Abstract
Abstract. The new mineral gobelinite, ideally CoCu4(SO4)2(OH)6⚫6H2O, is a new member of the ktenasite group and the Co analogue of ktenasite, ZnCu4(SO4)2(OH)6⚫6H2O. It occurs at Cap Garonne (CG), Var, France (type locality), and Eisenzecher Zug (EZ), Siegerland, North Rhine-Westphalia, Germany (cotype locality). The mineral forms pale green, bluish green or greyish green, blocky to thin, lath-like crystals. They are transparent and non-fluorescent, with a vitreous, sometimes also pearly, lustre and a white streak having a pale-green cast. Mohs hardness is about 2.5. The crystals are brittle with an irregular fracture; no cleavage was observed. D(meas.) is 2.95(2) and D(calc.) is 2.907 g cm−3 (for empirical formula, CG). Common associates are brochantite and various other hydrated metal sulfates. Electron-microprobe analyses of the CG material yielded (wt. %) CuO 42.45, CoO 6.58, NiO 3.37, ZnO 3.14, SO3 22.12, and H2O 22.62 (calculated on structural grounds), and total = 100.30 wt. %, giving the empirical formula (based on 20 O atoms) (Co0.63Ni0.32Zn0.28Cu3.83)Σ5.06S1.98O20H18.00. The simplified formula is (Co,Ni)(Cu,Zn)4(SO4)2(OH)6⚫6H2O, and the endmember formula is CoCu4(SO4)2(OH)6⚫6H2O. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS) analyses of the (Zn-free) EZ material gave the simplified average formula (Co0.92Ni0.21Mg0.01Cu3.79)Σ4.93(SO4)2.08(OH)6⚫6H2O. Optically, gobelinite (CG) is biaxial negative, with α=1.576(2), β=1.617(2) and γ=1.630(2); 2Vmeas=58(4)∘ and 2Vcalc=57.5∘. Dispersion is weak, r>v; orientation is X=β, Y=γ and Z≈α, with strong pleochroism X equaling colourless, Y equaling green and Z equaling pale green. The mineral is monoclinic, space group P21∕c, with a=5.599(1), b=6.084(1), c=23.676(5) Å, β=95.22(3)∘ and V=803.2(3) Å3 (at 100 K; CG) and a=5.611(1), b=6.103(1), c=23.808(5) Å, β=95.18(3)∘ and V=811.9(3) Å3 (at 298 K; EZ), respectively (Z=2). The eight strongest measured powder X-ray diffraction lines (d in Å (I) hkl (CG material)) are 11.870 (100) 002, 5.924 (40) 004, 4.883 (10) 102, 4.825 (15) 013, 3.946 (15) 006, 2.956 (15) 008, 2.663 (20) 202 and 2.561 (15) 1‾23. Single-crystal structure determinations gave R1=0.0310 (CG) and 0.0280 (EZ). The atomic arrangement is based on brucite-like sheets formed from edge-sharing, Jahn–Teller-distorted (4+2 coordination) CuO6 octahedra. These sheets are decorated on both sides with SO4 tetrahedra and linked via hydrogen bonds to interstitial, fairly regular Co(H2O)6 octahedra. The name alludes to the Old French word gobelin, equivalent to the German word kobold, from which the designation of the element cobalt was derived.
Highlights
The secondary hydrated Zn–Cu–sulfate ktenasite was first described from Lavrion, Greece, by Kokkoros (1950), who suggested the approximate formula (Cu,Zn)3SO4(OH)2 q2H2O
(1977) and Olsen and Lewis (1979) provided new data on ktenasite from Norway and Colorado, USA, respectively, and the crystal structure (monoclinic, P 21/c, a = 5.589(1), b = 6.166(1), c = 23.751(7) Å and β = 95.55(1)◦) was published by Mellini and Merlino (1978). They showed that the mineral can be viewed as having the general formula (M1)2(M2)4(M3)4(SO4)4(OH)12 q12H2O, where M1 = Zn, or another minor divalent cation with near regular octahedral coordination geometry, and with M2 and M3 being sites with more or less Jahn–Teller-distorted octahedral coordination geometries, which are occupied by a mixture of Cu, Zn or other cations
After the approval of the Mg analogue of ktenasite with the name fehrite, MgCu4(SO4)2(OH)6 q6H2O (IMA2018125a; Schlüter et al, 2020), the ideal formula of ktenasite was modified in an executive decision taken by the IMA
Summary
The secondary hydrated Zn–Cu–sulfate ktenasite was first described from Lavrion, Greece, by Kokkoros (1950), who suggested the approximate formula (Cu,Zn)3SO4(OH) q2H2O. 30 years later, Raade (1977) and Olsen and Lewis (1979) provided new data on ktenasite from Norway and Colorado, USA, respectively, and the crystal structure (monoclinic, P 21/c, a = 5.589(1), b = 6.166(1), c = 23.751(7) Å and β = 95.55(1)◦) was published by Mellini and Merlino (1978) They showed that the mineral can be viewed as having the general formula (M1)2(M2)4(M3)4(SO4)4(OH) q12H2O, where M1 = Zn, or another minor divalent cation with near regular octahedral coordination geometry, and with M2 and M3 being sites with more or less Jahn–Teller-distorted octahedral coordination geometries, which are occupied by a mixture of Cu, Zn or other cations. 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. Box 666, Melbourne, Victoria 3001, Australia, registration number M54565 (Cap Garonne), and the Naturhistorisches Museum, Burgring 7, 1010 Vienna, Austria, registration number O 1045 (Eisenzecher Zug)
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