Tennantite-(Cd), Cu6(Cu4Cd2)As4S13, from the Berenguela mining district, Bolivia: the first Cd-member of the tetrahedrite group

Abstract

AbstractTennantite-(Cd), Cu10Cd2As4S13, was approved as a new mineral species from the Berenguela mining district, Pacajes Province, La Paz, Bolivia. It occurs as black metallic anhedral grains, up to 1 mm across, associated with baryte, montmorillonite and secondary Cd-sulfates (aldridgeite, niedermayrite and voudourisite). In reflected light, tennantite-(Cd) is isotropic and grey with brownish tints. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 30.0; 546: 30.1; 589: 28.2; and 650: 25.8. Electron microprobe analysis gave (in wt.% – average of 11 spot analyses): Cu 40.56(32), Ag 0.05(5), Fe 0.04(1), Zn 1.91(63), Cd 11.32(1.09), Hg 0.04(7), As 19.04(43), S 26.78(30), total 99.74(53). On the basis of ΣMe = 16 atoms per formula unit (apfu), the empirical formula of tennantite-(Cd) is Cu9.98Ag0.01Cd1.57Zn0.46Fe0.01As3.97S13.05. Tennantite-(Cd) is cubic, I$\bar{4}$3m, with unit-cell parameters a = 10.3088(2) Å, V = 1095.53(6) Å3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0152 on the basis of 359 unique reflections with Fo > 4σ(Fo) and 22 refined parameters. Tennantite-(Cd) is isotypic with other tetrahedrite-group minerals. Its crystal-chemistry is discussed and previous findings of Cd-rich tetrahedrite-group minerals are briefly reviewed.