Tetrahedrite-(Cd), Cu₆(Cu₄Cd₂)Sb₄S₁₃, from Radětice near Příbram, Czech Republic: the new Cd member of the tetrahedrite group

Abstract

Tetrahedrite-(Cd), Cu6(Cu4Cd2)Sb4S13, is a new IMA-approved mineral species from Radětice near the Příbram deposit, Czech Republic. It occurs as black metallic anhedral grains, up to 200 µm in size in quartz–calcite gangue, associated with galena, bournonite, sphalerite, pyrite, geocronite, silver, stromeyerite, tetrahedrite-(Zn), tetrahedrite-(Fe), tetrahedrite-(Cu), and an unknown Pb-rich tetrahedrite-like phase. In reflected light, tetrahedrite-(Cd) is isotropic and grey with greenish tints. Reflectance data for the four COM wavelengths in air are as follows (λ (nm) : R (%)): 470 : 30.8, 546 : 31.1, 589 : 31.1 and 650 : 28.8. Electron microprobe analysis gave (in wt % – average of 13 spot analyses) Cu 34.85, Ag 2.09, Fe 0.18, Zn 0.26, Cd 11.03, Hg 0.75, Pb 0.31, Sb 28.07, and S 23.38, with a total of 100.92. On the basis of ΣMe = 16 atoms per formula unit, the empirical formula of tetrahedrite-(Cd) is Cu9.65Ag0.34Cd1.73Zn0.07Hg0.07Fe0.06Pb0.03Sb4.06S12.84. Tennantite-(Cd) is cubic, I4‾3m, with unit-cell parameters a=10.504(3) Å, V=1158.9(9) Å3, and Z=2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1=0.0252 on the basis of 257 unique reflections with Fo>4σ(Fo) and 23 refined parameters. Tetrahedrite-(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.