Tennantite-(In), Cu6(Cu5In)As4S13, was approved as a new mineral species from the Pefka epithermal ore deposit, Alexandroupolis, Evros, Western Thrace, Greece. It was identified as anhedral grains, up to 0.1 mm in size, intimately associated with roquesite, galena and tennantite-(Fe) in quartz gangue. In reflected light, tennantite-(In) is isotropic, pale grey in colour. Reflectance values for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 31.2; 546: 30.7; 589: 30.5; 650: 28.8. Electron microprobe analysis gave (in wt.% – average of 7 spot analyses): Cu 43.83(95), Mn 0.22(8), Fe 0.65(46), Zn 0.57(12), Cd 0.16(5), Pb 0.26(2), As 14.38(3.00), In 5.05(59), Sb 7.74(3.08), Te 0.48(9), S 26.18(79), total 99.52(78). The empirical formula of the sample studied, recalculated on the basis of ΣMe = 16 atoms per formula unit, is Cu10.82In0.69Fe0.18Zn0.14Mn0.06Cd0.02Pb0.02As3.01Sb1.00Te0.06S12.81. Tennantite-(In) is cubic, I
$\bar 4$3m, with a = 10.285(2) Å, V = 1088.1(6) Å3 and Z = 2. The five strongest reflection lines in the calculated powder X-ray diffraction pattern are [d in Å, (I), hkl]: 2.969, (100), 222; 2.571, (20), 400; 1.878, (7), 521; 1.818, (42), 440; and 1.551, (21), 622. The crystal structure of tennantite-(In) has been refined by single-crystal X-ray diffraction data to a final R1 = 0.0253 on the basis of 240 unique reflections with Fo > 4σ(Fo) and 23 refined parameters. Tennantite-(In) is isotypic with other members of the tetrahedrite group. Indium is hosted at the tetrahedrally coordinated M(1) site, in accord with the known preference of this element for tetrahedrally coordinated bonding environments.