Kenhsuite, gamma -Hg 3 S 2 Cl 2 , a new mineral species, occurs with cinnabar and mercury sulfo-halide minerals at the McDermitt mercury mine, Humboldt County, Nevada, where it is associated with the alpha form, corderoite, alpha -Hg 3 S 2 Cl 2 . Kenhsuite crystals, 1X10 mu m long, are dispersed along fractures in altered tuffaceous lacustrine rocks. Kenhsuite is orthorhombic, with possible space-groups Ammm, A2mm, Am2m, Amm2, or A222; a 9.332(5), b 16.82(2), c 9.108(5) Aa, V 1429.63 Aa 3 , a:b:c = 0.5548:1.0:0.5415, and Z = 8. The strongest four peaks in the X-ray powder-diffraction pattern [d in Aa(I)(hkl)] are 2.58(100)(242), 3.65(90)(122), 3.11(51)(300), and 2.60(49)(331). Color and streak are canary yellow. The mineral blackens on exposure to sunlight. Kenhsuite is transparent, with a glassy luster; it has a Mohs hardness of 2-3 and a conchoidal and hackly fracture. It fluoresces red and red-orange under 366 nm ultraviolet radiation. The measured density is 6.83 (5) g/cm 3 , and the calculated density, 6.87 g/cm 3 . Kenhsuite is biaxial (+), with 2V z greater than 70 degrees , and index of refraction 2.25+ or -0.01. It has pleochroic colors that are weak pale yellow to greenish yellow. In reflected light, kenhsuite has medium to low reflectance, estimated at about 15%, and is white with abundant bright canary yellow to palest yellow-white internal reflections. Its polishing hardness is about the same as cinnabar (soft, 2-2.5). Kenhsuite formed later than cinnabar and corderoite, and possibly, in part simultaneously with corderoite. The mineral is named after Dr. Kenneth Jinghwa Hsu, Professor Emeritus, Swiss Federal Institute of Technology, Zurich, Switzerland.