Gobelinite, the Co analogue of ktenasite from Cap Garonne, France, and Eisenzecher Zug, Germany

Stuart J. Mills1, Uwe Kolitsch2,3, Georges Favreau4, William D. Birch1, Valérie Galea-Clolus5, and Johannes Markus Henrich6 1Geosciences, Museums Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia 2Mineralogisch-Petrographische Abt., Naturhistorisches Museum, Burgring 7, 1010 Vienna, Austria 3Institut für Mineralogie und Kristallographie, Universität Wien, Althanstraße 14, 1090 Vienna, Austria 4independent researcher: 421 Avenue Jean Monnet, 13090 Aix-en-Provence, France 5independent researcher: 10 rue Combe Noire, 83210 Solliès-Toucas, France 6independent researcher: Im Großen Garten 3, 57548 Kirchen (Sieg), Germany

Studies of ktenasites from many worldwide localities over the last two decades by the authors and colleagues in Austria and Australia have shown that the crystal chemistry of the mineral is quite variable and that some ktenasite samples actually represent new mineral species, in which Zn may be partially or fully replaced by Mg, Co, Ni and Cd (nomenclature proposal in preparation). Originally independent studies of the Co analogue of ktenasite from a French locality (Cap Garonne) and a German locality (Eisenzecher Zug mine), led by the first and second author, respectively, were combined to propose a new mineral species, with the name gobelinite, to the IMA-CNMNC. Both the mineral and name were approved in late 2018 (IMA2018-167). The name, pronounced (/ /), is in reference to kobold (German), meaning "goblin", a superstitious term once used for the ore of cobalt by medieval miners. The etymology of both the element cobalt and cobaltite comes from the German word kobold (first used in the 16th century; Pfeifer, 2004), which itself originates from the Old French gobelin, used around 1195 in Ambroise of Normandy's Guerre sainte, and Medieval Latin gobelinus, used before 1141. Medieval miners in both Germany and France, as well as most of Europe, believed goblins to be expert miners and metalworkers who could be heard constantly drilling, hammering and shovelling. Legends suggest that goblins were evil creatures who poisoned ores in the mines, removed silver from ore and made it useless (kobold ore), and also made miners sick (Ashliman, 2006). The Cap Garonne ores contain Pb and As, which form part of this folklore. The Eisenzecher Zug mine is poor in Pb and As but locally rich in Co ores. In order to avoid confusion with cobaltite and because the Cap Garonne locality is in France, we prefer to honour the Old French roots of the word gobelin.

Occurrence
Gobelinite was found by Valérie Galea-Clolus on specimens from the famous Cap Garonne mine (Favreau and Galea-Clolus, 2014), Var, Provence-Alpes-Côte d'Azur, France (type locality; 43 • 4 53 N, 6 • 1 55 E), the type locality for 14 other minerals. More specifically, the type specimen of gobelinite was collected at pillar 78b in the north mine. Gobelinite is restricted to an area of the mine, called "fond de mine", where most of the new species were found at Cap Garonne. Gobelinite has also been found in pillars 78, 79 and 80.
Gobelinite was also found in 1997 on specimens from the Schlänger und Eichert vein of the Eisenzecher Zug mine (cotype locality; 50 • 49 9 N, 7 • 59 14 E), Eiserfeld, Siegerland, North Rhine-Westphalia, Germany, by Johannes Markus Henrich. The Eisenzecher Zug mine, which consolidated several older mines, worked on Fe−(Cu−Co) ores and was the most important mine in the Siegerland mining district (Henrich, 1998). The occurrence of gobelinite was restricted to a small, fissure-like area in an adit. In subsequent years, no additional material could be found.

Appearance and physical properties
At Cap Garonne, gobelinite occurs mostly as vitreous crystals, sometimes with a pearly lustre. Their colour is usually pale green and rarely bluish green. No colour zoning was observed in individual crystals. The crystal size is commonly 0.1 to 0.2 mm and rarely up to 0.5 mm in length. The crystals are blocky to thin and lath-like, with sharp rectangular outlines (Fig. 1) (Fig. 2). The crystals often form in the spaces between quartz grains, or between quartz pebbles and conglomerate. The crystals may be scattered or in groups forming fans or rosettes, sometimes densely covering the conglomerate matrix; they are mostly flat lying and more rarely free standing. The principal associated minerals are brochantite, cerussite and gordaite-group minerals ("guarinoite", thérèse-  magnanite and pale-green gordaite; Mills et al., 2019). The accessory associated species are serpierite, botallackite and an unknown copper sulfate currently under study. Sarp et al. (1990) also mentioned antlerite and anglesite at pillar 80 in association with what they called "cobalt-nickelktenasite" and for which they gave the empirical formula (Cu 3.53 Co 0.80 Ni 0.61 Zn 0.23 ) 5.17 (SO 4 ) 1.98 (OH) 6.38 q 5.7H 2 O. Gobelinite is likely to have formed from the weathering of cobaltite-gersdorffite and nearby tennantite under acidic conditions. At Eisenzecher Zug, gobelinite occurs as greyish green to pale greyish green, indistinct to lath-shaped, prismatic or acicular, subparallel, translucent crystals. The crystal length size ranges between 0.2 and 0.5 mm. On two other, noncotype specimens from the same material, pale greyish green, prismatic to acicular crystals form coral-like aggregates or sprays (flat lying, rarely free standing). The main associated mineral is brochantite (dark green globular aggregates, slightly Co-bearing). In the immediate vicinity of gobelinite, the following subordinate minerals were observed apart from brochantite: devilline (determined by powder X-ray diffraction), malachite, asbolane, spherocobaltite, langite, "sericite" and some unidentified phases. Gobelinite originated due to weathering, under acidic conditions, of a primary Cu-Co ore paragenesis composed of chalcocite, djurleite, carrollite (as aggregates, up to 3 cm in size, of interlocking single crystals); locally hematite; and, less commonly, pyrite, bornite, alloclasite and chalcopyrite. Gobelinite is directly grown on either quartz, siderite, carrollite or the slate country rock. Fewer than 10 gobelinite-bearing specimens were found.
Based on the two occurrences, gobelinite can be described as pale green, bluish green or greyish green with a white streak which has a pale-green cast. Crystals are vitreous, transparent -sometimes with a pearly lustre -and are nonfluorescent. Hardness is about 2.5 on the Mohs scale. Crystals are brittle with an irregular fracture; no cleavage was observed. The density measured by the sink-float method in Clerici solution is 2.95(2)g cm −3 (CG), while the calculated density based on the empirical formula and unit cell is 2.907 g cm −3 (CG). The X-ray density values from the single-crystal refinements (see Sect. 4) are 2.958 and 2.926 g cm −3 for CG and EZ material, respectively.

Chemical data
Quantitative wavelength-dispersive electron-microprobe analyses (10 points) of gobelinite from Cap Garonne were carried out using a JEOL Superprobe electron microprobe at the University of Melbourne. Operating conditions were 20 kV, 5 nA and a 20 µm defocused beam diameter. The ZAF correction method was used. No other elements apart from Co, Ni, Zn, Cu, Al, S and O were detected by energydispersive spectroscopy. Insufficient material is available for direct determination of water. The H 2 O content was therefore calculated on the basis of 6H 2 O+6(OH), in accordance with the results of the crystal-structure refinement. Analytical data are given in O + 6(OH) and excluding one outlier analysis with an anomalously high S : M ratio). Apart from minor substitutional differences, the two occurrences of gobelinite thus have very similar chemical compositions.

Cap Garonne
The single-crystal X-ray diffraction experiment was carried out on the microfocus macromolecular MX2 beamline at the Australian Synchrotron, part of the Australian Nuclear Science and Technology Organisation (ANSTO). A 10 × 10 × 2 µm light-green fragment was selected. A full Ewald sphere of intensity data was collected at 100 K by a Dectris EigerX 16M detector and monochromatic radiation with a wavelength of 0.71073 Å. The dataset was processed using XDS (Kabsch, 2010), XPREP (Bruker, 2001) and SAD-ABS (Bruker, 2001), leading to an R int of 0.0315. The crystal structure was solved by direct methods using SHELXT 2014/5 (Sheldrick, 2015b). All atoms were located, and the obtained structure model confirmed the ktenasite model of Mellini and Merlino (1978). The structure was refined with SHELXL-2016/6 (Sheldrick, 2015a), with anisotropic treatment of all non-H atoms. The U iso values of the hydrogen atoms were constrained to 0.029 Å 2 , and the O-H bond lengths were restrained to 0.90(2) Å. For the final refinement step, the coordinate set for ktenasite (Mellini and Merlino, The strongest lines are marked in bold. Indexing is based on the crystal-structure models. 1978) was adopted, and site populations were fixed based on EMPA; the calculations resulted in R 1 = 0.0310 for 2386 Powder X-ray diffraction data of the CG material were recorded using an Agilent SuperNova dual-source diffractometer with an Atlas charge-coupled device (CCD) detector. Data were measured at ambient temperature using MoKα radiation. Data are given in Table 2. Refined unitcell parameters, which are slightly larger than the singlecrystal ones due to the higher measurement temperature, are a = 5.6098(12), b = 6.0981 (7), c = 23.800(5) Å, β = 95.20(6) • , V = 810.8(9) Å 3 and Z = 2.
The final atomic positions and equivalent/isotropic displacement parameters for both CG and EZ crystals are given in Table 3. Tabulated anisotropic displacement parameters are provided in Table S1 in the Supplement. Selected bond lengths are given in Table 4.

Crystal structure
The structure of gobelinite is isostructural with that published by Mellini and Merlino (1978) for ktenasite (see also Mellini et al., 1981, for details on the crystal chemistry). The crystal structure of gobelinite is comprised of a brucite-like sheet formed from edge-sharing, Jahn-Teller-distorted (4+2 coordination) CuO 6 octahedra centred by the Cu1 and Cu2 sites. The sheets are decorated on both sides with SO 4 tetrahedra and linked via hydrogen bonds to interstitial, fairly regular Co(H 2 O) 6 octahedra (range of Co-O bond lengths: 2.061-2.092 for sample CG and 2.072-2.098 for sample EZ; Table 4). The Co-O bond lengths in gobelinite, 2.072 Å (Cap Garonne) and 2.082 Å (Eisenzecher Zug), are close to the grand mean value of 2.1115(621) Å for Co(II)O 6 polyhedra in inorganic compounds (Wildner, 1992), considering the partial substitutions of Co by Ni (and Zn at CG). These two different substitutions would both make the polyhedra slightly smaller because the ionic radii for octahedral coordination of both Zn 2+ (0.740 Å) and Ni 2+ (0.690 Å) are slightly smaller than that of Co 2+ (0.745 Å; values from Shannon, 1976). The corresponding Zn-O values in ktenasite are larger (2.089-2.097 Å for samples from various localities and with different Cu : Zn ratios and additional minor substituting elements (Mg, Ni and Cd); unpublished data of Kolitsch, Giester, Leverett, Sciberras, Williams and Hibbs). The Cu1-O bond lengths in gobelinite (Table 4) are similar to those in ktenasite, while the Cu2-O bond lengths are slightly shorter in gobelinite (2.101 and 2.105 Å for CG and EZ crystals, respectively) than in ktenasite (2.117-2.122 Å). However, both Cu1O 6 and Cu2O 6 octahedra are more (Jahn-Teller) distorted in gobelinite than in ktenasite. This is explained by the incorporation of minor Zn in both polyhedra in ktenasite. The S-O bond lengths obtained from both gobelinite refinements are comparable (1.478 Å for CG and 1.474 Å for EZ) and close to the expected value for sulfate minerals (1.473 Å; .  Two-sided tetrahedral decoration of brucite-like, MO 6based (M = Cu, Zn) sheets occurs in the crystal structures of schulenbergite, devilline, campigliaite, niedermayrite, christelite and serpierite Hawthorne and Schindler, 2000), all of which contain interlayer cations except schulenbergite. However, gobelinite has a different linkage of H bonds, and the interlayer cation forms an unconnected polyhedron which is held in the structure by hydrogen bonds of medium-weak to weak strength only (Fig. 3). These hydrogen bonds have donor-acceptor distances ranging from 2.706 to 3.399 Å (EZ; 298 K values) and 2.716 to 3.031 Å (CG; 100 K values; Table 5). The three shortest hydrogen bonds (O8-H81 q q q O5, O8-H82 q q q O6 and O9−H92 q q q O7) strengthen the structural network in the (010) plane. It is noteworthy that there is no difference between the hydrogenbonding network in gobelinite and that of ktenasite sensu stricto (Mellini et al., 1981). The positions of the hydrogen bonds are within experimental error between the two minerals, and there are no new linkages found in the structure of gobelinite. , parallel to the brucite-like sheets. Jahn-Tellerdistorted CuO 6 octahedra (blue), regular CoO 6 octahedra (magenta), SO 4 tetrahedra (yellow), O atoms (red) and H atoms (pale pink). The unit cell is outlined. Hydrogen bonds are shown as dashed lines. Drawing done with CrystalMaker (Palmer, 2009).

Relationship to other minerals
Gobelinite is the Co analogue of ktenasite, ideally ZnCu 4 (SO 4 ) 2 (OH) 6 q 6H 2 O, with Co replacing Zn in the interlayer octahedrally coordinated M1 site. It also