The new mineral shumwayite (IMA2015-058), [(UO2)(SO4)(H2O)2]2·H2O, was found in the Green Lizard and Giveaway-Simplot mines, White Canyon district, San Juan County, Utah, USA, where it occurs as a secondary alteration phase. At the Green Lizard mine, it is found in association with calcite, gypsum, plášilite, pyrite, rozenite and sulfur; at the Giveaway-Simplot mine, shumwayite is associated with rhomboclase and römerite. The mineral occurs as pale greenish-yellow monoclinic prisms, elongated on [100], up to ∼0.3 mm long and commonly in subparallel to random intergrowths. The mineral is transparent with a vitreous lustre and has a white streak. It fluoresces bright greenish white under both longwave and shortwave ultraviolet radiation. The Mohs hardness is ∼2. Crystals are brittle with perfect {011} cleavage and irregular fracture. The mineral is slightly deliquescent and is easily soluble in room temperature H2O. The calculated density is 3.844 g cm–3. Optically, shumwayite is biaxial (+/–), with α = 1.581(1), β = 1.588(1), γ = 1.595(1) (measured in white light). The measured 2Vx based on extinction data collected on a spindle stage is 89.8(8)°; the calculated 2Vx is 89.6°. Dispersion is strong, but the sense is not defined because the optic sign is ambiguous. No pleochroism was observed. The optical orientation is X = b, Y ≈ c, Z ≈ a. Energy-dispersive spectrometer analyses (with H2O based on the crystal structure) yielded the empirical formula U2.01S1.99O12.00·5H2O. Shumwayite is monoclinic, P21/c, a = 6.74747(15), b = 12.5026(3), c = 16.9032(12) Å, β = 90.919(6)°, V = 1425.79(11) Å3 and Z = 4. The crystal structure (R1 = 1.88% for 2936 F > 4σF) contains UO7 pentagonal bipyramids and SO4 tetrahedra that link by corner-sharing to form [(UO2)(SO4)(H2O)2] chains along [100]. The chains and isolated H2O groups between them are linked together only by hydrogen bonds. The mineral is named in honour of the Shumway family, whose members account for the discovery and mining of hundreds of uranium deposits on the Colorado Plateau, including the Green Lizard mine.

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