The new mineral chongite (IMA2015-039), Ca3Mg2(AsO4)(2)(AsO3OH)(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 native arsenic, arsenolite, gajardoite, talmessite and torrecillasite. Chongite occurs as prismatic crystals up to similar to 1 mm long grouped in tightly intergrown radial aggregates up to 2 mm in diameter. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is similar to 31/2, tenacity is brittle and fracture is conchoidal. Cleavage is good on {100}. The measured density is 3.09(2) g/cm(3) and the calculated density is 3.087 g/cm3. Optically, chongite is biaxial (-) with alpha = 1.612(1), beta = 1.626(1), gamma = 1.635(1) and 2V = 76.9(1)degrees (measured in white light). Dispersion is r < v, distinct. The optical orientation is X = b; Z (boolean AND) a = 27 degrees in obtuse angle beta. The mineral is slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is (Ca2.90Mg1.93Mn0.14)(Sigma 4.97)As4O20H10.07. Chongite is monoclinic, C2/c, a = 18.5879(6), b = 9.3660(3), c = 9.9622(7) angstrom, beta = 96.916(7)degrees, V = 1721.75(14) angstrom(3) and Z = 4. The eight strongest powder X-ray diffraction lines are [ d(obs) angstrom(I)(hkl)]: 8.35(29)(110), 4.644(62) (<(3)over bar>11,020,400,(2) over bar 02), 4.396(26)(311), 3.372(62)(022,312, (5) over bar 11), 3.275(100)(420, (2) over bar 22, (4) over bar 21), 3.113(57)(222), 2.384(30)(711,530, (7) over bar 12) and 1.7990(22)((9) over bar 13,334,(5) over bar 34). The structure determination (R-1 = 1.56% for 1849 F-o > 4 sigma F reflections) confirms that chongite is a member of the hureaulite group.