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

Abstract

The Restauradora vein, is one of the 20 veins of Capillitas (27270 S, 66300 W),an epithermal precious- and base-metal vein deposit belonging to the FarallonNegro Mining District, northwestern Argentina. One of the main minerals ofthe paragenesis of Restauradora vein is sphalerite, showing a large substitutionof Zn by In and Cu with small participation of Cd and Fe; the fluctuating Zncomposition 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 withdecreasing contents of Zn, especially when Zn is ≤0.60 apfu, a new variety differentiatesfrom regular sphalerite and develops ribbons with paler colors,higher reflectance, softer relative hardness and no internal reflections. Themain 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 powderdiffractionlines 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), thespace group is F-43m, with a = 5.49692(10) Å, V = 166,100(10) Å3, Z = 4.Electron-microprobe analyses of the In-Cu-rich sphalerite grains (n = 10) givean average composition (wt%) of: S 29.51, Zn 31.03, In 24.89, Cu 13.49, Ga0.96, Cd 0.54, Fe 0.05; the average (n = 7) chemical composition of the syntheticphase 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 thatof sphalerite, the mineral studied here is considered as the richest In-Cu varietyof sphalerite in the world.