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

Published online by Cambridge University Press:  02 January 2018

Anthony R. Kampf*
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
Barbara P. Nash
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
Maurizio Dini
Pasaje San Agustin 4045, La Serena, Chile
Arturo A. Molina Donoso
Los Algarrobos 2986, Iquique, Chile


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, die, 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) (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 > 4σF reflections) confirms that chongite is a member of the hureaulite group.

Research Article
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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