The new mineral gajardoite (IMA2015-040), KCa0.5As43+O6Cl2 center dot 5H(2)O, was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with native arsenic, arsenolite, chongite, talmessite and torrecillasite. Gajardoite occurs as hexagonal plates up to similar to 100 mu m in diameter and 5 mu m thick, in rosette-like subparallel intergrowths. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is similar to 11/2, tenacity is brittle and fracture is irregular. Cleavage is perfect on {001}. The measured density is 2.64 g/cm(3) and the calculated density is 2.676 g/cm3. Optically, gajardoite is uniaxial (-) with omega = 1.780(3) and epsilon = 1.570(5) (measured in white light). The mineral is very slowly soluble in H2O and slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is (K0.77Ca0.71Na0.05Mg0.05)(Sigma 1.58)As4O11Cl1.96H9.62. Gajardoite is hexagonal, P6/mmm, a = 5.2558(8), c = 15.9666(18) angstrom, V = 381.96(13) angstrom(3) and Z = 1. The eight strongest powder X-ray diffraction lines are [d(obs) angstrom(I)(hkl)]: 16.00(100)(001), 5.31(48)(003), 3.466 (31)(103), 3.013(44)(104), 2.624(51)(006,110,111), 2.353(36)(113), 1.8647(21)(116,205) and 1.4605(17) (119,303,216). The structure, refined to R-1 = 3.49% for 169 F-o > 4 sigma F reflections, contains two types of layers. One layer of formula KAs43+O6Cl2 consists of two neutral As2O3 sheets, between which are K+ cations and on the outside of which are Cl-anions. This layer is topologically identical to a slice of the lucabindiite structure and similar to a slice of the torrecillasite structure. The second layer consists of an edge-sharing sheet of Ca(H2O)(6) trigonal pyramids with isolated H2O groups centred in the hexagonal cavities in the sheet.