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Mineralogical ClassificationIMA 2015-039 = chongite

30th Nov 2016 05:05 UTCMarco E. Ciriotti Manager

Reference:

▪ Kampf, A.R., Nash, B.P., Dini, M., Molina Donoso, A.A. (2016): Chongite, Ca3Mg2(AsO4)2(AsO3OH)2·4H2O, a new arsenate member of the hureaulite group from the Torrecillas mine, Iquique Province, Chile. Mineralogical Magazine, 80, 1255-1263.


Abstract:

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 to about 1 mm in length 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 ca. 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 X-ray powder 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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