The new mineral chongite (IMA2015–039), Ca3Mg2(AsO4)2(AsO3OH)2·4H2O, 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 ∼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 ∼3½, tenacity is brittle and fracture is conchoidal. Cleavage is good on {100}. The measured density is 3.09(2) g/cm3 and the calculated density is 3.087 g/cm3. Optically, chongite is biaxial (–) with α = 1.612(1), β = 1.626(1), γ = 1.635(1) and 2V = 76.9(1)° (measured in white light). Dispersion is r < v, distinct. The optical orientation is X = b; Z ^ a = 27° in obtuse angle β. The mineral is slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is (Ca2.90Mg1.93Mn0.14)Σ4.97As4O20H10.07. Chongite is monoclinic, C2/c, a = 18.5879(6), b = 9.3660(3), c = 9.9622(7) Å, β = 96.916(7)°, V = 1721.75(14) Å3 and Z = 4. The eight strongest powder X-ray diffraction lines are [dobs Å(I)(hkl)]: 8.35(29)(110), 4.644(62)(-311,020,400,-202), 4.396(26)(311), 3.372(62)(022,312,-511), 3.275(100)(420,-222,-421), 3.113(57)(222), 2.384(30)(711,530,-712) and 1.7990(22)(-913,334,-534). The structure determination (R1 = 1.56% for 1849 Fo > 4σF reflections) confirms that chongite is a member of the hureaulite group.

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