The new mineral juansilvaite (IMA2015-080), Na5Al3[AsO3(OH)](4) [AsO2(OH)(2)](2)(SO4)(2) center dot 4H(2)O, was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with anhydrite, canutite, halite, sulfur and a mahnertite-like phase. Juansilvaite occurs as bright pink blades up to similar to 0.5 mmlong grouped in tightly intergrown radial aggregates and also as opaque dull pale pink rounded aggregates. Blades are flattened on {001}, elongated on [100] and exhibit the forms {001}, {111} and {201}. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is similar to 2(1/2), tenacity is brittle and fracture is irregular. Cleavage is very good on {001}. The measured density is 3.01(2) g cm(-3) and the calculated density is 3.005 g cm(-3). Optically, juansilvaite is biaxial (+) with alpha = 1.575(1), beta = 1.597(1), gamma = 1.623(1) and 2V = 86(1)degrees (measured in white light). Dispersion is r < v, slight, and the orientation is X = b; Z <^> c = 27 degrees in the obtuse angle beta. The pleochroism is X > Y approximate to Z in shades of pale pink. The mineral is slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is Na4.95Al2.28Fe0.503+ Mn-0.21(3+) Cu0.04As5.92S1.83O36H17.37. Juansilvaite is monoclinic, C2/c, a = 18.1775(13), b = 8.6285(5), c = 18.5138(13) angstrom, beta= 90.389(6)degrees, V = 2903.7(3)angstrom(3) and Z = 4. The eight strongest powder X-ray diffraction lines are [d(obs) angstrom(I)(hkl)]: 9.25(100)(002), 7.20(34)((1) over bar 11), 4.545(34)(400), 3.988(39)((1) over bar 14), 3.363(42)(314), 3.145(66)((5) over bar 12,420), 2.960(68)((4) over bar 22,422) and 2,804(33)(131,(4) over bar 23). The structure of juansilvaite (R-1 = 3.82% for 2040 F-o > 4 sigma F reflections) contains layers made up of alternating corner-linked Al-O octahedra and acid-arsenate tetrahedra. Sodium cations occur both peripheral to the layers and within cavities in the layers. An SO4 tetrahedron and an H2O group also are in the interlayer region.