Keutschite, Cu ₂ AgAsS ₄ , a new mineral with a stannite structure from the Uchucchacua polymetallic deposit, Lima Department, Peru

Abstract. Keutschite, Cu2AgAsS4, is a new mineral from the Ag–Pb–Zn deposit at Uchucchacua, Oyon District, Catajambo, Lima Department, Peru. The mineral occurs as metallic, highly lustrous, blocky, free-standing crystals measuring up to 3 mm. These crystals exhibit a grey colour with a slight green-brassy tint and a grey-black streak, and they are present on both manganoquadratite and proustite. It was observed that keutschite was brittle, and no fractures or cleavages were identified. In plane-polarised light, keutschite exhibits a grey hue devoid of any discernible internal reflections. It demonstrates a minimal manifestation of pleochroism and exhibits a negligible degree of bireflectance. Between crossed polars, the mineral is weakly anisotropic with rotation tints in shades of greenish grey to grey. Reflectance measurements in air yield the following Rmin/Rmax values for wavelengths recommended by the Commission on Ore Mineralogy of the International Mineralogical Association: 25.2/26.1 (470 nm), 29.6/29.4 (546 nm), 29.4/29.2 (589 nm), and 28.5/28.6 (650 nm). Keutschite crystallises in a tetragonal geometry and is classified as space group I4‾2m. The unit cell parameters are as follows: a=5.5834(15), c=10.021(3) Å, V=312.40(14) Å3, a:b:c=1:1:0.897, and Z=2. The crystal structure was refined to R1=0.0199 for 286 reflections with I>3σ(I). The structure of keutschite is derived from that of sphalerite by ordered substitution of Zn atoms, analogous to the substitution pattern for deriving stannite from sphalerite. The crystal structure of the mineral can be derived from that of luzonite through the complete substitution of one of the two copper sites with silver. The five strongest intensities in the X-ray powder diagram are [d in Å (intensity) hkl]: 3.101 (100) 110; 2.792 (11) 200; 1.974 (20) 220; 1.665 (34) 204; and 2.846 (27) 312. The chemical formula, as determined by electron microprobe analysis, is Cu2.05Ag0.96(As0.95Sb0.04)Σ0.99S4.00 (based on eight atoms). The ideal formula, derived from the crystal structure, is Cu2AgAsS4. The name honours Frank Keutsch (born 1971) for his contribution to the mineralogy of the Uchucchacua deposit.