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IMA 2013-111 = jahnsite-(CaFeMg)

Posted by Marco E. Ciriotti  
Marco E. Ciriotti January 11, 2017 06:19PM
▪ Elliott, P. (2016): Jahnsite-(CaFeMg), a new mineral from Tom’s quarry, South Australia: description and crystal structure. European Journal of Mineralogy, 28, 991-996.

Jahnsite-(CaFeMg) is a new member of the whiteite–jahnsite group from Tom’s Quarry, Kapunda, South Australia, Australia where it occurs as a low-temperature secondary mineral. Jahnsite-(CaFeMg) forms brownish orange prismatic crystals to 0.2 mm in length. Crystals are transparent, have a vitreous lustre and a good cleavage on {001}. The streak is white. The Mohs hardness is ∼4, the tenacity is brittle and the fracture is splintery. The measured density is 2.76(4) g/cm3 and the calculated density is 2.772 g/cm3. Optically jahnsite-(CaFeMg) is biaxial negative with α = 1.629(4), β = 1.658(4), γ = 1.677(4) and 2Vcalc = 76.8°. Electron-microprobe analyses provided: Na2O 0.65, CaO 4.74, Al2O3 0.06, Fe2O3 20.18, FeO 9.85, MgO 6.47, MnO 5.02, P2O5 34.41, H2Ocalc 19.46 (from crystal-structure analysis), total 100.84 wt.%. The empirical formula (based on 26 oxygen atoms) is: (Ca0.70Na0.17Mn2+0.16)Σ1.03Fe2+1.00(Mg1.33Mn2+0.43Fe3+0.24)Σ2.00(Fe3+1.99Al0.01)Σ2.00(PO4)4.01(OH)2.10H2O7.88. Jahnsite-(CaFeMg) is monoclinic, P2/a, a = 14.975(5), b = 7.1645(14), c = 9.928(2) Å, β = 110.65(3)°, V = 996.7(5), Å3 and Z = 2. The eight strongest lines in the powder diffraction pattern are [d(Å)(I)(hkl)]: 9.339(100)(001); 4.923(20)(21–1, 111, 310); 3.562(20)(400); 3.518(20) (112, 40–2); 3.453(20)(202); 2.965(20)(221); 2.839(35)(5-1-1); 2.592(20)(203). The crystal structure has been refined using a single crystal twinned on {001} to R1 = 0.0517 on the basis of 1616 observed reflections with Fo > 4σ(Fo) and confirms a whiteite–jahnsite-type structure.
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