Cuyaite (IMA2019-126), Ca2Mn3+As143+O24Cl, is a new arsenite mineral from near Cuya in the Camarones Valley, Arica Province, Chile. It is associated with anhydrite, native arsenic, arsenolite, calcite, claudetite, ferrinatrite, gajardoite-3R, leiteite, magnesiocopiapite, phosphosiderite, pyrite, realgar and talmessite and formed from the oxidation of As-bearing primary phases and alteration by saline fluids derived from evaporating meteoric water under hyperarid conditions. Cuyaite occurs as pale brown thin needles (elongated on [010]), typically in divergent sprays and subparallel intergrowths. The streak is white. Crystals are transparent with adamantine lustre; subparallel intergrowths exhibit silky lustre. The mineral has Mohs hardness of 21/2, is brittle, exhibits no cleavage and has irregular fracture. The calculated density is 4.140 g cm(-3). Cuyaite is optically biaxial (-), with alpha = 1.87(1), beta = 1.956(calc) and gamma = 1.98(1), determined in white light; 2V(meas) = 60(1)degrees; and orientation: X = b and Y boolean AND a= 53 degrees in obtuse beta . Electron microprobe analyses provided the empirical formula Ca2.03Mn0.9533+(As13.663+Sb0.653+)(Sigma 14.31)O24Cl0.88. The six strongest powder X-ray diffraction lines are [d(obs) angstrom(I)(hkl)]: 4.73(45) (111, (1) over bar 12), 3.162(100)((3) over bar 14), 3.035(28)(213), 3.004(37)(204), 2.931(90)((2) over bar 15, 312) and 2.779(28)(020). Cuyaite is monoclinic, Pn, a= 14.7231(6), b = 5.58709(19), c = 17.4185(12) angstrom, beta =112.451(8)degrees, V= 1324.23(14) angstrom(3) and Z = 2. In the crystal structure of cuyaite (R-1 = 0.0369 for 2095 I > 2 sigma I reflections), AsO3 pyramids share O corners to form a 'loose' 3D framework; Jahn-Teller distorted Mn3+O6 octahedra and CaO8 polyhedra link by edges and corners to form columns; the columns also link by edge- and corner-sharing to the AsO3 pyramids in the framework; Cl occupies channels along [010] in the framework. The Raman spectrum is consistent with the presence of multiple (As+O3)-O-3 groups.