Abstract: Juabite, ideally Cu₅(Te⁶⁺O₄)₂(As⁵⁺O₄)₂·3H₂O, is triclinic, space-group choices P1(1) or P1¯(2), with unit-cell parameters refined from powder data: a = 8.984(5), b = 10.079(7), c = 8.975(5) Å, α = 102.68(7)°, β =92.45(6)°, γ = 70.45(5)° V = 746.8(8) ų, a:b:c =0.8914:1:0.8905, Z = 2. The strongest seven reflections of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 9.28 (70)(010), 4.65 (70)(020), 3.097 (100)(030,2¯11), 3.018 (60)(212), 2.658 (50)(3¯01), 2.468 (50)(2¯22¯) and 1.740 (50)(1¯15¯, 521, 1¯51¯). The mineral is an extremely rare constituent on the dumps of the Centennial Eureka mine, Juab County, Utah, U.S.A., where it occurs as crystalline platy masses that average 0.2–0.3 mm in longest dimension within small interconnected vugs of drusy quartz. Associated minerals are enargite, beudantite, and an undefined, possible Pb-analogue of arsenobismite. Individual crystals are subhedral to euhedral and average 125 × 100 × 1–2 µm in size. Cleavage {010} perfect. Forms are: {010} major; {100}, {1¯01}, and {101} minor. The mineral is translucent (masses) to transparent (crystals), emerald-green, with a pale green streak, and an uneven to subconchoidal fracture. Juabite is vitreous to adamantine (almost gemmy) on cleavage faces, brittle, and nonfluorescent; H (Mohs) 3–4; D (calc.) 4.59 g/cm³ for the idealised formula. In polished section, juabite is white in plane-polarised reflected light in air with ubiquitous turquoise-blue internal reflections; bireflectance and anisotropy are unknown (due to interference from internal reflections). Averaged electronmicroprobe analyses yielded CuO 38.25, PbO 0.57, TeO₃ 32.58, As₂O₅ 22.81, H₂O (calc. assuming 3H₂O) [5.19], total [99.40] wt.%, leading to the empirical formula (Cu_5.01Pb_0.03)_Σ5.04(TeO₄)_l.93(AsO₄)_2.07·3.00H₂O based on O = 19. The infrared absorption spectrum shows definite bands for structural H₂O with an O-H stretching frequency centred at 3283 cm⁻¹ and a H-O-H flexing frequency centred at 1642 cm⁻¹. The mineral name is for the county within the state of Utah in which the Centennial Eureka mine is located.