Hanswilkeite, KFe3+S2, is a new potassium-rich natural sulfide discovered in the pyrometamorphic suite of the Hatrurim Formation, southern Negev Desert, Dead Sea basin, Israel. The mineral occurs in sulfide-calcite assemblages confined to black-colored calcite-spurrite marbles. It forms single-crystal grains up to 1 mm in size, isometric to lath-like, often intergrown with a less-common rasvumite, KFe2S3. Associated minerals include srebrodolskite, tilleyite, fluormayenite, cuspidine, fluorapatite, oldhamite, pyrite, and andradite. Macroscopically, hanswilkeite has a deep-purple color, dull metallic luster and brown-black streak. The Mohs’ hardness is 2. Moderate cleavage was observed along the c-axis. The calculated density is 2.654 g·cm-3. The Raman spectrum contains the following bands: 379, 357, 289, 236, 167, 131 and 124 cm-1. In reflected light, the mineral has very strong pleochroism from yellow-pink to dark-grey. Anisotropy is very strong, ∆R589 = 69 %. Reflectance values for COM required wavelengths measured in air, Rmax/Rmin (λ, nm) (%): 16.0/9.2 (470); 19.6/9.3 (546); 18.5/9.0 (589); 32.0/9.3 (650). Chemical composition (electron microprobe, average of 6 points, wt. %): K 23.78, Ca 0.44, Fe 34.75, Mn 0.60, Zn 0.47, S 39.46, Total 99.5, that corresponds to empirical formula (K0.98Ca0.02)1.00(Fe1.00Mn0.02Zn0.01)1.03S1.98 (Σ = 4 apfu) or ideally KFe3+S2. Single-crystal X-ray diffraction study shows that the mineral is monoclinic, space group C2/c (#15), with unit-cell parameters a = 7.0914(5), b = 11.3154(5), c = 5.3992(3) Å, β = 113.244(7)°, V = 398.08(4) Å3 and Z = 4. Strongest lines of X-ray powder diffraction pattern [d in Å(I)(hkl)]: 5.68(100)(020,110); 3.270(31)(130); 3.227(29)(111); 2.921(45)(–221); 2.510(12)(131); 2.198(12)(–132); 1.880(10)(330). The crystal structure has been solved and refined to R1 = 0.038 for 454 unique observed reflections [I ≥ 2σ(I)]. The structure consists of infinite chains of edge-sharing tetrahedra [FeS4]– centered with Fe3+; the sulfide chains are linked by K+ ions. Hanswilkeite is the third discovered dithioferrate mineral: a sulfosalt that contains [FeS2]– anion with iron in Fe3+ state. Other known natural dithioferrates are erdite, NaFeS2·2H2O, and raguinite, TlFeS2. Hanswilkeite has a synthetic counterpart and a group of related synthetic sulfides and selenides, well studied due to specific electrical and magnetic properties owed to their quasi-one-dimensional structures. The mineral can be considered as an indicator of an extreme potassium-rich environment superimposed onto anhydrous and oxidizing formation conditions. The association with oldhamite is herein discussed in view of super-reduced conditions previously supposed for oldhamite geosynthesis.

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