:01.13/, b D 4:15.2/, c D ◦ 3 3 Physical properties 15:36.14/Å, β D 109:4.6/ , V D 783.16/Å , Z D 2, space group P 21=m. Luxembourgite forms tiny fibres deposited on dolomite crys- The X-ray structural study was carried out on a fibre of tals; the fibres reach 200 µm in length but only 5 µm in di- luxembourgite measuring 0:005 × 0:005 × 0:150 mm. A to- ameter (Fig. 1). The colour is grey, the lustre is metallic, and tal of 361 frames with a spatial resolution of 1◦ were col- the streak is black. No cleavage has been observed, and the lected by the '=! scan technique, with a counting time of tenacity is brittle. The Mohs hardness, estimated by compar- 20 s per frame, in the range 5:00◦ < 2θ < 57:68◦. A total ison with litochlebite (Sejkora et al., 2011), is 3. The density, of 5904 reflections were extracted from these frames, cor- calculated from the chemical analysis and the single-crystal responding to 2117 unique reflections. The unit cell param- unit cell parameters, is 8.00 g cm−3. Optical properties were eters refined from these reflections are in very good agree- not determined due to small grain size. ment with those refined from the X-ray powder data (see above). Data were corrected for Lorentz, polarisation and ab- sorption effects, the latter with an empirical method using the SCALE3 ABSPACK scaling algorithm included in the CrysAlisRED package (Oxford Diffraction, 2007). More de- Eur. J. Mineral., 32, 449–455, 2020 https://doi.org/10.5194/ejm-32-449-2020 S. Philippo et al.: Luxembourgite, AgCuPbBi4Se8 451 Table 2. X-ray powder diffraction pattern of luxembourgite (d in Å). ∗ ∗ Iobs dobs dcalc Icalc hkl 20 4.61 4.588 11 2 0 −3 – – 4.330 5 3 0 −1 – – 3.790 8 3 0 −3 – – 3.647 22 3 0 1 – – 3.621 12 0 0 4 20 3.59 3.604 37 0 1 2 – – 3.443 4 2 1 0 – – 3.296 5 3 0 −4 – – 3.204 20 1 0 4 100 2.984 2.998 100 3 1 −1 – – 2.953 60 3 1 −2 – – 2.897 44 0 0 5 – – 2.800 16 3 1 −3 – – 2.582 4 3 1 −4 20 2.425 2.446 21 2 1 −5 – – 2.344 4 3 1 −5 – – 2.276 30 3 1 3 – – 2.172 6 4 1 2 – – 2.165 6 6 0 −2 – – 2.155 25 6 0 −3 – – 2.115 23 3 1 −6 60 2.085 2.077 36 0 2 0 – – 2.053 16 3 1 4 – – 1.969 6 1 1 6 – – 1.920 5 6 1 −2 1.909 9 3 1 −7 20 1.916 1.894 12 6 1 −1 – – 1.824 10 6 0 2 – – 1.805 7 3 2 1 – – 1.772 6 6 0 −7 – – 1.743 7 1 2 4 – – 1.688 18 0 2 5 20 1.490 1.496 16 6 2 −3 – – 1.475 5 6 1 4 – – 1.431 6 2 1 −10 – – 1.415 5 4 1 7 – – 1.370 8 6 2 2 1.363 12 9 1 −4 30 1.355 1.348 5 6 2 −7 1.345 4 8 1 −8 30 1.188 1.188 2 0 2 10 Data collected with a Rigaku Xcalibur four-circle diffractometer, MoKα radiation (λ D 0:71073 Å), EOS Rigaku detector.