Indium-copper-rich sphalerite from the Restauradora vein, Capillitas, Catamarca, Argentina

The Restauradora vein, is one of the 20 veins of Capillitas (27°27′ S, 66°30′ W), an epithermal precious- and base-metal vein deposit belonging to the Farallón Negro Mining District, northwestern Argentina. One of the main minerals of the paragenesis of Restauradora vein is sphalerite, showing a large substitution of Zn by In and Cu with small participation of Cd and Fe; the fluctuating Zn composition ranges from 0.99 to 0.49 apfu, while In and Cu ranges (apfu) are: 0.22–0.27 and 0.21–0.25, respectively. Under the polarizing-light microscope, using reflected light, the optical properties are those of sphalerite, but with decreasing contents of Zn, especially when Zn is ≤0.60 apfu, a new variety differentiates from regular sphalerite and develops ribbons with paler colors, higher reflectance, softer relative hardness and no internal reflections. The main reflectance values of this In-Cu-rich sphalerite are (Rair, λ/nm): 20.55, 470; 19.43, 546; 19.02, 589; and 18.42, 650. The strongest five x-ray powder-diffraction lines of the synthetic analogue [d in Å (I) (hkl)] are: 3.173 (100) (111); 1.943 (54) (202); 1.657 (37) (113); 1.122 (22) (224); 1.261 (19) (133), the space group is F-43m, with a = 5.49692(10) Å, V = 166,100(10) ų, Z = 4. Electron-microprobe analyses of the In-Cu-rich sphalerite grains (n = 10) give an average composition (wt%) of: S 29.51, Zn 31.03, In 24.89, Cu 13.49, Ga 0.96, Cd 0.54, Fe 0.05; the average (n = 7) chemical composition of the synthetic phase is: S 29.67, Zn 30.32, In 25.94, Cu 13.82. Despite of the differences, and given that Zn is the main cation and that the structure is the same as that of sphalerite, the mineral studied here is considered as the richest In-Cu variety of sphalerite in the world.