Chongite, Ca3Mg2(AsO4)2(AsO3OH)2·4H2O, a new arsenate member of the hureaulite group from the Torrecillas mine, Iquique Province, Chile

Abstract

AbstractThe 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/cm3and 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 isr < v,distinct. The optical orientation isX= 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,die, a =18.5879(6),b =9.3660(3),c =9.9622(7) Å, β = 96.916(7)°,V=1721.75(14) Å3and Z=4. The eight strongest powder X-ray diffraction lines are[dobsÅ(I)(hkl)]: 8.35(29)(110), 4.644(62) (3ˉ11,020,400,2̄02), 4.396(26)(311), 3.372(62)(022,312,5̄11), 3.275(100)(420,22ˉ2,421), 3.113(57)(222), 2.384(30)(711,530,7̄12) and 1.7990(22)(9̄13,334,5̄34). The structure determination(R1= 1.56% for 1849 Fo> 4σFreflections) confirms that chongite is a member of the hureaulite group.